admin – Page 3

Diagonal Difference

Problem from Hackerrank.

function diagonalDifference(arr) {
    // Write your code here
    let arrayLength = arr.length;
    let diag1 = 0;
    let diag2 = 0;

    for( let i = 0; i < arrayLength; i++) {
        for ( let j = 0; j < arrayLength; j++) {
            //main diagonal
            if ( i === j ) {
                diag1 += arr[i][j];
            };
            //second diagonal
            if ( i + j === arrayLength - 1 ) {
                diag2 += arr[i][j];
            };

        };
    };

    return Math.abs(diag1 - diag2);

}

function diagonalDifference(arr) {

// Write your code here

let arrayLength = arr.length;

let diag1 = 0;

let diag2 = 0;

for( let i = 0; i < arrayLength; i++) {

for ( let j = 0; j < arrayLength; j++) {

//main diagonal

if ( i === j ) {

diag1 += arr[i][j];

};

//second diagonal

if ( i + j === arrayLength - 1 ) {

diag2 += arr[i][j];

};

return Math.abs(diag1 - diag2);

}

Simply Array Sum

Problem from HackerRank.

Easy problem but I got confused with the index of the array and its value. I was adding i to my result variable and not ar[i].

function simpleArraySum(ar) {
    let result = 0;

    for (let i = 0; i < ar.length; i++) {
        result += ar[i];
    };

    return result;
}

function simpleArraySum(ar) {

let result = 0;

for (let i = 0; i < ar.length; i++) {

result += ar[i];

};

return result;

}

JS Cheatsheet (work in process)

JavaScript engine makes multiply passes over the same code:

First pass to parse the syntax. During this pass the engine undertakes a process called automatic semicolon insertion.
Second pass to initialise variables and functions and store them in memory. Hoisting: is a JS default behaviour of moving all declarations to the top of the current scope ( to the top of the current script or the current function). Variables and constants declared with let or const are not hoisted.
The third pass is to actually execute the code line-by-line.

First and second passes are called the compilation phase and the third phase is called the execution phase.

Debugging:

No matter what combination of types you write, JS won’t throw an error and will return something, even a non sense and weird something.

alert(“Hola!”);
console.log(“Hola!”);
console.error(“Hola!”);
console.warn(“Hola!”);
console.table

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}
var me = new Person("Diana", "Badas");
console.table(me);

function Person(firstName, lastName) {

this.firstName = firstName;

this.lastName = lastName;

}

var me = new Person("Diana", "Badas");

console.table(me);

try statement lets you test a block of code for errors.

catch statement lets you handle the error.

function Myfunc() {
  this.twice=function(num){
    if(num<2){
      throw "num should be greater than 2"
    }
  return num*2;
  }
}

var myfunc = new Myfunc();
var list = new Array(0,4,1,3);
for(var i=0; i<4; i++){
  try{
    var res = myfunc.twice(list[i]);
    console.log(res);
  }
  catch(e){
    console.log(e);
  }
}

function Myfunc() {

this.twice=function(num){

if(num<2){

throw "num should be greater than 2"

}

return num*2;

}

var myfunc = new Myfunc();

var list = new Array(0,4,1,3);

for(var i=0; i<4; i++){

try{

var res = myfunc.twice(list[i]);

console.log(res);

}

catch(e){

console.log(e);

}

throw statement lets you create custom errors.

function getRectArea(width, height) {
  if (isNaN(width) || isNaN(height)) {
    throw "Parameter is not a number!";
  }
}

try {
  getRectArea(3, 'A');
}
catch(e) {
  console.error(e);
  // expected output: "Parameter is not a number!"
}

function getRectArea(width, height) {

if (isNaN(width) || isNaN(height)) {

throw "Parameter is not a number!";

}

try {

getRectArea(3, 'A');

}

catch(e) {

console.error(e);

// expected output: "Parameter is not a number!"

}

finally statement lets you execute code, after try and catch, regardless of the result.

let letsTry = () => {

  try {
    // there is a SyntaxError
    eval('alert("Hello world)');
    
  } catch(error) {
    console.error(error);
	
    // break the function completion
    return;
  } finally {
      console.log('finally')
  }

  // This line will never get executed
  console.log('after try catch')
}

letsTry();

let letsTry = () => {

try {

// there is a SyntaxError

eval('alert("Hello world)');

} catch(error) {

console.error(error);

// break the function completion

return;

} finally {

console.log('finally')

}

// This line will never get executed

console.log('after try catch')

}

letsTry();

Tests:

Unit testing
Integration testing
End-to end testing

Operators:

//Arithmetic operator **. The exponentiation operator returns the left number //raised to the power of the right number.
3 ** 3;
  => 27

//The remainder operator returns the remainder when the left number is divided //by the right number

5 % 2;
  => 1

//Order of the operations
()
**
* / %
+-

//Incrementing and Decrementing
//JS has a pair of operators that we can use to increment and decrement a //numerical value stored in a variable ++ --. 
//If the ++ operator comes after the variable, the variable's value is returned first and then incremented.

let count = 0;
count++;
  => 0
count;
  => 1

Otherwise:

let count = 5;

--count;
  => 4
count;
  => 4

//Assignment operators
let count = 0;

count += 10;
  => 10

count -= 2;
  => 8

//Equality operators:
== // Loose equality operator
=== /// Strict equality operator
!= // Loose inequality operator
!== // Strict inequality operator

//Relational operators
> //Greater than
>= //Greater than or equals
< //Less than
<= //Less than or equals

//Logical operators
//Use ! to negate an expression
//Convert an expression to a boolean using !!

//Link conditions with && and ||

//Return value of && is the falsy expression. The return value of the && //operator is always one of the two expressions. If the first expression is //falsy, && returns the value of the first expression. If the first expression //is truthy, && returns the value of the second expression.

