- Reading these docs
- Creating streams
- Handling errors
- Transforming streams
- Filtering streams
- Slicing streams
- Consuming streams
- Combining streams
- Combining higher order streams
- Awaiting promises
- Sampling streams
- Rate limiting streams
- Delaying streams
You'll see diagrams like the following:
stream1: -a-b-c-d->
stream2: -a--b---c|
stream3: -abc-def-X
These are timeline diagrams that try to give a simple, representative notion of how a stream behaves over time. Time proceeds from left to right, using letters and symbols to indicate certain things:
-- an instant in time where no event occurs- letters (a,b,c,d,etc) - an event at an instant in time
|- end of streamX- an error occurred at an instant in time>- stream continues infinitely- Typically,
>means you can assume that a stream will continue to repeat some common pattern infinitely
- Typically,
stream: a|
A stream that emits a and then ends immediately.
stream: a-b---|
A stream that emits a, then b, and some time later ends.
stream: a-b-X
A stream that emits a, then b, then fails.
stream: abc-def->
A stream that emits a, then b, then c, then nothing, then d, then e, then f, and then continues infinitely.
####most.of(x) -> Stream
most.of(x): x|
Create a stream containing only x.
var stream = most.of('hello');
stream.forEach(console.log.bind(console)); // logs hello####most.fromPromise(promise) -> Stream
promise: ----a
most.fromPromise(promise): ----a|
Create a stream containing the outcome of a promise. If the promise fulfills, the stream will contain the promise's value. If the promise rejects, the stream will be in an error state with the promise's rejection reason as its error. See flatMapError for error recovery.
####most.from(iterable) -> Stream
most.from([1,2,3,4]): 1234|
Create a stream containing all items from an iterable. The iterable can be an Array, Array-like, or anything that supports the iterable protocol or iterator protocol, such as a generator. Providing a finite iterable, such as an Array, creates a finite stream. Providing an infinite iterable, such as an infinite generator, creates an infinite stream.
// Logs 1 2 3 4
most.from([1,2,3,4])
.forEach(console.log.bind(console));// Strings are Array-like, this works
// Logs a b c d
most.from('abcd')
.forEach(console.log.bind(console));function* numbers() {
for(i=0 ;; ++i) {
yield i;
}
}
// Create an infinite stream of numbers
var stream = most.from(numbers());
// Limit the stream to the first 100 numbers
stream.take(100)
.forEach(console.log.bind(console));####most.repeat(x) -> Stream
Create a stream containing infinite occurrences of x.
most.repeat(x): xxxxxxx->
####most.periodic(period) -> Stream
most.periodic(2): a-b-c-d-e-f->
most.periodic(5): a----b----c->
Create an infinite stream containing events that arrive every period milliseconds. The value of each event is its arrival time in milliseconds.
####most.empty() -> Stream
most.empty(): |
Create an already-ended stream containing no events.
####most.never() -> Stream
most.never(): ---->
Create a stream that contains no events and never ends.
####most.iterate(f, initial) -> Stream
Build an infinite stream by computing successive items iteratively. Conceptually, the stream will contain: [initial, f(initial), f(f(initial)), ...]
// An infinite stream of all integers >= 0, ie
// 0, 1, 2, 3, 4, 5, ...
most.iterate(function(x) {
return x + 1;
}, 0);The iterating function may return a promise. This allows most.iterate to be used to build asynchronous streams of future values. For example:
var when = require('when');
// An infinite stream of all integers >= 0, each delayed by 1 more
// millisecond than the previous.
// IOW, a stream that decelerates as it produces values:
// 0 (immediately)
// 1 (1 millisecond after 0)
// 2 (2 millisecond after 1)
// 3 (3 millisecond after 2)
// ... etc
most.iterate(function(x) {
var y = x + 1;
return when(y).delay(y);
}, 0);####most.unfold(f, seed) -> Stream
Build a stream by computing successive items. Whereas reduce tears down a stream to a final value, unfold builds up a stream from a seed value.
