Difference between var, let, and const

It is the most common question asked in a JavaScript interview. var was the only option for variable declaration in Javascript before ES6 but after ES6 let and const were announced. I have covered var, let and const in their seperate blogs in-depth.

Q.1) What is the difference between var and ES6 new declarations (let/const)?

var	let/const
var can't create block scope	let/const creates block scope
var can be re-declared	let/const can't be re-declared
var declarations are hoisted and initialized with undefined	let/const declarations are hoisted but are not initialized

Q.2) What is the difference between let and const?

let	const
variable declared with let can be re-assigned.	const declarations can't be re-assigned.
If the variable(declared with let) is not initialized with any value, JavaScript assigns undefined to it.	If the variable(declared with const) is not initialized with any value, JavaScript throws an error.
let a; //a = undefined	const a; //SyntaxError: Missing initializer in const declaration
let a = 10; a = 5; //a = 5	const a = 5; a = 10; //TypeError: Assignment to constant variable

Q.3) Can you change the value of a variable declared with const?

Yes, only if it is a reference type.

const declaration can't be assigned again that doesn't mean you can't change the existing value. Take the example of assigning an array to a const declaration. We can't assign another array but we can change the value of the existing array because the reference is not changing.

const arr = [1,2,3];
arr.push(4);
console.log(arr); //[1,2,3,4]

const arr = [1,2,3];
arr = [4,5,6];
TypeError: Assignment to constant variable

So use let in such cases where array values will be changed in the future like in the above example to avoid the confusion created by the meaning of const. You can use const anywhere if the values will not be changed in the future.

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Conclusion:

• In my current coding style I don't use var.
• I use let where values can be changed in the future.
• I use const for cases where values will not be changed like imports, exports, constant values, etc.

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This blog will be 2nd blog in the series JavaScript Interview Guide. I covered Hoisting in JavaScript in last blog. Do check it out.

Thanks for reading the blog!