//Return value of || is the truthy expression.
// The return value of the || operator is always one of the two expressions. //If the first expression is truthy, || returns the value of the first //expression. If the first expression is falsy, || returns the value of the //second expression.

//Arithmetic operator **. The exponentiation operator returns the left number //raised to the power of the right number.

3 ** 3;

=> 27

//The remainder operator returns the remainder when the left number is divided //by the right number

5 % 2;

=> 1

//Order of the operations

()

* / %

//Incrementing and Decrementing

//JS has a pair of operators that we can use to increment and decrement a //numerical value stored in a variable ++ --.

//If the ++ operator comes after the variable, the variable's value is returned first and then incremented.

let count = 0;

count++;

=> 0

count;

=> 1

Otherwise:

let count = 5;

--count;

=> 4

count;

=> 4

//Assignment operators

let count = 0;

count += 10;

=> 10

count -= 2;

=> 8

//Equality operators:

== // Loose equality operator

=== /// Strict equality operator

!= // Loose inequality operator

!== // Strict inequality operator

//Relational operators

> //Greater than

>= //Greater than or equals

< //Less than

<= //Less than or equals

//Logical operators

//Use ! to negate an expression

//Convert an expression to a boolean using !!

//Link conditions with && and ||

//Return value of && is the falsy expression. The return value of the && //operator is always one of the two expressions. If the first expression is //falsy, && returns the value of the first expression. If the first expression //is truthy, && returns the value of the second expression.

//Return value of || is the truthy expression.

// The return value of the || operator is always one of the two expressions. //If the first expression is truthy, || returns the value of the first //expression. If the first expression is falsy, || returns the value of the //second expression.

Data types:

Everything in JavaScript is data except:

operators:+, -, <=, etc.
reserved words:function, for, debugger, etc.

A basic type checking with the typeof operator: typeof(42);

The JS’s seven data types:

1.Numbers (primitive data type).

2. strings (primitive data type).

We use ‘single quotes’, “double quotes”, or `backticks.

String concatenation:

//Using the + operator:

"This" + "is" + "a" + "string";
//Interpolation of variables and other JS expressions

const number = 5;

"This" + "is" + "my string number " + number + "."

//Using template literals, using backticks.
This is a string using a template literal, introduced with ES2015 I can interpolate my varirable ${number} here too!

//Using the + operator:

"This" + "is" + "a" + "string";

//Interpolation of variables and other JS expressions

const number = 5;

"This" + "is" + "my string number " + number + "."

//Using template literals, using backticks.

This is a string using a template literal, introduced with ES2015 I can interpolate my varirable ${number} here too!

3. Booleans (primitive data type).

The following values are falsy:

false

null

undefined

0

NaN

'', ""

Every other value is truthy.

4.Symbols(primitive data type) are primarily used as an alternate way to add properties to objects.

5. Objects:

JS defines 5 types of primitive data types: string, number, boolean, null, undefined. All JS values, except primitives, are objects. In JS almost everything is an object: booleans, numbers and strings, if defined with the new keyword can be objects. Primitive data types represent single values, such as a number, a string or false, instead of a collection of values.

var x = new Boolean(false);

//typeof x returns object

JS objects: dates, maths, regular expressions, arrays, functions and objects.

Objects are similar to a hash in Ruby or a dictionary in Python. Arrays in JS are objects too.

All keys in an object are strings.

const diana = {
  name: "Diana",
  surname: "Badas",
  city: "London",
  favouriteColour: "blue"
};

const diana = {

name: "Diana",

surname: "Badas",

city: "London",

favouriteColour: "blue"

};

Access a value stored in an Object:

//.notation
diana.name;

//bracket notation
diana["name"];

//.notation

diana.name;

//bracket notation

diana["name"];

Add a property to an Object:

//.notation
diana.university = "USC";

//bracket notation
diana["age"] = "enough";

//.notation

diana.university = "USC";

//bracket notation

diana["age"] = "enough";

Delete an property

//.notation
delete diana.age;

//bracket notation
delete diana["university"];

//.notation

delete diana.age;

//bracket notation

delete diana["university"];

Arrays:

let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"];

1	let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"];

Add an element:

//Destructively
dogs.push("Snoopy");
  =>  ["Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]
dogs.unshift("Lassie");
  => ["Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]
//Using arr[arr.length] = b 
let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"];
const moreDogs = ["Toby", "Luna"]; 
dogs[dogs.length] = moreDogs;
  => (2) ["Toby", "Luna"] 
dogs;
  => (5) ["Byron", "Cubby", "Boo Radley", "Luca", Array(2)]

//Non-destructively
//Using ... operator
const allDogs = ["Rex", ...dogs];
allDogs;
  => ["Rex", "Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]
dogs;
  => ["Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]

//Using.concat 
const moreDogs = ["Toby", "Luna"]; 
const allConcatDogs = dogs.concat(moreDogs);

//Destructively

dogs.push("Snoopy");

=> ["Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]

dogs.unshift("Lassie");

=> ["Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]

//Using arr[arr.length] = b

let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"];

const moreDogs = ["Toby", "Luna"];

dogs[dogs.length] = moreDogs;

=> (2) ["Toby", "Luna"]

dogs;

=> (5) ["Byron", "Cubby", "Boo Radley", "Luca", Array(2)]

//Non-destructively

//Using ... operator

const allDogs = ["Rex", ...dogs];

allDogs;

=> ["Rex", "Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]

dogs;

=> ["Lassie", "Byron", "Cubby", "Boo Radley", "Luca", "Snoopy"]

//Using.concat

const moreDogs = ["Toby", "Luna"];

const allConcatDogs = dogs.concat(moreDogs);