The unfolding function accepts a seed value and must return a tuple: {value:*, seed:*, done:boolean}, or a promise for a tuple. Returning a promise allows most.unfold to be used to build asynchronous streams of future values.
tuple.valuewill be emitted as an event.tuple.seedwill be passed to the next invocation of the unfolding function.tuple.donecan be used to stop unfolding. Whentuple.done == true, unfolding will stop.
Note that if the unfolding function never returns a tuple with tuple.done == true, the stream will be infinite.
var rest = require('rest');
var urlPrefix = 'product/';
// Unfold an infinite stream of products, producing a stream of:
// [rest('product/1'), rest('product/2'), rest('product/3'), ...]
most.unfold(function(id) {
return rest(urlPrefix + id).then(function(content) {
return { value: content, seed: id + 1 };
});
}, 1);####most.fromEvent(eventType, source) -> Stream
source: -a--b-c---d->
most.fromEvent(eventType, source): -a--b-c---d->
Create a stream containing events from the provided EventTarget, such as a DOM element, or EventEmitter. This provides a simple way to coerce existing event sources into streams.
Note that when the stream ends (for example, by using take, takeUntil, etc.), it will automatically be disconnected from the event source. For example, in the case of DOM events, the underlying DOM event listener will be removed automatically.
var clicks = most.fromEvent('click', document.querySelector('.the-button'));####most.fromEventWhere(predicate, eventType, source) -> Stream
Like most.fromEvent, create a stream containing, but apply a predicate function synchronously to each event. This allows preventDefault, filtering based on CSS selectors using element.matches, and any other filtering or side effects that must be performed immediately in the DOM event call stack.
As with most.fromEvent, when the stream ends, it will automatically be disconnected from the event source.
// Using preventDefault
var form = document.querySelector('form');
most.fromEventWhere(function(e) { e.preventDefault(); }, 'submit', form)
.map(parseForm)
.map(JSON.stringify)
.forEach(postToServer);// Using event delegation with Element.matches
// This allows only events with the .toggle-button class
// It also only calls preventDefault on allowed events
var form = document.querySelector('.container');
most.fromEventWhere(function(e) {
return e.matches('.toggle-button') && e.preventDefault();
}, 'click', container)
.forEach(doSomething);####most.create(publisher) -> Stream
Create a push-stream for imperatively pushing events, primarily for adapting existing event sources.
function publisher(add:function(x:*), end:function(x:*), error:function(e:Error))
-> function()
The publisher function receives 3 functions as arguments, which it can use to publish events, end the stream, or signal an error. It may return a dispose function. The dispose function will be called once all consumers have lost interest in the stream, and should free any resources held by the publisher.
Note that the publisher will not be called until there is demand for the stream's events. Specifically, the publisher will be called when the number of observers goes from zero to one. Likewise, the dispose function will be called when the number of observers again returns to zero. The publisher would then be called again if the number of observers subsequently goes from zero to one, and so on.
The publisher function can use add, end, and error:
add(x)- Addxto the streamend()- End the stream. Any later calls toadd,end, orerrorwill be no-ops.error(e)- Signal that the stream has failed and cannot produce more events.
Note that if you never call end or error, the stream will never end, and consumers will wait forever for additional events.
If the publisher returns a dispose function, it will be called when the stream ends or errors--for example, when the publisher explicitly calls end or error, or when all consumers lose interest.
dispose- free resources held by the publisher
Note that if the stream neither ends nor fails, the dispose function will never be called.
Using add and end to push events and then end the stream.
// Add events and then end
var stream = most.create(function(add, end, error) {
setTimeout(add, 1000, 'event 1');
setTimeout(add, 3000, 'event 2');
setTimeout(function(x) {
add('event 3');
end();
}, 10000);
// OPTIONAL: Return a dispose function to clean up
// resources when the stream ends
return function() {
console.log('dispose');
}
});
// Logs
// 'event 1' after 1 second
// 'event 2' after 3 seconds
// 'event 3' after 10 seconds
// 'dispose' after 10 seconds
stream.forEach(console.log.bind(console));Using error to fail the stream and propagate an Error:
// Add events and then fail
var stream = most.create(function(add, end, error) {
setTimeout(add, 1000, 'event 1');
setTimeout(function() {
error(new Error('oops!'));
}, 3000);
});
// Logs
// 'event 1' after 1 second
// '[Error: oops!]' after 3 seconds
stream
.forEach(console.log.bind(console))
.catch(console.log.bind(console)); // Catch the error as a promise####stream.startWith(x) -> Stream
####most.startWith(x, stream) -> Stream
Create a new stream containing x followed by all events in stream.
