RxJS in Angular page

Learn how to read and write observables in Angular.

Video

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The problem

In this section, we will:

  • Create a CodePen setup with Angular and the HTML of our form.
  • Write values to a Subject and write out the value of the subject in the template.

Observables and the $ suffix

At Bitovi, we follow the ts.dev Style Guide and prefer to suffix Observables with $.

Although not enforced, it is useful to visually differentiate between observable and non-observable values.

How to solve this problem

  • Create a userCardNumber$ BehaviorSubject.
  • Write the value of the cardNumber input to the userCardNumber$ BehaviorSubject on the input event.
  • Write the value of cardNumber out in the template under the </form> element like: 
    </form>
UserCardNumber: {{ GET-CARD-NUMBER-HERE }} <br />

What you need to know

Click "Run in your browser" at the bottom of the following example to launch a CodePen with Angular and the HTML of the credit card form. You will edit this CodePen for the remainder of this tutorial.

<my-app></my-app>
<script type="typescript">
  // app.js
  const { Component, VERSION } = ng.core;

  @Component({
    selector: 'my-app',
    template: `
      <form>
        <div class="message"></div>

        <input type="text" name="cardNumber" placeholder="Card Number" />

        <input type="text" name="expiry" placeholder="MM-YY" />

        <input type="text" name="cvc" placeholder="CVC" />

        <button>
          PAY
        </button>
      </form>
    `
  })
  class AppComponent {

      constructor() {

      }
  }

  // main.js
  const { BrowserModule } = ng.platformBrowser;
  const { NgModule } = ng.core;
  const { CommonModule } = ng.common;

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

  const { platformBrowserDynamic } = ng.platformBrowserDynamic;

  platformBrowserDynamic()
    .bootstrapModule(AppModule)
    .catch(err => console.error(err));
</script>
<style>
  @import url('https://fonts.googleapis.com/css?family=Raleway:400,500');

Initialize a BehaviorSubject instance on a class like the following:

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/7.4.0/rxjs.umd.min.js"></script>
<script type="typescript">
  const { BehaviorSubject } = rxjs;

  class AppComponent {
    userCardNumber$ = new BehaviorSubject<string>();
  }

  const app = new AppComponent();
  console.log(app.userCardNumber$) // logs Observable
</script>

A BehaviorSubject is just like a Subject except that it remembers its last value. Any new subscribers will immediately be sent the last value.

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/7.4.0/rxjs.umd.min.js"></script>
<script type="typescript">
  const { BehaviorSubject } = rxjs;

  const subject = new BehaviorSubject<string>();

  subject.next("THE PAST");

  subject.subscribe((value) => console.log(value));
  // logs "THE PAST"
</script>

Emit a value on a Subject by calling subject.next():

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/7.4.0/rxjs.umd.min.js"></script>
<script type="typescript">
  const { BehaviorSubject } = rxjs;

  class AppComponent {
    userCardNumber$ = new BehaviorSubject<string>();
  }

  const app = new AppComponent();

  app.userCardNumber$.subscribe(console.log); // logs undefined

  app.userCardNumber$.next("1111-2222-3333-4444"); // logs "1111-2222-3333-4444"
</script>

You can call subject.next() in the DOM as follows:

<input
  type="text"
  name="cardNumber"
  placeholder="Card Number"
  (input)="userCardNumber$.next($event.target.value)"
/>

Use | async to write out an observable’s value in a template as follows:

UserCardNumber: {{ userCardNumber$ | async }} <br />

Read more about this technique here.

The solution

Click to see the solution

<script type="typescript">
  // app.js
  const { Component, VERSION } = ng.core;
  const { BehaviorSubject } = rxjs;

  @Component({
    selector: 'my-app',
    template: `
      <form>
          <div class="message"></div>

          <input
            type="text"
            name="cardNumber"
            placeholder="Card Number"
            (input)="userCardNumber$.next($event.target.value)"
          />

          <input type="text" name="expiry" placeholder="MM-YY" />

          <input type="text" name="cvc" placeholder="CVC" />

          <button>
            PAY
          </button>
      </form>
      UserCardNumber: {{ userCardNumber$ | async }} <br />
    `
  })
  class AppComponent {
    userCardNumber$ = new BehaviorSubject<string>();

    constructor() {
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