Delete an element:

let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"]

//Destructively
dogs.pop(); //deletes the last element
  => "Luca"
dogs;
  => ["Byron", "Cubby", "Boo Radley"]

dogs.shift(); //deletes the first element
  => "Byron"
dogs;
  => ["Cubby", "Boo Radley"]

Array.splice() //Add and delete elements
var arrDeletedItems = array.splice(start[, deleteCount[, item1[, item2[, ...]]]])

const numbers = [1, 3, 4, 6];

numbers.splice(1, 0, 2); // at index 1, remove 0 elements, add number 2
  => [] //elements deleted
numbers;
  => [1, 2, 3, 4, 6]

numbers.splice(4, 1, 5);
  => [6] //elements deleted
numbers;
  => [1, 2, 3, 4, 5]


//Non-destructively
Array.slice
arr.slice([begin[, end]])
//begin index is included and end index is not included in the new array.

const colours = ["red", "blue", "yellow", "green"];

colours.slice(2);
  => ["yellow", "green"]
colours;
  => ["red", "blue", "yellow", "green"]
colours.slice(1, 3)
  => ["blue", "yellow"]
colours;
  => ["red", "blue", "yellow", "green"]

let dogs = ["Byron", "Cubby", "Boo Radley", "Luca"]

//Destructively

dogs.pop(); //deletes the last element

=> "Luca"

dogs;

=> ["Byron", "Cubby", "Boo Radley"]

dogs.shift(); //deletes the first element

=> "Byron"

dogs;

=> ["Cubby", "Boo Radley"]

Array.splice() //Add and delete elements

var arrDeletedItems = array.splice(start[, deleteCount[, item1[, item2[, ...]]]])

const numbers = [1, 3, 4, 6];

numbers.splice(1, 0, 2); // at index 1, remove 0 elements, add number 2

=> [] //elements deleted

numbers;

=> [1, 2, 3, 4, 6]

numbers.splice(4, 1, 5);

=> [6] //elements deleted

numbers;

=> [1, 2, 3, 4, 5]

//Non-destructively

Array.slice

arr.slice([begin[, end]])

//begin index is included and end index is not included in the new array.

const colours = ["red", "blue", "yellow", "green"];

colours.slice(2);

=> ["yellow", "green"]

colours;

=> ["red", "blue", "yellow", "green"]

colours.slice(1, 3)

=> ["blue", "yellow"]

colours;

=> ["red", "blue", "yellow", "green"]

6. Null (primitive data type). Null is an absent object, but when called with typeof returns object.

7. Undefined (primitive data type). It is like a “not yet assigned a value”.

Spread operator (…)

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);

returnedTarget
  return value => {a: 1, b: 4, c: 5}

//Object.assign changes the original array, it is destructive.
target
  return value => {a: 1, b: 4, c: 5}

const mergedObj = { ...target, ...source };

mergedObj
  return value => {a: 1, b: 4, c: 5}

//... operator is not destructive.
target
  return value => {a: 1, b: 2}

const target = { a: 1, b: 2 };

const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);

returnedTarget

return value => {a: 1, b: 4, c: 5}

//Object.assign changes the original array, it is destructive.

target

return value => {a: 1, b: 4, c: 5}

const mergedObj = { ...target, ...source };

mergedObj

return value => {a: 1, b: 4, c: 5}

//... operator is not destructive.

target

return value => {a: 1, b: 2}

Variables:

Start with a lower case and don’t use spaces, use camelCase.

Mind reserved words or future reserved words.

We can package both of the initialisation steps, declaration and assignment in a single line of code:

const name = "Diana";
  => undefined

1 2	const name = "Diana"; => undefined

Differences between var, const and let.

*Variables declared without a const, let, or var keywords are always globally scoped. You can avoid this using strict mode.

var:

var is function scoped. The scope is limited to the function within which it is defined. If it is defined outside any function, the scope of the variable is global. They are not block-scoped.

Can be reassigned.

var comes with a ton of baggage in the form of scope issues, for example, with var no error is thrown if you declare a variable twice.

let:

let is block scoped. The scope is limited to the block defined by curly braces {}.

Can be reassigned.

const:

const is block scoped. The scope is limited to the block defined by curly braces {}.

Can not be reassigned to anew value, but can be mutated.

const colours = ["blue", "red"];

colours.push("yellow");
  => 3
colours;
  => ["blue", "red", "yellow"]

colours = ["blue", "red", "yellow", "green"];
  => VM2182:1 Uncaught TypeError: Assignment to constant variable.

const colours = ["blue", "red"];

colours.push("yellow");

=> 3

colours;

=> ["blue", "red", "yellow"]

colours = ["blue", "red", "yellow", "green"];

=> VM2182:1 Uncaught TypeError: Assignment to constant variable.

Conditional statements:

if (condition) {
  doThis;
} else {
  doThat;
}

if (condition) {
  doThis;
} else if (condition2) {
  doThat;
} else {
  doOtherThing;
}

//Switch
switch(expression) {
  case x:
    //code block
    break;
  case y:
    //code block
    break;
  default:
    //code block
}

//Ternary operator
condition ? expresion1 : expresion2;
//I the condition returns a truthy value, run the code in expression1. I the condition returns a falsy value, run the code in expression2.

if (condition) {

doThis;

} else {

doThat;

}

if (condition) {

doThis;

} else if (condition2) {

doThat;

} else {

doOtherThing;

}

//Switch

switch(expression) {

case x:

//code block

break;

case y:

//code block

break;

default:

//code block

}

//Ternary operator

condition ? expresion1 : expresion2;

//I the condition returns a truthy value, run the code in expression1. I the condition returns a falsy value, run the code in expression2.

