I'm looking at this example from Angular's docs for $q but I think this probably applies to promises in general. The example below is copied verbatim from their docs with their comment included:
promiseB = promiseA.then(function(result) {
return result + 1;
});
// promiseB will be resolved immediately after promiseA is resolved and its value
// will be the result of promiseA incremented by 1
I'm not clear how this works. If I can call .then() on the result of the first .then(), chaining them, which I know I can, then promiseB is a promise object, of type Object. It is not a Number. So what do they mean by "its value will be the result of promiseA incremented by 1"?
Am I supposed to access that as promiseB.value or something like that? How can the success callback return a promise AND return "result + 1"? I'm missing something.
promiseA's then function returns a new promise (promiseB) that is immediately resolved after promiseA is resolved, its value is the value of the what is returned from the success function within promiseA.
In this case promiseA is resolved with a value - result and then immediately resolves promiseB with the value of result + 1.
Accessing the value of promiseB is done in the same way we accessed the result of promiseA.
promiseB.then(function(result) {
// here you can use the result of promiseB
});
Edit December 2019: async/await is now standard in JS, which allows an alternative syntax to the approach described above. You can now write:
let result = await functionThatReturnsPromiseA();
result = result + 1;
Now there is no promiseB, because we've unwrapped the result from promiseA using await, and you can work with it directly.
However, await can only be used inside an async function. So to zoom out slightly, the above would have to be contained like so:
async function doSomething() {
let result = await functionThatReturnsPromiseA();
return result + 1;
}
When a promise is resolved/rejected, it will call its success/error handler:
var promiseB = promiseA.then(function(result) {
// do something with result
});
The then method also returns a promise: promiseB, which will be resolved/rejected depending on the return value from the success/error handler from promiseA.
There are three possible values that promiseA's success/error handlers can return that will affect promiseB's outcome:
1. Return nothing --> PromiseB is resolved immediately,
and undefined is passed to the success handler of promiseB
2. Return a value --> PromiseB is resolved immediately,
and the value is passed to the success handler of promiseB
3. Return a promise --> When resolved, promiseB will be resolved.
When rejected, promiseB will be rejected. The value passed to
the promiseB's then handler will be the result of the promise
Armed with this understanding, you can make sense of the following:
promiseB = promiseA.then(function(result) {
return result + 1;
});
The then call returns promiseB immediately. When promiseA is resolved, it will pass the result to promiseA's success handler. Since the return value is promiseA's result + 1, the success handler is returning a value (option 2 above), so promiseB will resolve immediately, and promiseB's success handler will be passed promiseA's result + 1.
pixelbits answer is correct and you should always use .then() to access the value of a promise in production code.
However, there is a way to access the promise's value directly after it has been resolved by using the following unsupported internal node.js binding:
process.binding('util').getPromiseDetails(myPromise)[1]
WARNING: process.binding was never meant to be used outside of nodejs core and the nodejs core team is actively looking to deprecate it
https://github.com/nodejs/node/pull/22004 https://github.com/nodejs/node/issues/22064