Mini-Max Sum

Problem from Hackerrank.

My solution:



Problem from Hackerrank.

My solution:


Plus Minus

Problem from Hackerrank.

I was making the mistake of checking if the index was bigger or smaller than zero, instead of checking the value of the integer in that index. I was comparing i instead ,arr[i].

My solution:


Diagonal Difference

Problem from Hackerrank.


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].


JS Cheatsheet (work in process)

JavaScript engine makes multiply passes over the same code:

  1. First pass to parse the syntax. During this pass the engine undertakes a process called automatic semicolon insertion.
  2. 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.
  3. 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.


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

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

catch statement lets you handle the error.

  • throw statement lets you create custom errors.

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


  • Unit testing
  • Integration testing
  • End-to end testing


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:

3. Booleans (primitive data type).

The following values are falsy:






'', ""

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.

Access a value stored in an Object:

Add a property to an Object:

Delete an property


Add an element:


Delete an element:


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 (…)


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:

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

Can be reassigned.


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.

Conditional statements:


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

Function calling or execution:

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

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

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.

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.

Creating object with JS


  • 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.

  • 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.

  • 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).

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

  • 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.

  • 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‚Äôre not trying to change the data in your array, but instead want to just¬†do something¬†with it ‚ÄĒ 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 ‚ÄĒ 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.

  • 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 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.‚Ää‚ÄĒ‚Ä䬆

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.¬†‚Ää‚ÄĒ‚Ä䬆

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

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

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:

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

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

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

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:

I create a seeds file and run my migration:

Generate my controller:

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

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:

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

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

And I got this output:

So now I can stopped it using its pid:

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!