Functions:

A function is an object that contains a sequence of JavaScript statements. Declaration:

const sayMyName = () => {

  console.log("Diana");

}

const sayMyName = () => {

console.log("Diana");

}

Function calling or execution:

sayMyName();

1	sayMyName();

Parameters. They are locally scoped variables that are usable, scoped, to inside the function. Here name is our parameter.

const sayAname = (name) => {
  console.log(name);
}

const sayAname = (name) => {

console.log(name);

}

JS will assign the argument of “Diana” to the parameter name when this function is called.

sayAname("Diana");

1	sayAname("Diana");

Function declaration vs function expression

The difference lies in how the browser loads them into the execution context.

Function declarations load before any code is executed.

Function expressions load only when the interpreter reaches that line of code.

If you try to call a function expression before it is loaded you get an error.

//Function declaration
function myFunc () {
  return 5;
}
  => undefined

//Anonymous function expression
const myFunc = function () {
  return 5;
}
  => undefined
myFunc;
  => f(){
       return 5;
      }
myFunc();
  => 5 

//Named function expression
const myFunc = function myFunc () {
  return 5;
}

//Function declaration

function myFunc () {

return 5;

}

=> undefined

//Anonymous function expression

const myFunc = function () {

return 5;

}

=> undefined

myFunc;

=> f(){

return 5;

}

myFunc();

=> 5

//Named function expression

const myFunc = function myFunc () {

return 5;

}

myFunc();

function myFunc () {
  return 5;
}
  => 5
/////////////

myFunc();
const myFunc = function () {
  return 5;
}
  => Uncaught ReferenceError: Cannot access 'myFunc' before initialization

myFunc();

function myFunc () {

return 5;

}

=> 5

/////////////

myFunc();

const myFunc = function () {

return 5;

}

=> Uncaught ReferenceError: Cannot access 'myFunc' before initialization

Arrow functions:

Arrow functions do not have their own this, it uses whatever this is defined within the scope it is in.

Looping and iteration:

Iteration is the number of times a loop can be executed, while loop is the code which generate or causes expressions to be iterated.

Looping is the process of executing a set of statements repeatedly until a condition is met. It’s great for when we want to do something a specific number of times (for loop) or unlimited times until the condition is met (while loop).

Iteration is the process of executing a set of statements once for each element in a collection.

//for loop
const colours = ["blue", "red", "yellow"];

for (let i = 0; i < colours.length; i++) {
  console.log(`Colour ${colours[i]}`);
}

//while loop
function printColours (colours) {
  let i = 0;
  while (i < colours.length) {
    console.log(`Colour ${colours[i]}`);
    i++;
  }
  return colours;
}

printColours(colours);

//for...of. Good for arrays. Allow us to use const
const colours = ["blue", "red", "yellow"];
for (const element of colours) {
  console.log(element);
}

//for...in. Good for objects. It iterates over the properties of //an object in an arbitrary order.
let result = '';
const myObject = {
  a: 1,
  b: 2,
  c: 3
}

for (const property in myObject) {
  result += myObject[property];
};
  => '123'

//for loop

const colours = ["blue", "red", "yellow"];

for (let i = 0; i < colours.length; i++) {

console.log(`Colour ${colours[i]}`);

}

//while loop

function printColours (colours) {

let i = 0;

while (i < colours.length) {

console.log(`Colour ${colours[i]}`);

i++;

}

return colours;

}

printColours(colours);

//for...of. Good for arrays. Allow us to use const

const colours = ["blue", "red", "yellow"];

for (const element of colours) {

console.log(element);

}

//for...in. Good for objects. It iterates over the properties of //an object in an arbitrary order.

let result = '';

const myObject = {

a: 1,

b: 2,

c: 3

}

for (const property in myObject) {

result += myObject[property];

};

=> '123'

Creating object with JS

//Constructor function<code>
function Person(first, last, email) {
  this.first = first;
  this.last = last;
  this.email = email;
  this.name = function() {return this.first + " " + this.last;};
}

Person.prototype.sayHello = function() {
  console.log(`Hi! My name is ${this.first}`);
};

let Nao = new Person("Nao", "Gonzalez", n"nao@gmail.com");

//Constructor function<code>

function Person(first, last, email) {

this.first = first;

this.last = last;

this.email = email;

this.name = function() {return this.first + " " + this.last;};

}

Person.prototype.sayHello = function() {

console.log(`Hi! My name is ${this.first}`);

};

let Nao = new Person("Nao", "Gonzalez", n"nao@gmail.com");

//Class syntax
class Person {
  constructor(data) {
    this.first = first;
     this.last = last;
     this.email = email;
     this.name = function() {return this.first + " " + this.last;};</code> } } Person.prototype.sayHello = function() { console.log(`<code>Hi, my name is ${this.first}</code>`); }

//Class syntax

class Person {

constructor(data) {

this.first = first;

this.last = last;

this.email = email;

this.name = function() {return this.first + " " + this.last;};</code> } } Person.prototype.sayHello = function() { console.log(`<code>Hi, my name is ${this.first}</code>`); }

Definitions:

S.O.L.I.D.: 5 principles of Object Oriented Design with JS.
- S: Single responsibility principle.
- O: Open closed principle. Open for extensions and closed for modifications, we shouldn’t introduce breaking changes to existing functionality.
- L: Liskov substitution principle. Every subclass should be substitutable for their parent class.
- I: Interface segregation principle. A client should never be forced to implement an interface that it doesn’t use or clients shouldn’t be forced to depend on methods they do not use.
- D: Dependency Inversion principle. Entities must depend on abstractions not on concretions. Importance of higher-order functions.
ES6: “ES” ECMAScript, the official name of the JavaScript specification (Harmony – ES5)
Higher Order function: Function that may receive a first-class function as an argument and can even return a function.
First-class functions: Functions that are treated as objects or assignable to a variable.
AJAX: Process of making requests for additional data became known as Asynchronous JavaScript and XML, or AJAX.
Closure: When a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope. A closure is a feature in JavaScript such that a function holds onto the variables that it had access to when it was declared. Closures can be used to declare functions that have specific variables always defined. JavaScript developers also take advantage of closures to encapsulate data, as we can declare our functions in such a way that the data is only accessible from the returned function, with no way to overwrite the variables captured by the closure.