stream: a-b-c-d->
stream.startWith(x): xa-b-c-d->####stream1.concat(stream2) -> Stream
####most.concat(stream1, stream2) -> Stream
Create a new stream containing all events in stream1 followed by all events in stream2.
stream1: -a-b-c|
stream2: -d-e-f->
stream1.concat(stream2): -a-b-c-d-e-f->Note that this effectively timeshifts events from stream2 past the end time of stream1. In contrast, other operations such as combine, merge, flatMap preserve event arrival times, allowing events from the multiple combined streams to interleave.
####stream.cycle() -> Stream
####most.cycle(stream) -> Stream
Tie a stream into a circle.
most.from([1,2,3]): 123|
most.from([1,2,3]).cycle(): 123123123123->
Makes an infinite stream from a finite one. If the input stream is infinite, then there will be no observable difference between stream and stream.cycle().
####stream.flatMapError(f) -> Stream
####most.flatMapError(f, stream) -> Stream
Recover from a stream failure by calling a function to create a new stream.
stream: -a-b-c-X
f(X): -d-e-f->
stream.flatMapError(f): -a-b-c-d-e-f->
When a stream fails with an error, the error will be passed to f. f must return a new stream to replace the error.
var rest = require('rest');
var stream = most.fromPromise(rest('http://myapi.com/things'));
// Try to process data from the real API, but fall back
// to some default data if that fails.
stream.map(JSON.parse)
.flatMapError(function(e) {
// console.error(e);
return most.of(defaultData);
})
.forEach(processData);####most.throwError(error) -> Stream
Create a stream in the error state. This can be useful for functions that need to return a stream, but need to signal an error.
most.throwError(X): X####stream.map(f) -> Stream
####most.map(f, stream) -> Stream
Create a new stream by applying f to each event of the input stream.
stream: -a-b-c-d->
stream.map(add1): -f(a)-f(b)-f(c)-f(d)->
// Logs 2 3 4 5
most.from([1,2,3,4])
.map(function(x) {
return x + 1;
})
.forEach(console.log.bind(console));####stream.constant(x) -> Stream
####most.constant(x, stream) -> Stream
Create a new stream by replacing each event of the input stream with x.
stream: -a-b-c-d->
stream.constant(x): -x-x-x-x->
// Logs 1 1 1 1
most.from([1,2,3,4])
.constant(1)
.forEach(console.log.bind(console));####stream.scan(f, initial) -> Stream
####most.scan(f, initial, stream) -> Stream
Create a new stream containing incrementally accumulated results, starting with the provided initial value.
function f(accumulated, x) -> newAccumulated
stream: -1-2-3->
stream.scan(add, 0): 01-3-6->
Unlike reduce which produces a single, final result, scan emits incremental results. The resulting stream is of the same proportion as the original. For example, if the original contains 10 events, the resulting stream will contain 11 (the initial value, followed by 10 incremental events). If the original stream is infinite, the resulting stream will be infinite.
// Logs a ab abc abcd
most.from(['a', 'b', 'c', 'd'])
.scan(function(string, letter) {
return string + letter;
}, '')
.forEach(console.log.bind(console);// Maintain a sliding window of (up to) 3 values in an array
// A stream containing all integers >= 0
var numbers = most.iterate(function(x) {
return x+1;
}, 0);
// Logs
// []
// [0]
// [0,1]
// [0,1,2]
// [1,2,3]
// [2,3,4]
// ... etc ...
numbers.scan(function(slidingWindow, x) {
return slidingWindow.concat(x).slice(-10);
}, [])
.forEach(console.log.bind(console));####stream.flatMap(f) -> Stream
####most.flatMap(f, stream) -> Stream
Transform each event in stream into a stream, and then merge it into the resulting stream. Note that f must return a stream.
function f(x) -> Stream
stream: -a----b----c|
f(a): 1--2--3|
f(b): 1----2----3|
f(c): 1-2-3|
stream.flatMap(f): -1--2-13---2-1-233|
Note the difference between concatMap and flatMap: concatMap concatenates, while flatMap merges.
// Logs: 1 2 1 1 2 1 1 2 2 2
most.from([1, 2])
.flatMap(function(x) {
return most.periodic(x * 1000).take(5).constant(x);
})
.observe(console.log.bind(console));####stream.concatMap(f) -> Stream
####most.concatMap(f, stream) -> Stream
Transform each event in stream into a stream, and then concatenate it onto the end of the resulting stream. Note that f must return a stream.
function f(x) -> Stream
stream: -a----b----c|
f(a): 1--2--3|
f(b): 1----2----3|
f(c): 1-2-3|
stream.flatMap(f): -1--2--31----2----31-2-3|
Note the difference between concatMap and flatMap: concatMap concatenates, while flatMap merges.
// Logs: 1 1 1 1 1 2 2 2 2 2
most.from([1, 2])
.concatMap(function(x) {
return most.periodic(x * 1000).take(5).constant(x);
})
.observe(console.log.bind(console));####streamOfFunctions.ap(stream) -> Stream
####most.ap(streamOfFunctions, stream) -> Stream
Apply all the functions in streamOfFunctions to all the values in stream.
streamOfFunctions: f-g-h|
stream: x-y-z|
streamOfFunctions.ap(stream): f(x)-f(y)-f(z)-g(x)-g(y)-g(z)-h(x)-h(y)-h(z)|
This effectively creates the cross-product of streamOfFunctions and stream. As shown in the diagram above, stream will be traversed multiple times--once for each event in streamOfFunctions.
####stream.timestamp() -> Stream
####most.timestamp(stream) -> Stream
Materialize event timestamps, transforming Stream<X> into Stream<{ time:number, value:X }>
// Logs
// { time: 1418740004085, value: 'hello' }
// { time: 1418740005085, value: 'hello' }
// { time: 1418740006085, value: 'hello' }
// { time: 1418740007085, value: 'hello' }
// ... etc
most.periodic(1000).constant('hello')
.timestamp()
.observe(console.log.bind(console));
####stream.tap(f) -> Stream
####most.tap(f, stream) -> Stream
Perform a side-effect for each event in stream.
stream: -a-b-c-d->
stream.tap(f): -a-b-c-d->
For each event in stream, f is called, but the value of its result is ignored. If f fails (ie throws), then the returned stream will also fail. The stream returned by tap will contain the same events as the original stream.
####stream.filter(predicate) -> Stream
####most.filter(predicate, stream) -> Stream
Create a stream containing only events for which predicate returns truthy.
stream: -1-2-3-4->
stream.filter(even): ---2---4->
####stream.distinct() -> Stream
####most.distinct(stream) -> Stream
Create a new stream with adjacent duplicates removed.
stream: -1-2-2-3-4-4-5->
stream.distinct(): -1-2---3-4---5->
Note that === is used to identify duplicate items. To use a different comparison, use distinctBy
####stream.distinctBy(equals) -> Stream
####most.distinctBy(equals, stream) -> Stream
Create a new stream with adjacent duplicates removed, using the provided equals function.
stream: -a-b-B-c-D-d-e->
stream.distinctBy(equalsIgnoreCase): -a-b---c-D---e->
The equals function should accept two values and return truthy if the two values are equal, or falsy if they are not equal.
function equals(a, b) -> boolean
####stream.slice(start, end) -> Stream
####most.slice(start, end, stream) -> Stream
Create a new stream containing only events where start <= index < end, where index is the ordinal index of an event in stream.
stream: -a-b-c-d-e-f->
stream.slice(1, 4): ---b-c-d|
stream: -a-b-c|
stream.slice(1, 4): ---b-c|
If stream contains fewer than start events, the returned stream will be empty.
