Hoisting in Javascript

In Javascript (JS), variables and functions can be accessed before initializing it without any error anywhere in the program. This is known as Hoisting.

Javascript creates a Global Execution Context, GEC even before a program starts running. In the GEC, memory allocation and code execution takes place.

Let us try to understand hoisting in the variables with the following code.

var a = 10;

console.log(a);

This function would display the value of "a" as "10" in the browser window.

Now let's move the console.log(a) at the top.

console.log(a);

var a = 10;

We could see that the console.log(a) displays undefined.

To understand it better we could move the debugger to console.log(a) in the Sources window in the browser. Here, in the Global Execution Context, we could see that even before initialising the variable var a =10, the memory is allocated to a as undefined.

The execution context is created before the code starts running and the memory is allocated to each and every variables and functions.

Now let us understand in the case of the function.

function qwerty() {
    console.log("Keys don't start with ABCD");
}

qwerty();
console.log(qwerty);

Here, the function qwerty() is called at the end of the program & it displays the output in the console as "Keys don't start with ABCD".

console.log(qwerty);

prints the entire function.

If we call the function "qwerty()" before it's being written then we see that it prints the contents of console.log inside of the function.

qwerty();

function qwerty() {
     console.log("Keys don't start with ABCD");
}

However, console.log(qwerty) will print the function qwerty() with the string "Keys don't start with ABCD"

console.log(qwerty);
function qwerty() {
       console.log("Keys don't start with ABCD");
}

During the compile phase, the Javascript engine stores the variables and function declaration in the memory allowing the user to access them in the code before they are actually written.

Moreover, only the declaration variables are hoisted and not their assignment.