How to access the value of a promise?

The Solution to How to access the value of a promise? is


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;
}

~ Answered on 2015-04-08 13:49:06


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