Angular2 why do we need the es6-shim

Question

Following the Angular2 Quickstart Guide we are instructed to include es6-shim in 2 places:

1) index.html

<script src="node_modules/es6-shim/es6-shim.min.js"></script>

2) typings.json

"ambientDependencies": {   "es6-shim": "github:DefinitelyTyped/DefinitelyTyped/es6-shim/es6-shim.d.ts#6697d6f7dadbf5773cb40ecda35a76027e0783b2" } 

I was under the impression that we are compiling our es6 code down to es5.

Configured in tsconfig.json

{   "compilerOptions": {     "target": "es5",     ... 

If the end result is that the browser is loading es5, why does the browser needs shims for es6?

Answer 1

Typings are used by your editor to give you code hinting/intellisense, and es6-shim.min.js is a code that emulates ES6 features for ES5 browsers. Some of those features are Promise, Array.from()...

While your code is translated to ES5, you need to include es6-shim so you can use those new features in it... Consider this ES6 code:

let test1 = () => 123 + 456; let test2 = new Promise((resolve, reject ) => {}); 

it will be translated to ES5 code:

var test1 = function () { return 123 + 456; }; var test2 = new Promise(function (resolve, reject) { }); 

but without es6-shim Promise would be undefined...

Answer 2

TypeScript does not come with built-in polyfills. There are certain features it doesn't transpile, this is where polyfills like es-shim come in.

The TypeScript definition file will provide you with typing support within your chosen editor and the es-shim will provide the implementation for the functionality that TypeScript doesn't transpile to ES5 code.

A few of such features TypeScript doesn't transpile are:

• Promises
• Functions hanging off of prototype objects (Array.from(),Array.of(),Number.isInteger(), etc)
• Module loading

The general approach is:

The rule of thumb is that if there's a canonical/sane emit that doesn't have a huge perf-hit, we'll try to support it. We don't add any runtime methods or data structures, which is more what core.js (or any pollyfil) serves to do. - source (TypeScript developer)