区别：回报承诺和回报承诺。

Question

I have such method in my factory

getTogether() {
        let promise = this._$q.all([this._$http.get('/groups'), this._$http.get('/tasks')]);
        return promise
            .then((data) => {
                //local variable for reduce server<->client
                this._localList = data;
                return this._localList;
            });
    };

and I call this method in my controller.

this._boardFactory.getTogether().then((list) => {
            console.log(list)
        });

all works perfectly, however I don't understand one thing.

what's the difference between return promise and return promise.then ?
thanks for your attantion

Answer 1

If you only do return promise in your code, you will return the resolution of the promise object. However, if you do return promise.then you will return the resolution of the promise from the then clause.

then returns a promise, so promise.then will execute promise then execute the then clause returning that promise.

More info on then here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then

Answer 2

I say Promise is object and then() is method on Promise object which reruns another Promise.

Promise.prototype.then(onFulfilled, onRejected) - Appends fulfillment and rejection handlers to the promise, and returns a new promise resolving to the return value of the called handler, or to its original settled value if the promise was not handled (ie if the relevant handler onFulfilled or onRejected is not a function). Below image explains thing :

Read : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

this is how it gose

-Promise - let this is ajax call,
  - then() - now this allows you to work on the output you received from first promise
     - then() - now if you want to work on the output of second promise which return by then(first) 
       - then - another promise allows to wok on ouput of third

so it kind of chaining ...go on ..till you complete it