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The Secret of Hoisting in JavaScript

godcrampy profile image Sahil Bondre ・4 min read

Hoisting is one of the most confused about concept of JavaScript. It's one of the things that makes people think JavaScript as a nonsensical language. But once you know what's happening under the hood, everything makes perfect sense.

Inside the Engine

JavaScript is an Interpreted language. This means that the JavaScript Engine runs the code line by line by converting it to machine code (machine code is binary code that the computer can understand). Similar to JavaScript, Python and Perl too are interpreted languages. But what makes JavaScript different from these programming language is hoisting.

Try to guess the output of the following code:

console.log(a)

// Some other code

var a = 5;

If you guessed 5, you are wrong.

If you guessed to get an Error then you are wrong too!

The actual output of the above is undefined! Weird isn't it? It all makes sense when hoisting makes sense. So let's make some sense out of it.

Interpreting of JavaScript

So we already know that JavaScript is interpreted line by line. Well, there are a few complications in that too. It helps to think that the JS engine goes thru your code line by line twice. First time, the engine goes thru the code and does the hoisting and some other things (like adding the missing semicolons). The second time it actually runs the code.

So, Hoisting is the process of setting up of memory space for our variables and functions. Before the code starts to execute, the JS engine goes thru the code and sets up blocks of memory for functions and variables. The values of variables are not stored but functions are stored entirely along with their definitions. It's like the engine writes on a piece of paper the variables and functions it needs to keep track of before actually running the code.

Let's put our understanding to the test:

Our previous example:

console.log(a)

// Some other code

var a = 5;

So when our engine goes thru our code first, it "writes" down on a piece of paper (metaphor for reserving memory block for the variable). The engine doesn't assign any value to the variables so it sticks with the value of undefined by default. So after this hoisting is done on our imaginary piece of paper (memory), the engine starts all over again to execute the code this time. So on the first line it encounters the variable a. It then looks into its paper reference (memory). Oh! a is defined, it thus prints the value which is undefined right now. Then on the next line, a is reassigned the value of 5.

Let's try another one:

b();

function b() {
    console.log('b called!');
}

Note that when hoisting is being done, the variables are stored with the value of undefined only whereas the functions are stored with their definitions too. So after going thru the code once, the engine knows what variables are there but not their values. It also knows what functions are there and what each of them does. Hence in the above example, when we call b, the engine already knows that there exists such a function and what this function does too. So we get the output as b called!.

Last one:

b();
console.log(a);


function b() {
    console.log('b called!');
}

This one is tricky one as there's one small thing you might skip over. Here, since a is not defined, we get an error. There is another interesting thing we notice on running this code. Let's walk thru step by step using our knowledge of hoisting. So in the first pass, the function b along with its definition get hoisted and stored in the memory. Now comes the second pass. On seeing the first line, the interpreter will call the function b and we will get b called! on our screen. After this on the next line we will get an error as a is not defined. It's important to note here that the code above the erroneous line will be executed and the output will be shown too. This highlights the very important feature of JavaScript being an interpreted language.

Thus in conclusion note the following things:

  • Unlike other languages, JavaScript doesn't give errors for calling variables and functions before declaration
  • The functions get executed in their entirety while the variables return as undefined until they are reassigned some value.

ES6 features

No JavaScript blog is complete without a word on the ES6 features. ES6 introduced two new keywords let and const for declaring variables. Variables declared using let and const are also hoisted but the only difference is that in the case of let/const, the variables are not initialized with undefined as in the case of var. The below code will throw a reference error as the variable a is in the temporal dead zone. If it was not hoisted then the value of a would have been 10.

a = 10;

console.log(a);
// Reference Error

let a = 5;

That's all folks! Thank You for reading and have a marvelous day 😄

Discussion

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bhupendra1011 profile image
bhupendra

variables declared using let and const are also hoisted but only difference is that in case of let/const , variables are not initialized with undefined as in case of var/function.

x=10;
console.log(x);
let x = 20; 

above code will throw reference error as variable x in temporal dead zone . If it was not hoisted then x would have been 10.

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godcrampy profile image
Sahil Bondre Author

Yes you are correct indeed. I've fixed my error. Thank You for pointing this major blunder out!