let

signature: let(function): Observable

Let me have the whole observable.

let is no longer available, or necessary, with pipeable operators! (RxJS 5.5+)

Ultimate RxJS

Examples

Example 1: Reusing error handling logic with let

( jsBin | jsFiddle )

// custom error handling logic
const retryThreeTimes = obs =>
obs.retry(3).catch(_ => Rx.Observable.of('ERROR!'));
const examplePromise = val =>
new Promise(resolve => resolve(`Complete: ${val}`));
//faking request
const subscribe = Rx.Observable.of('some_url')
.mergeMap(url => examplePromise(url))
// could reuse error handling logic in multiple places with let
.let(retryThreeTimes)
//output: Complete: some_url
.subscribe(result => console.log(result));
const customizableRetry = retryTimes => obs =>
obs.retry(retryTimes).catch(_ => Rx.Observable.of('ERROR!'));
//faking request
const secondSubscribe = Rx.Observable.of('some_url')
.mergeMap(url => examplePromise(url))
// could reuse error handling logic in multiple places with let
.let(customizableRetry(3))
//output: Complete: some_url
.subscribe(result => console.log(result));

Example 2: Applying map with let

( jsBin | jsFiddle )

//emit array as a sequence
const source = Rx.Observable.from([1, 2, 3, 4, 5]);
//demonstrating the difference between let and other operators
const test = source
.map(val => val + 1)
/*
this would fail, let behaves differently than most operators
val in this case is an observable
*/
//.let(val => val + 2)
.subscribe(val => console.log('VALUE FROM ARRAY: ', val));
const subscribe = source
.map(val => val + 1)
//'let' me have the entire observable
.let(obs => obs.map(val => val + 2))
//output: 4,5,6,7,8
.subscribe(val => console.log('VALUE FROM ARRAY WITH let: ', val));

Example 3: Applying multiple operators with let

( jsBin | jsFiddle )

//emit array as a sequence
const source = Rx.Observable.from([1, 2, 3, 4, 5]);
//let provides flexibility to add multiple operators to source observable then return
const subscribeTwo = source
.map(val => val + 1)
.let(obs =>
obs
.map(val => val + 2)
//also, just return evens
.filter(val => val % 2 === 0)
)
//output: 4,6,8
.subscribe(val => console.log('let WITH MULTIPLE OPERATORS: ', val));

Example 4: Applying operators through function

( jsBin | jsFiddle )

//emit array as a sequence
const source = Rx.Observable.from([1, 2, 3, 4, 5]);
//pass in your own function to add operators to observable
const obsArrayPlusYourOperators = yourAppliedOperators => {
return source.map(val => val + 1).let(yourAppliedOperators);
};
const addTenThenTwenty = obs => obs.map(val => val + 10).map(val => val + 20);
const subscribe = obsArrayPlusYourOperators(addTenThenTwenty)
//output: 32, 33, 34, 35, 36
.subscribe(val => console.log('let FROM FUNCTION:', val));

Additional Resources

  • let

    📰 - Official docs

📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/let.ts