What does filter() do?

AngularJunior~1 min read

The filter() operator in RxJS lets through only the values from the stream that pass a check.

A simple example:

import { of } from 'rxjs';
import { filter } from 'rxjs/operators';

of(1, 2, 3, 4, 5)
  .pipe(filter(x => x % 2 === 0))
  .subscribe(val => console.log(val));

The stream was: 1 → 2 → 3 → 4 → 5 What remains: 2 → 4

How it works:

Each value is checked by the function (the condition)
If true, it passes through
If false, it's ignored

Where it's used:

Keep only the values you need from a stream
Remove empty strings, zeros, errors
React only to certain events

Example in Angular:

this.searchInput.valueChanges
  .pipe(filter(value => value.length > 2))
  .subscribe(search => {
    // Run the search only if more than 2 characters were entered
  });

Conclusion:

filter() is like a checkpoint in the stream: it only lets through the data that fits what you need. Everything else is cut off.

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What does filter() do?

#A simple example:

#How it works:

#Where it's used:

#Example in Angular:

#Conclusion: