How to use Rx.Observable.prototype.let operator? - rxjs

Question

The example and explanation of the let operator (https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/let.md) is not clear. Anyone has a good example/explanation how the let operator works, and when we should use it?

Answer 1

&tldr;
It is a convenience function for being able to compartmentalize logic and inject it into a pipeline.
Longer Explanation
The source is probably the most definitive explanation. It is really just passing a function which gets called with a source Observable.
Rx.Observable.prototype.let = function(fn) {
return fn(this);
}
The utility of this is that we can create or pre-define a pipeline that you want to reuse for multiple sources. Consider a common trope for Rx, the reactive search bar:
// Listen to a key up event on the search bar
// and emit the value of the search
Rx.Observable.fromEvent(searchBar, 'keyup', e => e.target.value)
// Don't search too eagerly
.filter(text => text.length > 3)
.debounceTime(500)
//Search logic
.flatMap(text => $.getJSON(`my/search/api?q=${text}`))
.flatMap({results} => results)
//Handler
.subscribe(appendToList);
The above should give a basic sense of the structure of how a pipeline might be created. If we wanted to try and abstract some of this logic either to clean up the code or to be able to use it elsewhere it can be a little tricky, because it usually means creating a new operator (and that has its own headaches).
The solution is a relatively simple approach of pulling common logic into a function that can be passed a source Observable and will return a new Observable with that logic applied.
So the above might become:
//Defined in pipelines.js
function filterBuilder(minText, debounceTime) {
return (source) =>
source.filter(text => text.length > minText)
.debounce(debounceTime);
}
function queryBuilder(baseUrl) {
return (source) =>
source.flatMap(text => $.getJSON(`${baseUrl}?q=${text}`))
.flatMap({results} => results);
}
//In your application code
Rx.Observable.fromEvent(searchBar, 'keyup', e => e.target.value)
.let(filterBuilder(3, 500))
.let(queryBuilder('my/search/api'))
.subscribe(appendResults);