ES2015 tl;dr; - using const

Here’s a mind blowing piece of information: const comes from “constant”. You’re welcome. Now let’s dive into more details on how to use it and it’s behaviours so that you don’t get fooled by thinking it should be used only to things that are immutable.

const is not really “constant”

Although some might get misleaded by the name const, it does not indicate it can’t be changed:

const sonsOfNedStark = [];

// Outputs: ["Robb", "Sansa", "Arya", "Bran", "Rickon"]

On the other hand, what const does guarantee you is no new bindings. On the code above, the const has a reference to an Array and that will never change (or shouldn’t as you’ll see in a few moments). We’re simply modifying the contents of the array, but not rebinding or modifying a value directly:

const sonsOfNedStark = ['Robb', 'Sansa', 'Arya', 'Bran', 'Rickon'];
sonsOfNedStark = ['Robb', 'Sansa', 'Arya', 'Bran', 'Rickon', 'John'];
// Chrome and Firefox will output assignment errors

If you wanted to store an object on a const but you didn’t want it to have the ability to change, you could use Object.freeze({}) and it would take care of it, tho (imaging your object has only one level of deepness).

When environments colide

Depending on where you’re running your code, you might end with discrepancies on the output of the code.

const isJohnSnowDead = 'yes';
isJohnSnowDead = 'nope';

// Chrome: TypeError: Assignment to constant variable.
// Firefox: SyntaxError: invalid assignment to const isJohnSnowDead.
// Safari 9: Logs 'nope'. <- LOL


Both const & let have the same behaviour when it comes to hoisting and scope, as explained in ES2015 tl;dr; - using let, but the main difference is that const has a protective behaviour not allowing the reassignment.

Hoisting example:

const daenerysDragonsStatus = 'Free';

// ReferenceError: daenerysDragonsStatus is not defined

Scope example:

const daenerysDragonsStatus = 'Free';

if (true) {
  const daenerysDragonsStatus = 'Trapped';

// Outputs: Free

When to use it

Opinions will be very different among developers, but I’m particularly with Axel Rauschmayer and Mathias Bynens on this one.

  1. Favor const over let given it’s more controlled behaviour;
  2. Use let for all things that require storage of values that are passive of change;
  3. let var live constantly in the past. Stahp using it.