var, let, const

One of the first features you may have noticed in ES6 is a new way to declare variables (and now constants) with let and const instead of just var.

These new keywords are used to create Block Scoped variables and constants.

Variable declarations with var were only capable of Global Scope and Function Scope

All three keywords are capable of Global Scope

Keyword Behavior

Below is a table you can quickly reference explaining the differences. If you are looking for a recommendation for which ones to use in what scenarios here is my recommendation:

• Avoid using var
• Use const wherever possible
• When you have a variable you expect will change over time use let

Keyword	Scope	Redefinable	Redeclarable	Hoisting
var	function	✅	✅	✅
let	block	✅	❌	❌
const	block	❌	❌	❌

Function Scope

Notice in the following code snippet we can access all of the variables from within the function.

function func() { var var1 = "I am function scoped" let var2 = "Technically I'm block scoped" const var3 = "But right now you can't tell" console.log(var1) console.log(var2) console.log(var3) } func()

Now that we have moved the log statements outside of the function you'll see we end up with a reference error.

function func() { var var1 = "I am function scoped" let var2 = "Technically I'm block scoped" const var3 = "But right now you can't tell" } func() // Comment out each one to see that it returns a reference error console.log(var1) console.log(var2) console.log(var3)

Block Scope

In the following example var1 will be accessible outside of the if statement which is considered a block. Both let and const will be confined to the if block.

if (true) { var var1 = "You can access me outside the block" let var2 = "You can't access me outside the block" const var3 = "Me neither" console.log(var1) console.log(var2) console.log(var3) } // Uncomment each one separately, notice let and const throw reference errors // console.log(var1); // console.log(var2); // console.log(var3);

Redefining keywords

You will be able to redefine variables declared with var.

var foo = 1 console.log(foo) foo = "I can also be a string" console.log(foo)

You will also be able to redefine variables declared with let.

let bar = 1 console.log(bar) bar = "I can also be a string" console.log(bar)

This should go without saying but you can't redefine a constant declared with const it will throw a type error.

const baz = 1 console.log(baz) baz = "This is gonna fail" console.log(baz)