Shuhei Kagawa

The Best Part of Angular 2 Offline Compiler

@2016-06-03 22:47 - JavaScript, Angular 2

A couple of weeks ago, I gave a short talk about Angular offline compiler at ng-sake #3, which is a cozy meetup where Tokyo's Angular developers hang out drinking beer.

I won't write much about it here because it's still in its early stage and I couldn't make @angular/compiler-cli work without modification.

The only thing I want to stress here is that it enables us to statically type-check our templates with TypeScript, which is awesome. Templates have been one of the places where mistakes are made since Angular 1. Even if we introduce TypeScript or lint tools, we couldn't be able to detect mistakes in templates until they are evaluated at the runtime.

The steps would be the following:

  1. Angular 2 offline compiler compiles templates into TypeScript files with .ngfactory.ts extension.
  2. You prepare another bootstrap script that imports the root .ngfactory.ts and bootstraps your app with it.
  3. TypeScript compiler compiles the bootstrap script, other TypeScript files and .ngfactory.ts files into JavaScript.

The step 3 type-checks .ngfactory.ts, and detects typo and type errors in your templates if any. It is a great benefit in addition to skipping the runtime compilation and smaller library size. Looking forward to Angular 2 offline compiler's official release!

Angular 2 with Babel

@2016-05-08 15:47 - JavaScript, Angular 2, Babel

Although Angular 2's primary language is apparently TypeScript, many people want to use Babel as shown in a survey.

However, The official documentation targets only TypeScript and ES5. In addition, many pages are not yet available for ES5. That is because Angular 2 relies heavily on cutting-edge ES7 decorators and TypeScript's type annotations for annotating components and services.

To fill the gap, you can use babel-preset-angular2 that supports all the decorators and annotations available in TypeScript. With the preset, you can follow the official documentation for TypeScript to learn Angular 2 itself.

How to use it

npm install -D babel-preset-es2015 babel-preset-angular2

Add presets to .babelrc. Note that the presets' order is important.

  "presets": ["es2015", "angular2"]

See babel-angular2-app for more complete example.

Supported annotations

Name Example EcmaScript TypeScript Babel* Babel + angular2
Class decorator @Component() Stage 1 Yes Yes Yes
Property decorator @Input() Stage 1 Yes Partial* Yes
Parameter decorator @Optional() Stage 0 Yes No Yes
Type annotation foo: Foo - Yes No Yes

"Babel*" above means Babel with the following official plugins:

Property decorator in Babel is marked "Partial" because babel-plugin-transform-decorators-legacy ignores class properties without initializers.

You can emulate parameter decorator and type annotation with plain ES2015 like the following but it's a little bit counterintuitive.

class HelloComponent {
  constructor(foo: Foo, @Optional() bar: Bar) {
    // Do something with foo and bar.

class HelloComponent {
  static get parameters() {
    return [[Foo], [Bar, Optional()]];

  constructor(foo, bar) {
    // Do something with foo and bar.


Angular 2 beta versions had polyfill bundles but RC versions don't. But never mind. We can just import them before bootstrapping our app.

npm install -S babel-polyfill zone.js


// Import polyfills.
import 'babel-polyfill';
import 'zone.js/dist/zone.js';

// Bootstrap app!
import {provide} from '@angular/core';
import {bootstrap} from '@angular/platform-browser-dynamic';
import {ROUTER_PROVIDERS} from '@angular/router';
import {LocationStrategy, HashLocationStrategy} from '@angular/common';

import {HelloApp} from './app';

bootstrap(HelloApp, [
  provide(LocationStrategy, { useClass: HashLocationStrategy })
]).catch(err => console.error(err));

Note that we can use babel-polyfill that includes core-js instead of es6-shim and reflect-metadata. According to use core-js instead of es6-shim, we can use whatever ES6 + ES7 polyfill we like.

Module resolution

You can use any module resolver as long as it works with Babel. I'll pick Browserify here for its simplicity.

npm install -D browserify babelify

Add a build script to your package.json assuming that your bootstrap script locates at src/index.js.

  "scripts": {
    "build": "browserify -t babelify src/index > public/bundle.js"
npm run build

Isn't this simple? babelify automatically finds your .babelrc and uses the presets specified above.

Of course you can use other module resolvers like Webpack or SystemJS.

Offline compilation

This is not yet available for Babel. Not completed even for TypeScript.

The compiler_cli seems to be deeply integrated with TypeScript compiler. It statically collects metadata from the source and feed it to the compiler. I believe that it is achievable with Babel to do the same thing.

I'm thinking of working on it once the TypeScript version is published and the compiler API becomes more stable.


I've presented how to use TypeScript-specific annotations in Babel. You can enjoy Angular 2 with your favorite transpiler.

See babel-angular2-app for more complete example.

Incremental search with RxJS switchMap

@2016-05-02 02:18 - JavaScript, RxJS

RxJS leads us to better design separating data flow and side-effects. In addition, it provides powerful functionalities that sophisticate the outcome application. My favorite is switchMap of RxJS 5, which is equivalent to flatMapLatest in RxJS 4.


switchMap(func) is equivalent to map(func).switch(). It keeps subscribing latest observable at the moment and unsubscribing outdated ones so that it only streams events from latest observable at the moment. Take a look at the marble chart for switch. It illustrates the behavior well.

switchMap is convenient for properly implementing incremental search. Incremental search makes multiple requests to a server. The server can respond in a different order from requests'. Because of the order, a naive implementation may show a wrong result. However, you can effortlessly avoid the caveat if you use switchMap.

Here is an example. Type fast in the text fields. Without switchMap sometimes shows a wrong result while With switchMap always works fine.

JS Bin on

search function mocks an AJAX request. It returns a Promise that resolves after a random delay.

function search(keyword) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Result of ' + keyword);
    }, Math.random() * 1000);

A naive implementation always shows the last response at the time. A wrong result is shown if responses come in a different order from requests'. We could add some debouncing to decrease the chance of the wrong order but it still may happen when response time is longer than the debounce time.

const keyword = document.getElementById('keyword-without');
const result = document.getElementById('result-without');

keyword.addEventListener('keyup', e => {
  const value =;
    .then(data => result.textContent = data);

switchMap guarantees that the last keyword's result is finally shown.

const keyword = document.getElementById('keyword-with');
const result = document.getElementById('result-with');

const keyword$ = Rx.Observable.fromEvent(keyword, 'keyup')
  .map(e =>;
  .subscribe(data => result.textContent = data);