let - Learn RxJS

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let
signature: let(function): Observable
Let me have the whole observable.
⚠ let is no longer available, or necessary, with pipeable operators! (RxJS 5.5+)
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Examples
Example 1: Reusing error handling logic with let
( jsBin | jsFiddle )
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// custom error handling logic
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const retryThreeTimes = obs =>
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  obs.retry(3).catch(_ => Rx.Observable.of('ERROR!'));
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const examplePromise = val =>
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  new Promise(resolve => resolve(`Complete: ${val}`));
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​
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//faking request
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const subscribe = Rx.Observable.of('some_url')
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  .mergeMap(url => examplePromise(url))
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  // could reuse error handling logic in multiple places with let
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  .let(retryThreeTimes)
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  //output: Complete: some_url
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  .subscribe(result => console.log(result));
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​
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const customizableRetry = retryTimes => obs =>
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  obs.retry(retryTimes).catch(_ => Rx.Observable.of('ERROR!'));
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​
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//faking request
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const secondSubscribe = Rx.Observable.of('some_url')
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  .mergeMap(url => examplePromise(url))
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  // could reuse error handling logic in multiple places with let
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  .let(customizableRetry(3))
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  //output: Complete: some_url
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  .subscribe(result => console.log(result));
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Example 2: Applying map with let
( jsBin | jsFiddle )
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//emit array as a sequence
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const source = Rx.Observable.from([1, 2, 3, 4, 5]);
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//demonstrating the difference between let and other operators
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const test = source
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  .map(val => val + 1)
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  /*
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        this would fail, let behaves differently than most operators
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        val in this case is an observable
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    */
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  //.let(val => val + 2)
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  .subscribe(val => console.log('VALUE FROM ARRAY: ', val));
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​
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const subscribe = source
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  .map(val => val + 1)
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  //'let' me have the entire observable
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  .let(obs => obs.map(val => val + 2))
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  //output: 4,5,6,7,8
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  .subscribe(val => console.log('VALUE FROM ARRAY WITH let: ', val));
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Example 3: Applying multiple operators with let
( jsBin | jsFiddle )
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//emit array as a sequence
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const source = Rx.Observable.from([1, 2, 3, 4, 5]);
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​
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//let provides flexibility to add multiple operators to source observable then return
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const subscribeTwo = source
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  .map(val => val + 1)
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  .let(obs =>
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    obs
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      .map(val => val + 2)
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      //also, just return evens
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      .filter(val => val % 2 === 0)
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  )
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  //output: 4,6,8
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  .subscribe(val => console.log('let WITH MULTIPLE OPERATORS: ', val));
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Example 4: Applying operators through function
( jsBin | jsFiddle )
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//emit array as a sequence
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const source = Rx.Observable.from([1, 2, 3, 4, 5]);
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​
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//pass in your own function to add operators to observable
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const obsArrayPlusYourOperators = yourAppliedOperators => {
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  return source.map(val => val + 1).let(yourAppliedOperators);
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};
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const addTenThenTwenty = obs => obs.map(val => val + 10).map(val => val + 20);
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const subscribe = obsArrayPlusYourOperators(addTenThenTwenty)
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  //output: 32, 33, 34, 35, 36
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  .subscribe(val => console.log('let FROM FUNCTION:', val));
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Additional Resources
​let​
📰 - Official docs
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/let.ts​
finalize / finally
repeat
Last modified 1yr ago
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Contents
Let me have the whole observable.
Examples
Additional Resources