let

signature: let(function): Observable

Let me have the whole observable.

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

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