Different behavior of async functions when assigning temporary to variable

Question

Why a different result in the following cases? The first example works correctly, returns an array of three elements [\"qwe\", \"rty\", \"asd\"]. Second example

Answer 1

Concurrency. Or more precisely: A non atomic modification of values.

First of all, the values.concat(...) get evaluated, at that time values is an empty array. Then all the functions await. Then, all the values = get run, concatenating the awaited element to the empty array, and assigning those arrays with one value to values. The last resolved value wins.

To fix:

 await Promise.all(
  keys.map(
    async key => {
       const el = await this.getValue(key); // async operation
      values = values.concat(el); // atomic update
    }
  )
);