How to extract data out of a Promise

javascript promise ecmascript-6 bluebird es6-promise
Tobias Mühl picture Tobias Mühl · Apr 28, 2016 · Viewed 42k times · Source

I have a promise that returns data and I want to save that in variables. Is this impossible in JavaScript because of the async nature and do I need to use onResolve as a callback?

Can I somehow use this (e.g. wrap it with async/await):

const { foo, bar } = Promise.then(result => result.data, errorHandler);
// rest of script

instead of this?

Promise.then(result => {
   const { foo, bar } = result.data;
   // rest of script
 }, errorHandler);

Note: Bluebird library is used instead of native implementation, and I can't change from Promise to asnyc/await or Generators.

Answer

silkAdmin picture silkAdmin · Apr 28, 2016

NO you can't get the data synchronously out of a promise like you suggest in your example. The data must be used within a callback function. Alternatively in functional programming style the promise data could be map()ed over.

If your are OK using async/await (you should it's awesome) then you can write code that looks synchronous yet retain the asynchronicity of a promise (see @loganfsmyth comments).

const { foo, bar }  = await iAmAPromise.then(result => result.data);

Overall since you are already using ES6 I assume you are also using a transpiler. In which case you should definitely give async/await a try. Just be sure to weight in the decision that as today they are not yet a ratified specification.

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