JavaScript: var, let, const

In ES6, a new way to declare variable is introduced -- const and let. Before ES6, developers use var to declare different variables. However, it may cause potential issues or implicit bugs in the code. In this article, I'll introduce const, let, var and give a clear comparison.

Comparison Table

	var	let	const
reassigned	O	O	X
Scope	Function Scope	Block Scope	Block Scope
Reference before declaration	O	X	X

const

const is used when you don't want to reassign the variable. It stays constant once it's declared. As a result, const is always a default option for me if I don't need to reassign the variable. In this way, we can also avoid the case when we occasionally declare the same variable name in other files.

let

When it comes to the situation I need to reassign a variable, let is more welcome than var after ES6. The reason is that let is in block scope which means it only exists within its own scope. For Example,

let foo='outside';
if(true){
    let foo = 'inside'
    console.log(foo);            //print inside

}
console.log(foo);                //print outside

After if condition, foo is equal to 'outside' rather than the value we have inside the if statement ('inside').

var

var is an old way to declare variable in javaScript and it's now the weakest keyword to define a variable. The variable declared by var may or may not be reassigned and because it is in function scope, the variable may or may not be used for entire function. If the variable is reassigned accidentally within a for loop (block scope), then that's when things can easily go wrong. People can overwrite the parent assignments in this case. For example,

var foo='outside';
if(true){
    var foo = 'inside'
    console.log(foo);            //print inside

}
console.log(foo);                //print inside

To clarify, var is in function scope. So if people do reassignment in a function, the variable outside the function wouldn't be changed.

var foo='outside';
var fooFunction = function(){var foo='inside'}
fooFunction();
console.log(foo);    //print outside

Another potential issue with var is that it can be referenced before it's assigned. For example,

var x = 'foo';
console.log(x);         //foo
console.log(y);         //undefined, no errors!!
var y = 'late foo';     //late foo 
console.log(y);

There's no errors when using variable before declaration. Javascript engine reads the above script as

var x;             //variable declaration
var y;             //variable declaration
x = 'foo';         //variable initialization
console.log(x);    //foo
console.log(y);    //undefined, no errors!!
y ='late foo';     //variable initialization
console.log(y);    //late foo

This is because Javascript engine only hoists declaration not initialization. Although people can avoid this issue by using 'use strict', const and let are still stronger and can reduce potential error and make the code more clear.

Comments

[helloanandpatel]

Ming-Shiuan Tsai, nice article. It looks like there is a mistake in your last javascript snippet. Shouldn't it be -
var x = 'foo';
var y;
console.log(x);
console.log(y);
y = 'late foo';
console.log(y);

[abdallah0123]

I think your are right

[Ming-Shiuan Tsai]

Thanks for pointing out the mistake! I've updated the article!

[tyrellblackburn]

Ming-Shiuan Tsai, I'm loving your communication style. It's very clear and easy to understand. Thanks for writing your articles.

[Abdelaziz Mahdi]

Thanks Ming-Shiuan Tsai, your article was really clear and easy to understand, for those like me doesn't know about use strict :

Strict Mode was a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This strict context prevents certain actions from being taken.
More Info...

[Mark Rubanza]

now I understand

[Sanjar Afaq]

Ming-Shiuan Tsai, thanks a lot for the great article.
Correction: Your last snippet's 2nd comment is wrong. It should be
// late foo
instead of // undefined, no errors!!

[Husseini Mudi]

Excellent explanation. Thanks