function sayHi() {  // 'scope of sayHi' is the lexical scope for printHi
   var myVar = 'hi!!';
   return function printHi() {
      console.log(myVar);
   }
}
var myVar = null,
greetings = sayHi();
greetings(); // 'hi!!'

function sayHi() { // 'scope of sayHi' is the lexical scope for printHi

var myVar = 'hi!!';

return function printHi() {

console.log(myVar);

}

var myVar = null,

greetings = sayHi();

greetings(); // 'hi!!'

Lexical scope: Where functions and variables are declared.
Hoisting: It is JS’s default behaviour of moving declarations to the top of the current scope. Let and const are not hoisted. During a compilation, every declaration ( of variables or functions) are added to the relative scope. Function declaration hoisting differs from variables as the content of the function get hoisted too.

foo(); // ‘hola!’

function foo() {
  console.log(‘hola!’);
}

foo(); // ‘hola!’

function foo() {

console.log(‘hola!’);

}

Scope: Where something is available. In JS, where declared variables and methods are available within our code. Global scope, function scope, block scope, scope chain, lexical scope ( where functions and variables are declared).

function myFunction () {
  console.log(hello);
  var hello = "World!";
  return hello;

}

myFunction();

  => log: undefined
  => "World"

function myFunction () {

console.log(hello);

var hello = "World!";

return hello;

}

myFunction();

=> log: undefined

=> "World"

Recursion: A recursive function is a function that calls itself.

function countDown(n) {
  console.log(n);
  if(n >= 1) countDown(n-1);
}
countDown(5);

function countDown(n) {

console.log(n);

if(n >= 1) countDown(n-1);

}

countDown(5);

Call back: When we pass a function into another function where in it might be invoked, we refer to the passed function as a callback.

function doHomework(subject, callback) {
  alert(`Starting my ${subject} homework.`);
  callback();
}
function alertFinished(){
  alert('Finished my homework');
}
doHomework('math', alertFinished);

function doHomework(subject, callback) {

alert(`Starting my ${subject} homework.`);

callback();

}

function alertFinished(){

alert('Finished my homework');

}

doHomework('math', alertFinished);

forEach: Differences between map and forEach:
- forEach(): It simply calls a provided function on each element in your array. This callback is allowed to mutate the calling array. It returns undefined. forEach() affects and changes our original Array. forEach() may be preferable when you’re not trying to change the data in your array, but instead want to just do something with it — like saving it to a database or logging it out.
- map(): creates a new array with the results of calling a provided function on every element in the calling array. The difference is that map() utilises return values and actually returns a new Array of the same size. map() returns an entirely new Array — thus leaving the original array unchanged. And map() might be preferable when changing or altering data. Not only is it faster but it returns a new Array.

let arr = [1, 2, 3, 4, 5];
arr.forEach((num, index) => {
    arr[index] = num * 2;
});

arr;
  => [2,4,6,8,10]

let doubled = arr.map(num => {
    return num * 2;
});

doubled;
 => [2,4,6,8,10]
arr;
  => [1, 2, 3, 4, 5]

let arr = [1, 2, 3, 4, 5];

arr.forEach((num, index) => {

arr[index] = num * 2;

});

arr;

=> [2,4,6,8,10]

let doubled = arr.map(num => {

return num * 2;

});

doubled;

=> [2,4,6,8,10]

arr;

=> [1, 2, 3, 4, 5]

Statement vs expression: A statement is a unit of code that accomplish something but does not produce a value. An expression is a unit of code that produces a value.
- statements: variable declarations, iteration, control flow, debugging.
this: keyword that refers to the object it belongs to. Safer if using arrow functions, because arrow function uses whatever this is defined within the scope it is in.
- This, inside a standalone function will refer to the global object.
- Outside any function, this refers t the global object, in web browsers this is window.
- Inside an object method, this refers to the object that received the method call.
- Inside an standalone function, even one inside a method, this will default to the global object.
- When using strict mode in a standalone function, as we do inside classes, this will be undefined.
- We can use .bind, .call and .apply to control the value of this.

Boocket

Boocket is an app for book lovers that want to keep track of the books they want to read. You can search for books in the Google Book database and add them to your personal wishlist. If you can find anything you like you can create a new book and add it to your personal list and delete it when wished. All books are persisted in the Boocket database, so you will have always your list with you!

I used the create-react-app generator to start my project and in specific npx, a npm package runner, a tool to execute node.js packages.

npx create-react-app google-bookshelf

*I have change the name of the app after that.

My JS files, components and containers

index.js file will have my store with Redux that is connected with the rootReducer and thunk, in order to use async actions. index.js renders <Provider /> as the top level component and wrap my <App /> component.

App.js renders <NavBar />, <Footer /> and it is responsible for the React routes too using react-router-dom. I use react-router-dom, that is react-router plus some link improvements, <BrowserRouter> and <NavLink>.

<SearchableBookListContainer /> renders a form and takes care of its ‘query’ state. After the query, it renders a link list of books. This container is connected to the store to dispatch the action of searching and adding an item to the wishlist plus a list created after our search, the searchBookList. The list rendered with the results of the search has nested links to the book itself. Inside Link I pass a pathname and a state with the book object and in my representational Book component I will get this info as props from Link react-router. I can access them as props.location.state.

