What is JavaScript Hoisting? Explain it with example.

Hoisting is a mechanism in JavaScript that moves the declaration of variables and functions at the top. 

So, in JavaScript we can use variables and functions before declaring them.

JavaScript hoisting is applicable only for declaration not initialization. 

It is required to initialize the variables and functions before using their values.

Hoisting is JavaScript's default behavior of moving declarations to the top.

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JavaScript Declarations are Hoisted

In JavaScript, a variable can be declared after it has been used.

In other words; a variable can be used before it has been declared.

Example 1 gives the same result as Example 2:

Example 1

x = 5; // Assign 5 to x
elem = document.getElementById("demo"); // Find an elementelem.innerHTML = x;                     // Display x in the element
var x; // Declare x

Example 2

var x; // Declare xx = 5; // Assign 5 to x
elem = document.getElementById("demo"); // Find an elementelem.innerHTML = x;                     // Display x in the element

To understand this, you have to understand the term "hoisting".

Hoisting is JavaScript's default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function).

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The let and const Keywords

Variables and constants declared with let or const are not hoisted!

JavaScript Initializations are Not Hoisted

JavaScript only hoists declarations, not initializations.

Example 1 does not give the same result as Example 2:

Example 1

var x = 5; // Initialize xvar y = 7; // Initialize y
elem = document.getElementById("demo"); // Find an elementelem.innerHTML = x + " " + y;           // Display x and y

Example 2

var x = 5; // Initialize x
elem = document.getElementById("demo"); // Find an elementelem.innerHTML = x + " " + y;           // Display x and y
var y = 7; // Initialize y

Does it make sense that y is undefined in the last example?

This is because only the declaration (var y), not the initialization (=7) is hoisted to the top.

Because of hoisting, y has been declared before it is used, but because initializations are not hoisted, the value of y is undefined.

Example 2 is the same as writing:

Example

var x = 5; // Initialize xvar y;     // Declare y
elem = document.getElementById("demo"); // Find an elementelem.innerHTML = x + " " + y;           // Display x and y
y = 7;    // Assign 7 to y

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Declare Your Variables At the Top !

Hoisting is (to many developers) an unknown or overlooked behavior of JavaScript.

If a developer doesn't understand hoisting, programs may contain bugs (errors).

To avoid bugs, always declare all variables at the beginning of every scope.

Since this is how JavaScript interprets the code, it is always a good rule.

JavaScript in strict mode does not allow variables to be used if they are not declared.
Study "use strict" in the next chapter.