JS: How to Convert ES5 to ES2015

By Xah Lee. Date: . Last updated: .

Here is a guide to convert ES5 to ES2015.

〔see 2015 Features〕

Follow this checklist, in order.

Variables and Constants

Change var to let

Change some let to const.

in any for-loop, put a let for the counter variable.

Use const as much as possible.

Functions

Remove any use of the arguments Object in function.

use function default values.

〔see Function Argument Default Value〕

use function rest parameter.

〔see Function Rest Parameters〕

If a function does not use the this keyword 〔see this Binding〕 , change it to Arrow Function .

Change Function Blocks into Code Blocks

Remove the outmost function block of the from (function (){body})() that are just for creating local variables. Replace it with {body} and let.

replace all function f () {body} form to const f = function () {body}

You have to do this. Because we want to wrap a {} around our code. If you don't change function declaration into assignment form, you can't wrap the whole code into a block because function declaration must happen only at top level.

Now, wrap whole file into a block by wrapping curly brackets {} around it.

Dealing with Unicode

Replace String.fromCharCode to String.fromCodePoint

〔see String.fromCharCode〕

〔see String.fromCodePoint〕

Replace charCodeAt to codePointAt

〔see String.prototype.charCodeAt〕

〔see String.prototype.codePointAt〕

Use Map

Rewrite some data structure that use object, to use Map.

〔see the Map Object Tutorial〕

Use Set

Rewrite some data structure that use array or object, to use Set.

〔see the Set Object Tutorial〕

Use Class

Rewrite constructor functions to use the class keyword. This would be a big complex change. Better is to change the code to functional programing style.

〔see Class〕