<WishListContainer /> is connected to the store and gets the wishlist from here and renders it. It dispatch two actions, getMyWishList that fetches my API end point for a list of books and the deleteBook action. The list of books rendered follows same pattern as the one displayed in <SearchableBookListContainer />, and includes links to the book itself.

<CreateBook /> is a container connected to the store to dispatch the action of adding a new book and renders a form to create it.

<Book />, <Home />, <Footer /> and <NotFound /> components are representational components.

<NavBar /> takes care of all links importing from react-router-dom NavLink.

My source for the data is Google Books API. The Google Books API allows a maximum of 40 returned results and can sort only by search relevance and publish dates. I used the first functionality.

Using Redux and Thunk

Redux is a predictable state container for JavaScript applications. It helps you write applications that behave consistently, run in different environments (client, server, and native), and are easy to test. — https://redux.js.org/

I will use Redux to organize and have all my states in the same place and Thunk as a middleware to help me alter the behaviour of my actions, allowing me asynchronous requests. I will need to dispatch an action saying I am loading data, then to make a request to the API, and then to wait for the response and then dispatch another action with the response data.

With a plain basic Redux store, you can only do simple synchronous updates by dispatching an action. Middleware extend the store’s abilities, and let you write async logic that interacts with the store. — https://redux.js.org/

To install Redux and Thunk in my app I type into my console:

npm install --save redux

npm install --save react-redux

npm install --save redux-thunk

Initialize my store in index.js using createStore Redux function and pass it down to my top-level <App /> container.

import React from 'react';
import ReactDOM from 'react-dom';
import App from './containers/App';
import * as serviceWorker from './serviceWorker';
import { createStore } from 'redux';
import { Provider } from 'react-redux';
import booksReducer from './reducers/booksReducer';

const store = createStore(booksReducer);

ReactDOM.render(
    //We pass our store as a prop to Provider and wrap our App component so it will have access to our Redux store and/or dispatch actions from any component we want.
    <Provider store={store}>
        <App />
    </Provider>, 
    document.getElementById('root')
);

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

import React from 'react';

import ReactDOM from 'react-dom';

import App from './containers/App';

import * as serviceWorker from './serviceWorker';

import { createStore } from 'redux';

import { Provider } from 'react-redux';

import booksReducer from './reducers/booksReducer';

const store = createStore(booksReducer);

ReactDOM.render(

//We pass our store as a prop to Provider and wrap our App component so it will have access to our Redux store and/or dispatch actions from any component we want.

<App />

</Provider>,

document.getElementById('root')

);

// If you want your app to work offline and load faster, you can change

// unregister() to register() below. Note this comes with some pitfalls.

// Learn more about service workers: https://bit.ly/CRA-PWA

serviceWorker.unregister();

To have access to the store in any of our components we import the react-redux function connectinto the component where we want to have access.

If we want some state we can use:

const mapStateToProps = state => {
  return {
    whatEverWeWant: state.whatEverWeWant
  };
};

const mapStateToProps = state => {

return {

whatEverWeWant: state.whatEverWeWant

};

We can access these data as: this.props.whatEverWeWant.

If we want to dispatch an action and update our store we do something like:

const mapDispatchToProps = dispatch => {
  return {
    typeOfOurAction: () => dispatch({ type: 'TYPE_OF_OUR_ACTION' })
  };
};

const mapDispatchToProps = dispatch => {

return {

typeOfOurAction: () => dispatch({ type: 'TYPE_OF_OUR_ACTION' })

};

export default connect(
  mapStateToProps,
  mapDispatchToProps
)(App);

export default connect(

mapStateToProps,

mapDispatchToProps

)(App);

Then to be able to debug I have installed redux-devtools-extension and add this code to my index.js:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './containers/App';
import * as serviceWorker from './serviceWorker';
import { createStore } from 'redux';
import { Provider } from 'react-redux';
import booksReducer from './reducers/booksReducer';

const store = createStore(
    booksReducer,
    window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
);

ReactDOM.render(
    //We pass our store as a prop to Provider and wrapp our App component so it will have access to our Redux store and/or dispatch actions from any component we want.
    <Provider store={store}>
        <App />
    </Provider>, 
    document.getElementById('root')
);

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

import React from 'react';

import ReactDOM from 'react-dom';

import App from './containers/App';

import * as serviceWorker from './serviceWorker';

import { createStore } from 'redux';

import { Provider } from 'react-redux';

import booksReducer from './reducers/booksReducer';

const store = createStore(

booksReducer,

window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()

);

ReactDOM.render(

//We pass our store as a prop to Provider and wrapp our App component so it will have access to our Redux store and/or dispatch actions from any component we want.

<App />

</Provider>,

document.getElementById('root')

);

// If you want your app to work offline and load faster, you can change

// unregister() to register() below. Note this comes with some pitfalls.

// Learn more about service workers: https://bit.ly/CRA-PWA

serviceWorker.unregister();

Considering web accessibility in my app

I have followed some requirements to take into account web accessibility.

WAI-ARIA ( Web Accessibility Initiative – Accessible Rich Internet Applications). I am using Aria HTML attributes.
Using as possible fragments <> </>to group together multiple elements and don’t break the HTML semantic.
Every HTML form control, such as <input> and <textarea> is labeled accessibly.
Create react app has the eslint-plugin-jsx-a11y plugin with a subset of rules activated that checks your code as you develop.

Using docker

I wrapped up the project as a docker image in the Dockerfile.

