In this post i will try to give you full understanding about an important concept which is Javascript Hoisting, and how it works behind the scenes.

So without further preamble, let's start by this picture below



Now you thinking Why did you bring this crane here 😕.
Just take a minute and think about it, Isn't it the job of a crane to hoist something?
In our case its hoist variable and function declarations to the top of your code.

Variable hoisting

console.log(number); // undefined
var number = 12;

Why giving me undefined, isn't it supposed to giving me an error like

Uncaught ReferenceError: Cannot access 'number' before initialization.

However, the first line of code doesn’t cause an error because the JavaScript engine moves the variable declaration to the top of the script, The following figure illustrates this

Technically speaking, the JavaScript engine places the variable number in the memory and initializes its value to undefined.
Now you get it 😎

The let keyword

The following declares the variable number with the let keyword:

console.log(number);
let number = 12;

Now it don't giving me undefined, instead throw me an error

Uncaught ReferenceError: Cannot access 'number' before initialization.

The error message explains that the number variable is already in the heap memory. However, it hasn’t initialized.
Behind the scenes, the JavaScript engine hoists the variable declarations that use the let keyword. However, it doesn’t initialize those variables.
Notice that if you access a variable that doesn’t exist, the JavaScript will throw a different error:

console.log(name);
let number = 12;

Here is the error:

"ReferenceError: name is not defined

Function hoisting

Like variables, the JavaScript engine also hoists the function declarations. It moves the function declarations to the top of the script. For example:

let x = 20,
    y = 10;

let result = add(x,y);
console.log(result); // 30

function add(a, b){
return a + b;
}

This give me 30 because of hoisting , The following figure illustrates this

During the creation phase of the execution context, the JavaScript engine places the add() function declaration in the heap memory.
To be precise, the JavaScript engine creates an object of the function type and a function reference called add that refers to the function object.

Function expressions

The following example changes the add from a regular function to a function expression:

let x = 20,
    y = 10;

let result = add(x,y);
console.log(result); // 30

var add = function(x, y){
return a + b;
}

If you execute the code, the following error will occur:

"TypeError: add is not a function

During the creation phase of the global execution context, the JavaScript Engine creates the add variable in the memory and initializes its value to undefined.
When executing the following code, the add is undefined, hence, it isn’t a function.

Arrow functions

The following example changes the add function expression to the arrow function:

let x = 20,
    y = 10;

let result = add(x,y);
console.log(result);

var add = (x, y) => x + y;

The code also issues the same error as the function expression example because arrow functions are syntactic sugar for defining function expressions.

"TypeError: add is not a function

Similar to the functions expressions, the arrow functions aren’t hoisted.

Summary

• JavaScript hoisting occurs during the creation phase of the execution context that moves the variable and function declarations to the top of the script.

• The JavaScript engine hoists the variables declared using the let keyword, but it doesn’t initialize them as the variables declared with the var keyword.

• Function expressions and arrow functions aren’t hoisted.