generate

Examples

Produces sequences of number

import { generate } from 'rxjs';

const result = generate(0, x => x < 3, x => x + 1, x => x);

result.subscribe(x => console.log(x));

// Logs:
// 0
// 1
// 2

Use asap scheduler

import { generate } from 'rxjs';

const result = generate(1, x => x < 5, x => x * 2, x => x + 1, asap);

result.subscribe(x => console.log(x));

// Logs:
// 2
// 3
// 5

Use it instead of nexting values in a for loop.

generate allows you to create a stream of values generated with a loop very similar to a traditional for loop. The first argument of generate is a beginning value. The second argument is a function that accepts this value and tests if some condition still holds. If it does, then the loop continues, if not, it stops. The third value is a function which takes the previously defined value and modifies it in some way on each iteration. Note how these three parameters are direct equivalents of three expressions in a traditional for loop: the first expression initializes some state (for example, a numeric index), the second tests if the loop can perform the next iteration (for example, if the index is lower than 10) and the third states how the defined value will be modified on every step (for example, the index will be incremented by one).

Return value of a generate operator is an Observable that on each loop iteration emits a value. First of all, the condition function is ran. If it returns true, then the Observable emits the currently stored value (initial value at the first iteration) and finally updates that value with iterate function. If at some point the condition returns false, then the Observable completes at that moment.

Optionally you can pass a fourth parameter to generate - a result selector function which allows you to immediately map the value that would normally be emitted by an Observable.

If you find three anonymous functions in generate call hard to read, you can provide a single object to the operator instead where the object has the properties: initialState, condition, iterate and resultSelector, which should have respective values that you would normally pass to generate. resultSelector is still optional, but that form of calling generate allows you to omit condition as well. If you omit it, that means condition always holds, or in other words the resulting Observable will never complete.

Both forms of generate can optionally accept a scheduler. In case of a multi-parameter call, scheduler simply comes as a last argument (no matter if there is a resultSelector function or not). In case of a single-parameter call, you can provide it as a scheduler property on the object passed to the operator. In both cases, a scheduler decides when the next iteration of the loop will happen and therefore when the next value will be emitted by the Observable. For example, to ensure that each value is pushed to the Observer on a separate task in the event loop, you could use the async scheduler. Note that by default (when no scheduler is passed) values are simply emitted synchronously.

Examples

Use with condition and iterate functions

import { generate } from 'rxjs';

const result = generate(0, x => x < 3, x => x + 1);

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('Complete!')
});

// Logs:
// 0
// 1
// 2
// "Complete!"

Use with condition, iterate and resultSelector functions

import { generate } from 'rxjs';

const result = generate(0, x => x < 3, x => x + 1, x => x * 1000);

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('complete!')
});

// Logs:
// 0
// 1000
// 2000
// "complete!"

Use with options object

import { generate } from 'rxjs';

const result = generate({
  initialState: 0,
  condition(value) { return value < 3; },
  iterate(value) { return value + 1; },
  resultSelector(value) { return value * 1000; }
});

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('complete!')
});

// Logs:
// 0
// 1000
// 2000
// "Complete!"

Use options object without condition function

import { generate } from 'rxjs';

const result = generate({
  initialState: 0,
  iterate(value) { return value + 1; },
  resultSelector(value) { return value * 1000; }
});

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('complete!') // This will never run
});

// Logs:
// 0
// 1000
// 2000
// 3000
// ...and never stops.

Examples

Use options object with condition function

import { generate } from 'rxjs';

const result = generate({
  initialState: 0,
  condition: x => x < 3,
  iterate: x => x + 1,
});

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('complete!')
});

// Logs:
// 0
// 1
// 2
// "Complete!".

Examples

Use options object with condition and iterate function

import { generate } from 'rxjs';

const result = generate({
  initialState: 0,
  condition: x => x < 3,
  iterate: x => x + 1,
  resultSelector: x => x,
});

result.subscribe({
  next: value => console.log(value),
  complete: () => console.log('complete!')
});

// Logs:
// 0
// 1
// 2
// "Complete!".