Creating my API

I follow these instructions and as soon as I have the basics working I do the following:

Create a model for my Book:

rails generate model Book title:string author:text info:text

1	rails generate model Book title:string author:text info:text

I create a seeds file and run my migration:

rails db:migrate

rails db:seed

rails db:migrate

rails db:seed

Generate my controller:

rails generate controller Book

1	rails generate controller Book

Create a serializer for book and draw my endpoint inside config/routes.rb

Rails.application.routes.draw do
  # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
  resources :books
end

Rails.application.routes.draw do

# For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html

resources :books

end

How do I connect my react app with my rails API? In my actions. I fetch the end point of my API “books” and GET and POST to it.

This is how my end point looks with my wishlist:

And I think that that’s it! I have enjoyed a lot this project despite it took my a while to figure out Redux functionality. I went through a lot of different problems and learnt a lot. Ah, this bug was my favourite one . I manage to set undefined as my query so my search results were hilarious but very revealing too!

Compare the Triplets

Problem from Hackerrank.

Got some issues figuring out why it didn’t work my function. Two mental notes:

I am still have some problems with indentation and that can cause to put my return value inside the for loop without noticing.

I keep writing the operator “+=” as “=+”. How annoying! I read a good article about it and I finally got it.

My solution:

'use strict';

const fs = require('fs');

process.stdin.resume();
process.stdin.setEncoding('utf-8');

let inputString = '';
let currentLine = 0;

process.stdin.on('data', function(inputStdin) {
    inputString += inputStdin;
});

process.stdin.on('end', function() {
    inputString = inputString.split('\n');

    main();
});

function readLine() {
    return inputString[currentLine++];
}

// Complete the compareTriplets function below.
function compareTriplets(a, b) {
  let bob = 0;
  let alice = 0;
  for (let i=0; i < a.length; i++) {
    if (a[i] > b[i]) {
        alice += 1;
    } else if (a[i] < b[i]) {
          bob += 1;
    } 
  }
  let result = [alice, bob]; 
  return result;
}

function main() {
    const ws = fs.createWriteStream(process.env.OUTPUT_PATH);

    const a = readLine().replace(/\s+$/g, '').split(' ').map(aTemp => parseInt(aTemp, 10));

    const b = readLine().replace(/\s+$/g, '').split(' ').map(bTemp => parseInt(bTemp, 10));

    const result = compareTriplets(a, b);

    ws.write(result.join(' ') + '\n');

    ws.end();
}

'use strict';

const fs = require('fs');

process.stdin.resume();

process.stdin.setEncoding('utf-8');

let inputString = '';

let currentLine = 0;

process.stdin.on('data', function(inputStdin) {

inputString += inputStdin;

});

process.stdin.on('end', function() {

inputString = inputString.split('\n');

main();

});

function readLine() {

return inputString[currentLine++];

}

// Complete the compareTriplets function below.

function compareTriplets(a, b) {

let bob = 0;

let alice = 0;

for (let i=0; i < a.length; i++) {

if (a[i] > b[i]) {

alice += 1;

} else if (a[i] < b[i]) {

bob += 1;

}

let result = [alice, bob];

return result;

}

function main() {

const ws = fs.createWriteStream(process.env.OUTPUT_PATH);

const a = readLine().replace(/\s+$/g, '').split(' ').map(aTemp => parseInt(aTemp, 10));

const b = readLine().replace(/\s+$/g, '').split(' ').map(bTemp => parseInt(bTemp, 10));

const result = compareTriplets(a, b);

ws.write(result.join(' ') + '\n');

ws.end();

}

Recipe Manager Walkthrough

Address already in use – bind(2) for “127.0.0.1” port 3000 (Errno::EADDRINUSE) | Check what process is running in your localhost

I was trying to create a new rails app and got this error:

Address already in use – bind(2) for “127.0.0.1” port 3000 (Errno::EADDRINUSE)

To fix it I first check which process used my port 3000:

lsof -wni tcp:3000

1	lsof -wni tcp:3000

And I got this output:

So now I can stopped it using its pid:

kill -9 13901

1	kill -9 13901

I checked again if something was running in my port 3000 and I didn’t get any output so I run my rails s command and everything worked fine!

Rails with JavaScript – Recipe Manager

So… Starting with my project!

In first place, I want to build the JSON API using the Active Model Serializers gem. I add to my Gemfile:

gem ‘active_model_serializers’, that allows you to generate your JSON in an object-oriented and convention-driven manner.

How the JSON API works?

The user makes a request, clicking a link, filling a form… where we had added an event listener. That info is serialized, translated to JSON, and submitted via an AJAX POST or GET request. With that response or data we create a new JS object and then we append the info of that object that we want to show to the DOM using a jQuery selector.

Building a JSON API consists basically in creating the response with the right data to a request and the Active Model Serializers gem is the perfect tool for this. It allows us to format our JSON, select only the information we want, as well as get access to our relationships with a single request.

Setting up the Serializer files

One of the requirements for this project is render at least one index page and one show page via JS and an Active Model Serialization JSON Backend. I was thinking in rendering my recipes index and my recipes show pages.

At the moment I am displaying a list with all the recipes and links to their show page. I am not displaying more info than the title of the recipe in this page, so that’s what I want.

First I add to my Gemfile and bundle install:

gem ‘active_model_serializers’

Then I create my serializer file for my recipes. I tipe in my terminal:

rails g serializer recipe

And after creating it I specify which info, attributes and info of its relations I would like to have, in this case, the id and the name of the recipe.

If we specify in our contoller to “render :json” and because I have installed Serializer, Rails will now look to this folder before rendering a resource. Every object we pass to “render :json” in our controllers needs a corresponding serializer file.

