JS: How to Convert ES5 to ES2015

Here is a guide to convert ES5 to ES2015.

〔see 2015 Features〕

Follow this checklist, in order.

Change 「var」 to 「let」

Change var to let

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

Change 「let」 to 「const」 (optional)

Change some let to const

Use const as much as possible.

Remove 「arguments」 object in 「function」

Remove any use of the arguments Object in function.

use

• JS: Function Parameter Default Value
• JS: Function Rest Parameters

Change function declaration to function expression

If a function form

function name (params) {body}

and body does not contain this , change it to arrow function, like this

const name = ((params) => {body});

• JS: this (binding)
• JS: Arrow Function

Change Function Block as local variable block into code blocks

Remove the top-level function block of the from

(function (){body})()

that are just for creating local variables.

Replace it with

{body} and replace all var in body to let

Change 「function」 declaration into function expression

replace all form of

function f () {body}

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 If the string in question is ascii only or in Unicode Basic Multilingual Plane.

• JS: String.fromCharCode
• JS: String.fromCodePoint

Replace charCodeAt to codePointAt

• JS: String.prototype.charCodeAt
• JS: 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.

• JS: class (keyword)

Better is to change the code to functional programing style.