node / javascript () () syntax - how does it work exactly?

Question

Help me understand this code, how output of of (1,2,3) is piped to map( x => x*x) though map( x => x*x) is sequenced 1st in code line and of (1,2,3) is sequenced 2nd

map(x => x*x) (of (1,2,3)).subscribe((value)=> console.log(`value : ${value}`))

same can be written as below, that I understand well, but not above one..

of(1,2,3).pipe(map(x => x*x)).subscribe((value)=> console.log(`value : ${value}`))

FYI, both are correct, and return value 1,4,9

in case you are trying same in editor, include below imports

import {of} from 'rxjs'
import {map} from 'rxjs/operators'

Eternal1 · Accepted Answer · 2020-01-17 08:59:33Z

1

This is actually an example from RxJS documentation, and is explained above:

A Pipeable Operator is essentially a pure function which takes one Observable as input and generates another Observable as output. Subscribing to the output Observable will also subscribe to the input Observable.

So what this means is map(x => x*x) returns a kind of function, which takes one Observable as an argument and returns another Observable. Then we invoke that function with (of(1,2,3)) and get our final result, which is, in fact, equal to of(1,2,3).pipe(map(x => x*x))

edited Jan 17, 2020 at 8:59

answered Jan 17, 2020 at 7:14

Eternal1

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Comments

GLK · Accepted Answer · 2020-02-11 03:56:44Z

Its more of a JavaScript functionality. If you see function is called like foo()() or foo() ('hello') that means foo is returning another function, and parameter in second "()" like 'hello' in foo() ('hello') is passed to function returned by foo().

Example
save below code as sample.js, and execute it using node sample

foo()()                  // return undefined, as empty parameter ( parenthesis ) are passed though expecting one
foo()('hello')           // param 'hello' is passed to bar when it is returned inside foo
foo()('hello','world')   //both hello and world are passed, but only hello is printed, as bar expect only 1 param


function foo() {
    return bar
}


function bar (param) {
    console.log('bar : '+param)
}

Results

bar : undefined 
bar : hello 
bar : hello

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