* If you want deep nesting by default, you can set a configuration property in an active_model_serializer initializer. (stack overflow)

Create the file: app/config/initializers/active_model_serializer.rb with the following content:

<span class="typ">ActiveModelSerializers</span><span class="pun">.</span><span class="pln">config</span><span class="pun">.</span><span class="pln">default_includes </span><span class="pun">=</span> <span class="str">'**'</span>

1	<span class="typ">ActiveModelSerializers</span><span class="pun">.</span><span class="pln">config</span><span class="pun">.</span><span class="pln">default_includes </span><span class="pun">=</span> <span class="str">'**'</span>

I am going to create my serializers and specify which info I want.

Configuring the Controller to respond to JSON requests.

This is my new index action in the recipes controller:

Uhhm…I just realize that I have a problem. I want different info depending on where the object is rendered. For the index page I only want the title and the id of the recipe but for the show page I want all the info.

Lets make some changes and create two different serializers for recipes.

At the end I created these serializers:

category_serializer.rb
comment_serializer.rb
full_recipe_serializer.rb
ingredient_serializer.rb
rating_serializer.rb
recipe_ingredient_serializer.rb
simple_recipe_serializer.rb
user_recipe_serializer.rb

A lot of serializers…. I started playing with my app and encounter this error when I was trying to get one of the recipes of my index:

I think that this error is due to the info that the function fetch() is getting is not JSON.

Nice post about Unexpected token < in JSON at position 0.

My solution was in the serializers, I wasn’t displaying correctly the info and the relations between the objects. When I was trying to create my JS recipe object my function didn’t have the necessary info. It didn’t have the info for the has_many relations, so when I was trying to create these objects like the comments or the ingredients for the recipe I got this error.

I realize too that I didn’t need a serializer for my user class doing some methods inside the serializers where I wanted that info, for example in the comment serializer I added a method to display the name of the user who create that comment. This is better too in terms of security. I am not displaying my users data. Better not to have the associations because it will give access to the whole user object, including the authenticity token.

This is how my serializers look now:

Two different serializers for a recipe and I have created some methods to get the info I needed and delete the belongs_to relations. I kept the has_many relations trying to avoid defining methods for relations as much as possible, because it’s not clean, neither maintainable but didn’t do it in the case of the user info.

I have my serializers and the following images are my changes in my controllers to respond to JSON requests:

Comments controller:

Recipes controller:

Next steps are creating my event listener and JavaScript Object Models.

Creating my files for the JS code

I first add to my Gemfile: gem ‘sprockets-rails’, :require => ‘sprockets/railtie’, a Ruby library for compiling and serving web assets.

I continue adding a new file for all my JS: app/assets/javascripts/main.js. I don’t have to require this directive(main.js) in my manifest (application.js) because this manifest has //= require_tree, which will include all JS files in the same folder that application.js is located.

I’ll have to load the manifest file in my layout, so in the file: app/views/layouts/application.html.erb, inside the head tag, I write:

<%= javascript_include_tag “application” %>

javascript_include_tag is a Rails helper that generates a script tag instructing the browser to load that JS file.

Using arrow syntax of ES6

It differs with the normal JS function by the treatment of this. A normal JS function gets a thisaccording to the calling context (traditional semantics), but the arrow functions keep the this of the context of definition.

Creating a comment with Ajax.

In my comments/new.html.erb I have a form to create a comment. What I want to do is set it up so that I use this form, but use jQuery and AJAX to submit it, so I can handle a JSON response and display the newly created comment without redirecting to the comment show page.

Web users like fast websites and AJAX (Asynchronous JavaScript and XML) is a good technique to keep users engaged. In AJAX we first deliver a page using HTML and CSS and then use JS to add more to the DOM.

Ajax relies on several technologies: promises, XMLHttpRequestObjects, a serialization format called JSON, asynchronous Input / Output and the event loop.

Browsers and servers are only able to pass strings of text to each other. By using JSON, we can structure this text in a way that a browser or server can read as a regular JS object.

I will use the JavaScript fetch() function to apply the AJAX technique.

First step is to set up a document ready in order to detect when our HTML page has loaded, and the document is ready to be manipulated.

Then we hook up an event handler to the form’s submit event, and then block the form from doing an HTML submit. We add an event listener for $(‘div#new_comment’).on(‘submit’, function(event) {} to our document ready().

I am using the jQuery on() method. The on() method attaches one or more event handlers for the selected elements and child elements.

To display error message I read this tutorial.

To create the comment via Ajax I need a JavaScript Object Model too. I added two prototype functions.

This JMO allows me to get all of the attributes of a comment into javascript by passing the comment resource sent by the controller in response to the AJAX request for json and assigning its properties to a new javascript model object. We can then use the properties of this new javascript model object within my methods on the prototype to help me display the content to the page.

After creating this event listener I follow similar procedure to create another one to handle the comments when they are created in a nested resource, and after that one to get, not post, an index of recipes and show a recipe plus a JS Object model for Recipe, RecipeIngredient, Category and Rating.

Link to my repository.

Rails Portfolio Project – Recipe Manager – Walkthrough

Rails Portfolio Project – Recipe Manager

Recipe Manager was built using Ruby on Rails framework and Bootstrap. All users can safely signup, login and logout. I created authentication and authorization methods without using any gem. Recipe Manager include a third party signup/login via Github thanks to the OmniAuth gem.

Users can check the recipes, rate and comment them but only admin users can create and edit the recipes and the categories. The user can browse recipes by name and author and all the comments by author and recipe. The app include reasonable validations for its forms and an ActiveRecord scope method to get the newest recipe that can be seen in its own URL, in this case: http://localhost:3000/newest_recipe.

My models:

User

Has many recipes
Has many comments
Has many ratings
Attributes: name, email, password_digest, admin, and finally I added uid and provider for the signup/login with OmniAuth.

Recipe: