CODEWITHSUNDEEP
javascriptpromiseq
D.C.
D.C.

Reputation: 15588

How does then() feed its parameters

Trying to wrap my head around Javascript promises. I am experimenting with the Q library. I have some code that outputs what I expect, but I'm not sure how it is working:

'require strict';

var Q = require("q");

function getFirst(){
    var deferred = Q.defer();
    var first = 5;
    console.log("getting first: " + first);
    deferred.resolve(first);
    return deferred.promise;
}

function addSecond(first){
    var deferred = Q.defer();
    var second = 10;
    console.log("adding second: " + second);
    var result = first + second;
    deferred.resolve(result);
    return deferred.promise;
}

function printResult(result) {
    console.log("result is + " + result);
}

getFirst()
.then(addSecond)
.then(printResult);

Running this in Node i get:

getting first: 5
adding second: 10
result is + 15

What I'm confused about is how do addSecond and printResult get their parameters when they are eventually called? How did '5' magically make it into addSecond? What if addSecond needed a whole bunch of parameters?

Thanks for shedding light on this.

Upvotes: 1

Views: 72

Answers (4)

Jaromanda X
Jaromanda X

Reputation: 1

Your addSecond code can be simplified to

function addSecond(first){
    var second = 10;
    console.log("adding second: " + second);
    return first + second;
}

Upvotes: 0

Skarllot
Skarllot

Reputation: 754

When you call a promise it waits until underlying function "resolves", then call next function passing resolved value as parameter (this value is resolved on deferred.resolve(first)). Then whether the new function returns a promise the cycle is repeated, otherwise just call next function passing returned value.

If you need to pass more than one parameter, you can return a array.

Great presentation of promises:

Upvotes: 0

sebkeys
sebkeys

Reputation: 86

Any parameter that you add to deferred.resolve([params]) is sent to the deferred function or DoneCallback (function indicated in .then(function)). It does accept multiple parameters (https://api.jquery.com/deferred.resolve/).

Upvotes: 1

slebetman
slebetman

Reputation: 113866

addSecond gets its parameters from:

deferred.resolve(first);

so the value is 5.

printResult gets its parameters from:

var result = first + second;
deferred.resolve(result);

so the value is first + second.

You can experiment with this. Pass the values you want to pass to your function into .resolve() and see it changing. Basically, .resolve is acting like a pointer to the function you pass to .then().

Upvotes: 1

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