JavaScript const

Variable declaration: const

ES6 introduced the const keyword to declare variables for all types of data like a boolean, object, integer, etc.

The const

• Variable declared with const needs to be initialized at the time of declaration.
• Same variable can't be declared again.
• Same variable can't be initialized again.
• The const keyword declares a block-scoped variable in Javascript.
• const declarations are hoisted and but remain uninitialized.

1. Syntax

Variables declared with const can't be left uninitialized like var or let. They need to be initialized at the time of declaration.

const x = 5;
console.log(x); //5

const y;
y = 10;
// SyntaxError: Missing initializer in const declaration

2. Re-declaration and Re-initialization

When using const we can't re-declare or re-initialize variables. JavaScript throws error.

Re-declaration

const a = true;
const a = false;
console.log(a); 
// SyntaxError: Identifier 'a' has already been declared

Re-initialization

const a = true;
a = false;
//TypeError: Assignment to constant variable.

const arr = [1,2,3,4];
arr = [5,4,56];
//TypeError: Assignment to constant variable.

Note: Sometimes const can be referred to as constant value assignment. Once assigned can't be changed. But it is not the case. It should be considered as NO RE-INITIALIZATION. Like in the above example with the array arr we tried to assign a new array then JavaScript throws an error. But we tried to change the values in the already assigned array then Javascript won't throw an error. See the example below:

const arr = [1,2,3,4];
arr.push(5);
console.log(arr); // [1,2,3,4,5]
// JavaScript doesn't throw any error

3. Block scoping

variables declared with const create block scoping.

const const = 10;
if (a > 10) {
  const fav = "twitter";
  console.log(fav); //twitter
}
console.log(fav); // ReferenceError: fav is not defined

Here const created a block scope. That's why the fav variable is only accessible inside the if-statement not outside of it.

4. Hoisting

const declarations are hoisted but they are not initialized at the compile time. Due to this if we try to access a variable before it was declared and initialized, Javascript throws us an error.

Example:

console.log(isLoading);
const isLoading = true;
// ReferenceError: Cannot access 'isLoading' before initialization

Explanation: When our code is compiled const declarations are hoisted but they are not initialized. They will only get initialized when during runtime code execution reaches the assignment. So, we can't access the variable before it was declared and assigned in code.

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Conclusion: Including this article, I have covered all three variable declarations var, let, and const in JavaScipt. Now you can checkout most common asked Javascript Interview Question Difference between var/let/const here

Thanks for reading the article!