Js Map Promise All

As a result, you are returning an array of promises. This returned promise will resolve when all of the input's promises have resolved, or if the input iterable contains no promises. Await promise.all( customers.flatmap(async (customer) => { return customer.items.map(async (item) => { instead of returning an array of array of promises, it will flatten the content to a simple array of promises, which is what promise.all expects.

The first promise in the array will get resolved to the first element of the output array, the second promise will be a second element in the output array and so on. Javascript's map method uses a callback to apply the function over each element of the array, which is fine. Here, promise.all () method is the order of the maintained promises.

Fyi,.map () is just a plain loop.

Here the promise.all () method waits till all the promises resolve. Await promise.all( customers.flatmap(async (customer) => { return customer.items.map(async (item) => { instead of returning an array of array of promises, it will flatten the content to a simple array of promises, which is what promise.all expects. To use array.prototype.map with promise.all in javascript, we should call map with a callback that returns a promise. But recall that async functions always return a promise.