RxJS: The From Operator

The Rx from operator is used to transform data that can be iterated over to an observable. It can be useful especially when you want to normalize the types of data that’s being passed and shared in observable sequences or when a function expects to receive and act on an observable. Another use if for when you’d want to use an RxJS operator that wouldn’t normally be available on the original data type.

Example of iterable types that can be transformed into observables using from are arrays, maps, sets, promises, DOM nodes, and generator functions. Below you’ll find examples for a few of these types:

Arrays

Most often the from operator is used to convert an array to an observable:

let myArr = ['🐦', '😺', 'πŸ•', '🐊'];

Rx.Observable
  .from(myArr)
  .filter(x => x !== '🐦')
  .map(x => `Hello ${x}!`)
  .subscribe(console.log);

  // Hello 😺!
  // Hello πŸ•!
  // Hello 🐊!

Synchronous vs Asynchronous

By default the from operator returns a synchronous observable:

let myArr = ['😺', 'πŸ•', '🐊'];

console.log('Before');
Rx.Observable
  .from(myArr)
  .map(x => `Hello ${x}!`)
  .subscribe(console.log);
console.log('After');

// Before
// Hello 😺!
// Hello πŸ•!
// Hello 🐊!
// After

If you want however, you can make it asynchronous using an async scheduler:

let myArr = ['😺', 'πŸ•', '🐊'];

console.log('Before');
Rx.Observable
  .from(myArr, Rx.Scheduler.async)
  .map(x => `Hello ${x}!`)
  .subscribe(console.log);
console.log('After');

// Before
// After
// Hello 😺!
// Hello πŸ•!
// Hello 🐊!

Generator Functions

Generator functions are an iterable type, so they can also be transformed to an observable using the from operator. Here’s a simple example:

function* generateUnique() {
  let num = 0;
  while (true) {
    yield num++;
  }
}

Rx.Observable.from(generateUnique())
  .take(3)
  .subscribe(console.log);

  // 0
  // 1
  // 2

Here we use the take operator to complete the observable after a specified number of values. Otherwise we'd create an infinite observable and crash the page when subscribing.

And here’s a slightly more complex example that also uses the zip operator to combine the values of multiple observables:

function* generateName() {
  yield 'Cat';
  yield 'Dog';
  yield 'Bird';
  return;
}

function* generateEmoji() {
  yield '😺';
  yield 'πŸ•';
  yield '🐦';
  return;
}

function* generateSound() {
  yield 'Meow';
  yield 'Woof';
  yield 'Tweet';
  return;
}

const names = Rx.Observable.from(generateName());
const emojis = Rx.Observable.from(generateEmoji());
const sounds = Rx.Observable.from(generateSound());

const combined = Rx.Observable.zip(names, emojis, sounds, (name, emoji, sound) => {
  return `The ${name} ${emoji} goes ${sound.toUpperCase()}`;
})
.subscribe(console.log);

// The Cat 😺 goes MEOW
// The Dog πŸ• goes WOOF
// The Bird 🐦 goes TWEET

Note that the spawn operator is also used to combine generators and observables, but the operator is currently not available in RxJS 5+

Promises

Promises can also easily be transformed into observables, which will be asynchronous and wrap the resolved or rejected value:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Hello');
  }, 2000);
});

Rx.Observable
  .from(myPromise)
  .subscribe(x => console.log(x, ' World!'));

// Hello World! (after 2 seconds)

DOM Nodes

Here’s a quick example where a collection of 3 DOM nodes are transformed into an observable and mapped over to extract only the textContent:

<h2>Hey,</h2>
<h2>Hello</h2>
<h2>Alligator!</h2>


<script>
  const h2s = document.querySelectorAll('h2');

  Rx.Observable.from(h2s)
    .map(h2 => h2.textContent)
    .subscribe(console.log);

    // Hey,
    // Hello
    // Alligator!
</script>

A Word About Strings

Strings can be iterated over, so the from operator can be used, but every character in the string will be a separate value:

Rx.Observable
  .from('Hi!')
  .subscribe(console.log);

  // H
  // i
  // !

Instead, to convert a string as a single value, you’ll want to use the of operator:

Rx.Observable
  .of('Hi!')
  .subscribe(console.log);

  // Hi!
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