Introduction

• Well, we all start somewhere to start learning JavaScript, I wish this kind of beginner's guide would be available for me to get the very basics of JS.

• If you are not new to javascript, this will be a perfect read to brush up your skills or building a stronger foundation.

Enjoy Reading

What is Javascript

JS is synchronous single-threaded language

Synchronous - It executes its code line by line.
Single-Threaded - Executes one line at a time.

How does JS Works?

• Javascript works inside an execution context,
  they have 2 components namely Memory and Code

• Memory is also called as Variable Environment

• Code is also called Thread of Execution

Hoisting

• JS Code Hoisting is when a javascript code executes then even before the execution of the first line of the code it scans the entire code for declarations of functions & variables
• Allocates memory for it in the Global Execution Context as variables/identifiers as undefined and creates a reference in case of functions.

var x = 10; // x will be undefined before execution

function getName() {
  console.log("Hello World");
} // gets copied entirely

Undefined vs Not Defined

• Well as javascript creates a global execution context,
• It allocates memory to all the declared variables & functions
• By default, it allocates undefined to it. and when the variable assignment line is executed it assigns the respective value to it.

Eg undefined

var a;
console.log(a === undefined); //true

Not Defined

When JS tries to find a variable that was not found in the initial scan it gives the. implies that there was not memory allocated to the variable.

Eg Not Defined

var a = 10;
console.log(x); //here x is not defined in the scope

ReferenceError: x is not defined

Temporal Dead Zone -

let us look at this example

console.log(a); // Error
console.log(b); // undefined
let a = 10;
var b = 10;

ReferenceError: cannot access 'a' before initialization

let and const are not on the global object
instead they are stored in a different memory space than global called SCRIPT and since the console.log points to the global object, it does not find let a = 10; in it.

Example for Temporal Dead Zone

1. let c;
2. const a = 10;
3. if (a === 10) {
4.   c = "Temporal Dead Zone Ended";
5. }

Here, the above example shows that until the variable c or a is assigned any value, it will stay in a special zone called as Temporal Dead Zone 💀

let v/s const v/s var

1. In case of let and const there are no re declarations allowed.
2. Not even the console will work 😅

console.log("This will not print");
let a = 10;
var a = 10; // Redeclaration of identifier 'a'

Resulting in
SyntaxError: Identifier 'a' has already been declared.

var a = 10;
var a = "Please like this article";

No Error is there, re declarations are allowed in var

How to avoid temporal dead zone?

- Always put your declrations on the top of the file so that as soon as it starts executing it will get all the variables declared.
- By doing this we can shrinking the Dead Zone Window close to zero by moving all the declarations at the top.

Block in Javascript

Block is a set of lines covered by {} to provide it as a single statement.

Example

if (true) console.log("Single Statement");

That's the reason we dont need Curly Braces {} in if condition because it's a Single Statement
We use {} to provide a block, to have multiple statements

Example

if(condition){
  line1;
  line2;
  ...
} else {...}

function hello(){
 line1;
 line2;
 ...
}

Scopes in Block v/s Global

let & const are block scoped and var is global scoped.

How?

Example again! Let's say we execute this code

{
  var a = 10; //global
  let b = 20; //block
  const c = 30; //block
}

Proof

Shadowing

It is a concept of js where a variable declared using var hides the global scoped variable

var a = 10;
{
  var a = 100; // this 'a' shadows the global scoped 'a'
}
console.log(a); // results 100

But let and const works differently because they do not exist outside the current scope

let b = 10;
{
  let b = 100;
}
console.log(b); // results 10

let c = 1;
{
  let c = 10;
}
console.log(c); // results 1

Illegal Shadowing 😵‍💫

It is when we try to scope a let declared variable using a var.

let a = 100;
{
  var a = 10; // Error! Illegal Shadowing
  let a = 2; // works just fine!
}

SyntaxError: Identifier 'a' has already been declared

Closures 😶‍🌫️

A function bundling with its lexical scope is closure.

It's easy with an example

function first() {
  var a = 10;
  function second() {
    console.log(a);
  }

  return second;
}
...
let getFunction = first();

/* after this line, instance of 
first() function is popped out of 
the call stack */
...
getFunction(); // outputs: 10;

Theoretically it should not work as the identifier a needs to be outside the scope BUT when the second() function is returned it comes with its lexical scope as well. That's what exactly closure is.

Tricky Interview Question on setTimeout and Closures

Problem Statement - Print numbers from 1 to 5 with an interval of 1 second each.

You might get the confidence that this is a simple question and will come up with a code looking like this 👇

for (var i = 0; i <= 5; i++) {
  setTimeout(() => {
    console.log(i);
  }, i * 1000);
}

Above code would go wrong (why?) by printing 6 in each line. and can be fixed by just using a let instead of a var

What if?

The interviewer asked you to do it with var only?

Relax don't sweat out. I'll tell you what to code and explain it how it works.

Here's the code that you're interested in 😆
with the explaination

for (var i = 0; i <= 5; i++) {
  // used var
  function closure(i) {
    setTimeout(() => {
      console.log(i);
    }, i * 1000);
    closure(i);
  }
}

Identifying the problem

• Well the problem with the previous code using var was js maintains an execution context and the way the setTimeout() works is it stores the callback function and attaches a timer to it and goes to the next line.

• as the variable i does not exists in the callback function it points to the outside reference of i from the for loop.

• Now it creates 5 copies of functions with the timer and now by the timer expires js does not wait, for loop ends. it is just too late. the value of i becomes 6 at last.

Fixing the problem

Fix 1: using let declaration

We have discussed above that let is block scoped, the answer lies within that line.

When we use the let it creates separate blocks for its memory allocation. whereas in case of var all 5 functions were pointing to the same memory space available in the global execution context

Fix 2: using closures or functions

This will actually provide them a new copy of i in it everytime the setTimeout() was called

Types of Writing Functions

Function Declaration

function hello() {
  console.log("Like This Blog and star the repo");
}

Function Expression

const callAPI = async function () {
  const data = fetch("https://somesite.io/get/data/1232");
  return data;
};

Parameters v/s Arguments

function greet(name) {
  // identifier name is a parameter
  console.log(`Hello ${name}`);
}

greet("Gautam"); // "Gautam" is an Argument

First Class Functions

• functions having functions in their arguments.

• The ability of functions to be used as values and be able to return and pass as arguments to another function.
  

function first(doesSomething) {
  doesSomething();
}

first(() => {
  console.log("hello World");
});

Callback Functions in JS

These functions are sent in the arguments of another functions and are used respectively whenever needed,

setTimeout(function () {
  console.log("Timer");
}, 5000);
function x(y) {
  console.log("x");
}

x(function y() {
  console.log("y");
});

Arrow Functions

• These functions are introduced in ES6 or EcmaScript 6 (2015)
• They just allow us to write shorter syntaxes

Before

function addOne(addNumber) {
  return 1 + addNumber;
}

After

const addOne = (addNumber) => 1 + addNumber;

Above method allows us to write functions without using the function keyword.

If you liked reading till here. please like this blog and go to github for the codes and please star the repository.