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Matoy
Matoy

Reputation: 1818

javascript chaining promises clarification

I'm trying to figure the concept of promises in javascript.

I've looked at this code:

new Promise(function(res,rej) {
    res("aaa");
})
.then(function(result) {
    console.log(result);
    return "bbb";
})
.then(function(result) {
    console.log(result);
    return "ccc";
})
.then(function(result) {
    console.log(result);
});

it prints:

 aaa

 bbb

 ccc

to the console log.

a few questions:

  1. is the then() method 1'st parameter is a function that will be run as the resolve() method?

  2. is the then() method also returns a value which is a promise and this promise is the same promise as the one it is linked to (its parent) only the value of its resolve() method is the value return from the resolve() method inside the then()?

  3. is this promise:

    var myPromise = new Promise(function(res,rej) {
    res("aaa");
})
.then(function(result) {
    console.log(result);
    return "bbb";
})

is equivalents to this promise below?

var myPromise = new Promise(function(res,rej) {
    res("bbb");
})

in addition, what is happening when the then() accepts a promise?

like in this example?

var firstMethod = function() {
   var promise = new Promise(function(resolve, reject){
      setTimeout(function() {
         console.log('first method completed');
         resolve({data: '123'});
      }, 2000);
   });
   return promise;
};


var secondMethod = function(someStuff) {
   var promise = new Promise(function(resolve, reject){
      setTimeout(function() {
         console.log('second method completed');
         resolve({newData: someStuff.data + ' some more data'});
      }, 2000);
   });
   return promise;
};

var thirdMethod = function(someStuff) {
   var promise = new Promise(function(resolve, reject){
      setTimeout(function() {
         console.log('third method completed');
         resolve({result: someStuff.newData});
      }, 3000);
   });
   return promise;
};

firstMethod()
   .then(secondMethod)
   .then(thirdMethod);

Upvotes: 0

Views: 475

Answers (1)

Amir Popovich
Amir Popovich

Reputation: 29846

  1. The then method's first param is the resolve function and the second param is the rejection function.

var resolvedPromise = new Promise(function(res,rej){ res({data: 7}) });
var rejectedPromise = new Promise(function(res,rej){ rej('error!!!!') });
resolvedPromise.then(function(res){ console.log('resolved:' + JSON.stringify(res)); }, function (err){ console.log('rejected:' + err); });
rejectedPromise.then(function(res){ console.log('resolved:' + JSON.stringify(res)); }, function (err){ console.log('rejected:' + err); });

  1. The then method returns a promise based on the return type of the previous promise.

var promise = new Promise(function(res,rej){ res({data: 7}) });
promise.
then(function(res){ console.log(res); return res.data; }).
then(function(res){ console.log(res); return res + 1; }).
then(function(res){ console.log(res);});

  1. No. The first code will log "aaa" and return a promise with "bbb" while the second will return "bbb" without logging "aaa";

Upvotes: 3

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