Difference between var, let and const

Declaring variables in JavaScript since ES6 can be done in different ways. Most prominent of those are using var, let and const.

1. Basic example on var

Consider the examples below:

/* Example 1 */
x = 10;
function updatex() {
  x = 20;
}
updatex();
console.log(x); //20

/* Example 2 */
x = 10;
function updatex() {
  var x = 20;
}
updatex();
console.log(x); //10;

In these examples, we see that declaring variables as var limits their scope to the particular function, in this case the function updatex(). If we do not declare variable as var, it climbs up to global scope, declared and initializes itself there.

2. Example on var and let

var and let are similar in nature, but are not the same. Let's consider following examples.

console.log(x); //undefined
console.log(y); //ReferenceError

var x = 10;
let y = 15;

In this example, we try to console log variables before they are declared in the program. var variable logs undefined and let variable throws an Uncaught ReferenceError. This happens because of hoisting.

According to W3schools,

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).

In this example, var variable is hoisted to the top of the block, declared there, but not initialized, whereas let variable is hoisted to the top of the block (i.e. the block of the code is aware of the variable) but it cannot be used until it has been declared. The time from which the variable is used till the time where the variable is initialized is called temporal dead zone.

3. Example on let and const

So far we have seen direct declaration of variables, using var and using let. Now let's see const. As you might have guessed, const stands for constant. A constant is a variable which once declared and initialized remains the same and cannot be redeclared or changed through assignment. Does this mean const is immutable? Not really, but we will check that out later. First let's see the behavior of let and const. Let's consider these examples:

/* Example 1 */
// we've seen this before
x = 10; //ReferenceError
let x;

/* Example 2 */
y = 20;
const y; //SyntaxError

/* Example 3 */
y = 20; //ReferenceError
const y = 20;

We've seen example 1. It throws a Reference Error. In example 2, y being a const variable throws a SyntaxError because after declaration, a const variable must be immediately initialized. We cover this case in example 3, but as const variables cannot be redeclared or reassigned, it throws a ReferenceError. So a proper way to declare const variable would be:

const y = 20;

Both let and const have same scope properties. Unlike var variables, let and const variables are hoisted but cannot be used until declared, and in case of const, initialized. Now that you have a good understanding of scope and hoisting of different types of variables in JS, let's talk about immutability of const variables.

4. Immutability of const

JS has 2 types of variables:

1. Primitive types: undefined, null, boolean, number, string, and symbol
2. Reference types: object, array and function

Declaring a const variable to a primitive makes it immutable. But if it is initialized with a non-primitive or reference type, then it is possible to mutate the const variable. Let's take a few examples:

/* Example 1 */
let arr1 = [10, 20];
arr2 = [30, 40]; // No error

const arr2 = [50, 60];
arr2 = [70, 80]; // SyntaxError, redeclaraion

/* Example 2 */
const arr2 = [50, 60];
arr2[0] = 70;
arr2[1] = 80;
console.log(arr2); // [70, 80]

As you can see, with respect to reference type variables, it's not possible to preserve their immutability.

5. Conclusion

var variables are function scoped, let and const variables are block scoped. var, let and const are hoisted, but let cannot be used until declared and const variables cannot be used until they are declared + initialized. const variables, if initialized with reference types, are not immutable.

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