How to use `Array#map` within `Promise.all` in JavaScript

Question

I am trying to create a Promise.all with an array of items. So if I create it like this it works fine

Promise.all([   Query.getStuff(items[0]),   Query.getStuff(items[1]) ]).then(result => console.log(result)) 

If I try to create the Promise.all like this, it doesn't work

Promise.all([     items.map(item => Query.getStuff(item)) ]).then(result => console.log(result)) 

The then block is run before the Query.getStuff(item). What am I missing?

Answer 1

You should be writing

Promise.all(items.map(...)) 

instead of

Promise.all([ items.map(...) ]) 

Array#map returns an array, which means that the way you wrote your code originally, you were actually passing a multidimensional array to Promise.all — as in [ [promise1, promise2, ...] ] — instead of the expected one-dimensional version [promise1, promise2, ...].

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Revised Code:

Promise.all(     items.map(item => Query.getStuff(item)) ).then(result => console.log(result))