How to reduce cortisol: It's actually the JavaScript

It's actually the JavaScript array reduce function rather than being something specific to TypeScript. As described in the docs: Apply a function against an accumulator and each value of the array (from left- to -right) as to reduce it to a single value. The problem is that your accumulator values are promises - they're return values of async function s. To get sequential evaluation (and all but the last iteration to be awaited at all), you need to use const data = await array. reduce (async (accumP, current, index) => { const accum = await accumP; … }, Promise.resolve(initialValue)); That said, for async / await I would in general recommend to use plain loops instead of array iteration methods, they're more performant and often simpler. As far as I understand, the reduce function takes a list l and a function f. Then, it calls the function f on first two elements of the list and then repeatedly calls the function f with the next l... What scenarios would warrant the use of the "Map and Reduce " algorithm? Is there a .NET implementation of this algorithm?