####stream.take(n) -> Stream
####most.take(n, stream) -> Stream
Create a new stream containing at most n events from stream.
stream: -a-b-c-d-e-f->
stream.take(3): -a-b-c|
stream: -a-b|
stream.take(3): -a-b|
If stream contains fewer than n events, the returned stream will be effectively equivalent to stream.
####stream.skip(n) -> Stream
####most.skip(n, stream) -> Stream
Create a new stream that omits the first n events from stream.
stream: -a-b-c-d-e-f->
stream.skip(3): -------d-e-f->
stream: -a-b-c-d-e|
stream.skip(3): -------d-e|
stream: -a-b-c|
stream.skip(3): ------|
If stream contains fewer than n events, the returned stream will be empty.
####stream.takeWhile(predicate) -> Stream
####most.takeWhile(predicate, stream) -> Stream
Create a new stream containing all events until predicate returns false.
stream: -2-4-5-6-8->
stream.takeWhile(even): -2-4-|
####stream.takeWhile(predicate) -> Stream
####most.takeWhile(predicate, stream) -> Stream
Create a new stream containing all events after predicate returns false.
stream: -2-4-5-6-8->
stream.skipWhile(even): -----5-6-8->
Alias: takeUntil
####stream.until(endSignal) -> Stream
####most.until(endSignal, stream) -> Stream
Create a new stream containing all events until endSignal emits an event.
stream: -a-b-c-d-e-f->
endSignal: ------z->
stream.until(endSignal): -a-b-c|
If endSignal is empty or never emits an event, then the returned stream will be effectively equivalent to stream.
// Log mouse events until the user clicks. Note that DOM event handlers will
// automatically be unregistered.
most.fromEvent('mousemove', document)
.until(most.fromEvent('click', document)
.forEach(console.log.bind(console));Alias: skipUntil
####stream.since(startSignal) -> Stream
####most.since(startSignal, stream) -> Stream
Create a new stream containing all events until startSignal emits an event.
stream: -a-b-c-d-e-f->
startSignal: ------z->
stream.since(startSignal): -a-b-c|
If startSignal is empty or never emits an event, then the returned stream will be effectively equivalent to stream.
// Start logging mouse events when the user clicks.
most.fromEvent('mousemove', document)
.since(most.fromEvent('click', document)
.forEach(console.log.bind(console));EXPERIMENTAL
####stream.during(timeWindow)
####most.during(timeWindow, stream)
Create a new stream containing only events that occur during a dynamic time window.
stream: -a-b-c-d-e-f-g->
timeWindow: -----s
s: -----t
stream.during(timeWindow): -----c-d-e-|
This is similar to slice, but uses time signals rather than indices to limit the stream.
// After the first click, log mouse move events for 1 second.
// Note that DOM event handlers will automatically be unregistered.
var start = most.fromEvent('click', document);
var end = most.of().delay(1000);
// Map the first click to a stream containing a 1 second delay
// The click represents the window start time, after which
// the window will be open for 1 second.
var timeWindow = start.constant(end);
most.fromEvent('mousemove', document)
.during(timeWindow)
.observe(console.log.bind(console));####stream.reduce(f, initial) -> Promise
####most.reduce(f, initial, stream) -> Promise
Reduce a stream, returning a promise for the ultimate result.
stream: -1-2-3-4-|
stream.reduce(sum, 0): 10
The returned promise will fulfill with the final reduced result, or will reject if a failure occurs while reducing the stream.
The reduce function (f above)
TODO: Example
Alias: forEach
####stream.observe(f) -> Promise
####stream.forEach(f) -> Promise
####most.observe(f, stream) -> Promise
####most.forEach(f, stream) -> Promise
Start consuming events from stream, processing each with f. The returned promise will fulfill after all the events have been consumed, or will reject if the stream fails and the error is not handled.
// Log mouse movements until the user clicks, then stop.
most.fromEvent('mousemove', document)
.takeUntil(most.fromEvent('click', document))
.observe(console.log.bind(console))
.then(function() {
console.log('All done');
});####stream.drain() -> Promise
####most.drain(stream) -> Promise
Start consuming events from stream. This can be useful in some cases where you don't want or need to process the terminal events--e.g. when all processing has been done via upstream side-effects. Most times, however, you'll use observe to consume and process terminal events.
