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let, var, and const

ES6 introduced two new ways to define variables: let and const.

They’ve been the source of a fair bit of confusion, particularly around when to use which. Let’s clear that up.

let #

let does the almost the same exact thing as var.

The big difference between let and var is that you can’t redefine a variable set with let in the same scope.

// The value of `sandwich` is "tuna"
var sandwich = 'tuna';

// The value of `sandwich` is now "chicken"
var sandwich = 'chicken';

// The value of `chips` is "Cape Cod"
let chips = 'Cape Cod';

// Throws an error: "Uncaught SyntaxError: Identifier 'chips' has already been declared"
let chips = 'Lays';

You can still change the value of chips. You just can’t define it as a new variable once it’s already been defined within the current scope.

You can use let to define a new variable with the same name in a different scope, though.

// The value of `chips` is "Cape Cod"
let chips = 'Cape Cod';

// The value of `chips` is now "Lays"
chips = 'Lays';

var getChips = function () {

    // This works because it's a different scope
    let chips = 'Wise';

    // Returns "Wise"
    return chips;

};

// Logs "Lays" in the console
console.log(chips);

const #

Unlike var and let, if you define a variable with const, it cannot be given a new value. It is, as the term implies, constant.

// The value of sandwich is "tuna"
const sandwich = 'tuna';

// Throws an error: "Uncaught TypeError: Assignment to constant variable."
sandwich = 'chicken';

Browser Compatibility #

let and const work in all modern browsers, and IE11 and up. They cannot be polyfilled.

To push support back further, you would need to use a compiler like Babel. Babel does actually have an “in the browser” version you can load with a script tag, but… it requires you to inline your entire script, so it’s not really a good solution for production sites.


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Have any questions or comments about this post? Email me at chris@gomakethings.com or contact me on Twitter at @ChrisFerdinandi.

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