How to return additional values from flatMap in RxJs

Question

I need to return an additional value other than Observable, key string from flatMap from below setup:

this.subscribeInit = this.tradingService.getInProgress()
.pipe(flatMap((s) => {
    this.keys = s.map((tdi) => tdi.key);
    return this.keys;
}),
flatMap((key) =>{
    var res = this.tradingService.getTradingData(key);//returns `Observable<any>`
    return res;
}))
.subscribe(res => {
    console.log('Key is : ' + res.key);
}, 
undefined,
() => this.onComplete());

Something like:

flatMap((key) =>{
    var res = this.tradingService.getTradingData(key);
    return {res,key};
}))

How to achieve this?

Davy · Accepted Answer · 2018-08-02 05:37:38Z

2

Just use map ...

flatMap((key) =>{
    return this.tradingService.getTradingData(key).pipe(map((res) => {
        return {res,key};
    }));
}))

ps: i am a bit confused: isnt flatMap deprecated and replaced my mergeMap, switchMap, ... ?

edit: apparently the problem was caused by using flatMap instead of mergeMap.

edited Aug 2, 2018 at 5:37

answered Aug 1, 2018 at 13:20

Davy

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7 Comments

User3250 Over a year ago

Doesn't work Argument of type '(key: string) => void' is not assignable to parameter of type '(value: string, index: number) => ObservableInput<{}>'.

Davy Over a year ago

what is the returntype of getTradingData?

User3250 Over a year ago

It returns Observable<any>

User3250 Over a year ago

You are right as mentioned here, " flatMap is an alias for mergeMap!" and once I changed from flatMap to mergeMap. It worked. Thanks a ton! You might want to change ur answer to use mergeMap.

Davy Over a year ago

Do you still have the import statement for the flatMap? Could it be that you were importing it from the wrong location?

|

ponury-kostek · Accepted Answer · 2018-08-01 13:07:39Z

0

You can destructur object as function parameters

this.subscribeInit = this.tradingService.getInProgress()
    .pipe(flatMap((s) => {
            this.keys = s.map((tdi) => tdi.key);
            return this.keys;
        }),
        flatMap((key) =>{
            var res = this.tradingService.getTradingData(key);
            return {res,key}; // pack res and key into object
        }))
    .subscribe(({res,key}) => { // destructuring previously returned object
            console.log('Key is : ' + res[key]);
        },
        undefined,
        () => this.onComplete());

const val1 = 'some val1', val2 = 'some val2';
Promise.resolve({val1, val2}).then(({val1, val2}) => {
	console.log(val1);
	console.log(val2);
});

edited Aug 1, 2018 at 13:07

answered Aug 1, 2018 at 13:00

ponury-kostek

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3 Comments

User3250 Over a year ago

Already tried this. Error: Argument of type '(key: string) => { res: Observable<any>; key: string; }' is not assignable to parameter of type '(value: string, index: number) => ObservableInput<{}>'

User3250 Over a year ago

Pls validate it doesn't work, as I mentioned I get the error as in my previous comment.

ponury-kostek Over a year ago

You need to turn off type check for this.

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