The returned promise will fulfill after all the events have been consumed, or will reject if the stream fails and the error is not handled.
####stream1.merge(stream2) -> Stream
####most.merge(stream1, stream2) -> Stream
Create a new stream containing events from stream1 and stream2.
stream1: -a--b----c--->
stream2: --w---x-y--z->
stream1.merge(stream2): -aw-b-x-yc-z->
Merging multiple streams creates a new stream containing all events from the input stream without affecting the arrival time of the events. You can think of the events from the input streams simply being interleaved into the new, merged stream. A merged stream ends when all of its input streams have ended.
In contrast to concat, merge preserves the arrival times of events. That is, it creates a new stream where events from stream1 and stream2 can interleave.
####stream1.combine(f, stream2) -> Stream
####most.combine(f, stream1, stream2) -> Stream
Create a new stream that emits the set of latest event values from all input streams whenever a new event arrives on any input stream.
stream1: -0--1----2--->
stream2: --3---4-5--6->
stream1.combine(add, stream2): --3-4-5-67-8->
Combining creates a new stream by applying a function to the most recent event from each stream whenever a new event arrives on any one stream. Combining must wait for at least one event to arrive on all input streams before it can produce any events. A combined stream ends with any one of its input streams ends.
// Add the current value of two inputs
// Updates the result whenever *either* of the inputs changes!
// Create a stream from an <input> value
function fromInput(input) {
return most.fromEvent('change', input)
.map(function(e) { return e.target.value })
.map(Number);
}
// Add two numbers
function add(x, y) {
return x + y;
}
// Create streams for the current value of x and y
var xStream = fromInput(document.querySelector('input.x'));
var yStream = fromInput(document.querySelector('input.y'));
// Create a result stream by adding x and y
// This always adds the latest value of x and y
var resultStream = xStream.combine(add, yStream);
var resultNode = document.querySelector('.result');
result.forEach(function(z) {
resultNode.textContent = z;
});####most.lift(f) -> function
Lifts a function to act on streams. Lifting returns a function that accepts streams as arguments, and returns a stream as a result.
One way to think of lifting is that it takes a function that operates on "normal" values, like two strings, and creates a function that operates on "time-varying" values--on the "current value" of two <input> elements, for example.
Lifting should be done at "initialization time": you should lift a function once and then use it many times.
// return the concatenation of 2 strings
function append(s1, s2) {
return s1 + s2;
}
var s1 = 'foo';
var s2 = 'bar';
// result is a string
var result = append(s1, s2);
// Logs 'foobar'
console.log(result);
// Lift the append function to operate on values that change over time
var liftedAppend = most.lift(append);
// A stream representing the "current value" of <input name="s1">
var input1 = most.fromEvent('change', document.querySelector('[name="s1"]'))
.map(function(e) {
return e.target.value;
});
// A stream representing the "current value" of <input name="s2">
var input2 = most.fromEvent('change', document.querySelector('[name="s2"]'))
.map(function(e) {
return e.target.value;
});
// resultStream is a stream of strings
// Whenever either input changes, resultStream will emit a new event
// It's like a live-updating value
var resultStream = liftedAppend(input1, input2);
// Logs the concatenated value of input1 and input2
// *whenever either input changes*
resultStream.forEach(console.log.bind(console));####stream1.zip(f, stream2) -> Stream
####most.zip(f, stream1, stream2) -> Stream
Create a new stream by applying a function to corresponding pairs of events from the inputs streams.
stream1: -1--2--3--4->
stream2: -1---2---3---4->
stream1.zip(add, stream2): -2---4---6---8->
Zipping correlates by index corresponding events from two or more input streams. Fast streams must wait for slow streams. For pull streams, this does not cause any buffering. However, when zipping push streams, a fast push stream, such as those created by most.create and most.fromEvent will be forced to buffer events so they can be correlated with corresponding events from the slower stream.
