06_Hoisting.md

CHAPTER 06: Hoisting

Code Example 01

var x = 7;

function getName() {
  console.log("Namaste JavaScript");
}

getName();
console.log(x);

Output:

Namaste JavaScript
7

Code Example 02

getName(); // in most languages, both the lines which are above their declaration will give error. But not in JS though.
console.log(x);

var x = 7;

function getName() {
  console.log("Namaste JavaScript");
}

Output:

Namaste JavaScript
undefined

Code Example 03

getName();
console.log(x);

function getName() {
  console.log("Namaste JavaScript");
}

Output:

Namaste JavaScript
Error: x is not defined // note that not defined here and "undefined" in
sample 2 are totally different.

• Not defined: We have not initialised the value for variable anywhere in the entire code and in memory space.
• Undefined: It is a placeholder that is assigned to a variable by the Javascript Engine until the variable is assigned with some other value.

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Hoisting is a concept which enables us to extract values of variables and functions even before initialising/assigning value without getting error

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Code Example 04

function getName() {
  console.log("Namaste JavaScript");
}

console.log(getName);

Output:

f getName(){
    console.log("Namaste JavaScript);
}

Code Example 05

getName();
console.log(x);
console.log(getName);

var x = 7;

function getName() {
  console.log("Namaste JavaScript");
}

Output:

Namaste JavaScript

undefined

f getName(){
    console.log("Namaste JavaScript);
}

Code Example 06

console.log(getName);

var getName = function () {
  console.log("Namaste JavaScript");
};

var getName = () => {
  // use fat arrow function
  console.log("Namaste JavaScript");
};

Output:

undefined //it is because arrow function behave as variable and not function.

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REASON OF WEIRDNESS

• The answer lies in the Global Execution Context. In the memory phase, the variables will be initialized as undefined and functions will get the whole function code in their memory.

• This is the reason why we are getting these outputs.

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