estus estus - 3 months ago 76
Javascript Question

Angular 2 dependency injection in ES5 and ES6

Here is a basic TypeScript/ES.next example that uses decorators for DI and follows the syntax suggested by the framework manual:

import {Component, Inject, Injectable, NgModule, OpaqueToken} from '@angular/core';
import {BrowserModule} from '@angular/platform-browser';
import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';

const CONSTANT = { value: 'constant' };
const CONSTANT_TOKEN = new OpaqueToken;
const CONSTANT_PROVIDER = { provide: CONSTANT_TOKEN, useValue: CONSTANT };

@Injectable()
class Service {
constructor(@Inject(CONSTANT_TOKEN) constant) {
console.log('Service constructor', constant);
}
}

@Component({
selector: 'app',
template: '...',
providers: [Service, CONSTANT_PROVIDER]
})
class AppComponent {
constructor(@Inject(Service) service: Service, @Inject(CONSTANT_TOKEN) constant) {
console.log('AppComponent constructor', service, constant);
}
}

@NgModule({
imports: [BrowserModule],
declarations: [AppComponent],
bootstrap: [AppComponent]
})
class AppModule {}

platformBrowserDynamic().bootstrapModule(AppModule);


How would it be written in in ES5?

How would the same thing be done in untranspiled ES6/ES2015?

How are
Injectable
and
Inject
decorators translated in these cases?

The question particularly applies to real-world ES6 browser implementations that have classes but may use
require
or
System.import
instead of ES6 imports.

Answer

Injectable decorator is specific to TypeScript flavour of Angular 2. It enables a class constructor to be implicitly annotated for DI through TypeScript type annotations. It is redundant in TS and unneeded in JS for injected dependencies that are annotated with Inject.

Angular 2 injectables (classes and constructor functions) are supposed to be annotated with annotations and parameters static properties under the hood.

annotations is an array that contains newed decorators for injectable class:

function SomeComponent(...) {}
SomeComponent.annotations = [new Componenent(...)];

parameters is an array that contains decorators for constructor parameters, each element is an array that contains a list of newed decorators for respective constructor property (similarly to $inject property explicit annotation in Angular 1.x):

function Service(someService, anotherService) {}
Service.parameters = [
  [new Inject(SomeService)],
  [new Inject(AnotherService), new Optional, new SkipSelf]
];

All class decorators are extended from TypeDecorator, meaning that they can be called as functions. In this case so-called DSL syntax is used that allows to chain a decorator with Class helper function:

var SomeComponent = Componenent(...).Class(...);

Class is also available separately, it constructs a new class from given definition object and allows to annotate constructor method with array (similarly to inline array explicit annotation in Angular 1.x):

var SomeService = Class({
  constructor: [[new Inject(SomeService)], function (someService) {}]
});

This is how ES6 with System.import may look like:

Promise.all([
  System.import('@angular/core'),
  System.import('@angular/platform-browser'),
  System.import('@angular/platform-browser-dynamic')
])
.then(([
  {Component, Inject, Injectable, Optional, NgModule, OpaqueToken},
  {BrowserModule},
  {platformBrowserDynamic}
]) => {

  const CONSTANT = { value: 'constant' };
  const CONSTANT_TOKEN = new OpaqueToken;
  const CONSTANT_PROVIDER = { provide: CONSTANT_TOKEN, useValue: CONSTANT };

  class Service {
    constructor(constant) {}
  }
  Service.parameters = [[new Inject(CONSTANT)]];

  class AppComponent {
    constructor(service, constant) {}
  }
  AppComponent.annotations = [new Component({
    selector: 'app',
    template: '...',
    providers: [Service, CONSTANT_PROVIDER]
  })];
  AppComponent.parameters = [[new Inject(Service)], [new Inject(CONSTANT_TOKEN)]];

  class AppModule {}
  AppModule.annotations = [new NgModule({
    imports: [BrowserModule],
    declarations: [AppComponent],
    bootstrap: [AppComponent]
  })];

  platformBrowserDynamic().bootstrapModule(AppModule);

})
.catch((err) => console.error(err));

This is how ES5 with UMD modules and ng global may look like:

var Class = ng.core.Class;
var Component = ng.core.Component;
var Inject = ng.core.Inject;
var Injectable = ng.core.Injectable;
var NgModule = ng.core.NgModule;
var OpaqueToken = ng.core.OpaqueToken;

var BrowserModule = ng.platformBrowser.BrowserModule;
var platformBrowserDynamic = ng.platformBrowserDynamic.platformBrowserDynamic;

var CONSTANT = { value: 'constant' };
var CONSTANT_TOKEN = new OpaqueToken;
var CONSTANT_PROVIDER = { provide: CONSTANT_TOKEN, useValue: CONSTANT };

// Class helper function that uses A1-flavoured inline array DI annotations
// and creates an annotated constructor
var Service = ng.core.Class({
  constructor: [[new Inject(CONSTANT_TOKEN)], function (constant) {
    console.log('Service constructor', constant);
  }]
});
// can also be
// function Service(constant) {};
// Service.parameters = [[new Inject(...)], ...];

// when not being `new`ed, Component is a chainable factory that has Class helper method
var AppComponent = ng.core.Component({
  selector: 'app', 
  template: '...',
  providers: [Service, CONSTANT_PROVIDER]
})
.Class({
  constructor: [
    [new Inject(Service)],
    [new Inject(CONSTANT_TOKEN)],
    function (service, constant) {
      console.log('AppComponent constructor', service, constant);
    }
  ]
});
// can also be
// function AppComponent(...) {};
// AppComponent.annotations = [new Component(...)];
// AppComponent.parameters = [[new Inject(...)], ...];

var AppModule = NgModule({
  imports: [BrowserModule],
  declarations: [AppComponent],
  bootstrap: [AppComponent]
})
.Class({ constructor: function () {} });
// can also be
// function AppModule() {};
// AppModule.annotations = [new NgModule(...)];

platformBrowserDynamic().bootstrapModule(AppModule);