A zipped stream ends when any one of its input streams ends.
function add(x, y) {
return x + y;
}
// Logs 5 7 9
// In other words: add(1, 4) add(2, 5) add(3, 6)
most.from([1,2,3])
.zip(add, most.from([4,5,6,7,8]))
.forEach(console.log.bind(console));####stream.switch() -> Stream
####most.switch(stream) -> Stream
Given a higher-order stream, return a new stream that adopts the behavior of (ie emits the events of) the most recent inner stream.
s: -a-b-c-d-e-f->
t: -1-2-3-4-5-6->
stream: -s-----t----->
stream.switch(): -a-b-c-4-5-6->
TODO: Example
####stream.join() -> Stream
####most.join(stream) -> Stream
Given a higher-order stream, return a new stream that merges all the inner streams as they arrive.
s: ---a---b---c---d-->
t: -1--2--3--4--5--6->
stream: -s------t--------->
stream.join(): ---a---b--4c-5-d6->
TODO: Example
####stream.await() -> Stream
####most.await(stream) -> Stream
Given a stream of promises, ie Stream<Promise<X>>, return a new stream containing the fulfillment values, ie Stream<X>.
promise p: ---1
promise q: ------2
promise r: -3
stream: -p---q---r->
stream.await(): ---1--2--3->
Note that event order is preserved, regardless of promise fulfillment order. The fulfilled event values will arrive at the later of the original event time and the promise fulfillment time.
If a promise rejects, the stream will be in an error state with the rejected promise's reason as its error. See flatMapError for error recovery. For example:
promise p: ---1
promise q: ------X
promise r: -3
stream: -p---q---r->
stream.await(): ---1--X
Functionally, stream.await() and stream.flatMap(most.fromPromise) are equivalent, but await is much more efficient.
var urls = [url1, url2, url3, ...];
function fetchContent(url) {
// return a promise
}
var streamOfPromises = Stream.from(urls).map(fetchContent);
var streamOfContent = streamOfPromises.await();
streamOfContent.forEach(console.log.bind(console));####values.sampleWith(sampler)
####most.sampleWith(sampler, values)
When an event arrives on sampler, emit the latest event value from values.
values: -1---2-3---4-5---6-7---8->
sampler: ---a---a---a---a---a---a->
values.sampleWith(sampler): ---1---3---4---5---7---8->
values: -1----2----3----4----5--->
sampler: -a-a-a-a-a-a-a-a-a-a-a-a->
values.sampleWith(sampler): -1-1-1-2-2-3-3-3-4-4-5-5->
Sampling can "smooth" an erratic source, or can act as a dynamic throttle to speed or slow events from one stream using another.
// Log mouse position whenever the user presses a key
most.fromEvent('mousemove', document)
.sampleWith(most.fromEvent('keydown', document))
.observe(console.log.bind(console));####stream.debounce(debounceTime) -> Stream
####most.debounce(debounceTime, stream) -> Stream
Wait for a burst of events to subside and emit only the last event in the burst.
stream: abcd----abcd---->
stream.debounce(2): -----d-------d-->
Debouncing can be extremely useful when dealing with bursts of similar events, for example, debouncing keypress events before initiating a remote search query in a browser application.
var searchInput = document.querySelector('[name="search-text"]');
var searchText = most.fromEvent('input', searchInput);
// Logs the current value of the searchInput, only after the
// user stops typing for 500 millis
searchText.debounce(500)
.map(function(e) {
return e.target.value;
})
.observe(console.log.bind(console));See the type-to-search example for a more complete example of using debounce.
####stream.throttle(throttlePeriod) -> Stream
####most.throttle(throttlePeriod, stream) -> Stream
Limit the rate of events to at most one per throttlePeriod.
stream: abcd----abcd---->
stream.throttle(2): a-c-----a-c----->
In contrast to debounce, throttle simply drops events that occur more often than throttlePeriod, whereas debounce waits for a "quiet period".
####stream.delay(delayTime) -> Stream
####most.delay(delayTime, stream) -> Stream
Timeshift a stream by delayTime.
stream: -a-b-c-d->
stream.delay(1): --a-b-c-d->
stream.delay(5): ------a-b-c-d->
Delaying a stream timeshifts all the events by the same amount. Delaying doesn't change the time between events.
TODO: Example