javascriptAsync race conditionHard

Reducer outputting invalid strings

Time: 0:00

What it should do

sumAsync(nums) should add up values fetched asynchronously, so sumAsync([1, 2, 3]) returns 6.

What goes wrong

sumAsync([1, 2, 3]) returns the string "[object Promise]3" instead of 6.

Hints

This is an async and race conditions bug

A missing await, results in the wrong order, and updates that overwrite each other under load.