promise-handling.js

const chunk = (arr, n) => {
 if(n<=0)throw new Error('First argument to splitEvery must be a positive integer')
 var result = [],idx = 0;
 while(idx<arr.length)result.push(arr.slice(idx,idx+=n))
 return result
}
const fetchProxy=()=> new Promise((resolve, reject) => {
 const randomWait = 300*Math.random();
 setTimeout(() => resolve(randomWait), randomWait);
});

const reqCount = 50;
const arrayToWork = […Array(reqCount).keys()];
console.log(startTime=Date.now())
Promise.all(arrayToWork.map(fetchProxy)).then(() => console.log({'Parallel ~ 300ms':Date.now()-startTime}));
Promise.resolve(
 arrayToWork.reduce((memo,i)=>memo.then(fetchProxy),Promise.resolve())
).then(()=>console.log({[`Sequential ~ ${reqCount}*300ms`]:Date.now()-startTime}))

const chunkSize = 25; // number that reflects your bandwidth's capacity
Promise.resolve(chunk(arrayToWork,chunkSize).reduce((memo,pieceOfChunk)=>
 memo.then(()=>Promise.all(pieceOfChunk.map(fetchProxy))),Promise.resolve()
)).then(()=>console.log({[`Parallel and Sequential ~ < ${reqCount}/${chunkSize} * 300ms`]:Date.now()-startTime}))