函数之间错误的 promise.all() 用法？

Question

I made this dummy code to have a better understanding on how promises work, mimicking a more complex software that I have to "promisify". In the attached code, I wanted the events to fire and be logged in the following order:

• the "before" string
• the list of "... detected"
• the list of "done with ..."
• the "after" string

But as you can see if you run it, the "after" string is printed between steps 2 and 3. Clearly, I must be doing something wrong in handling async logic. Thanks for your help!

 const obj = { "rows": [{ "type": "A", "value": 0 }, { "type": "B", "value": 0 }, { "type": "C", "value": 0 }] } let promises = []; function delay(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } const alter_value = async(row, to_add, time) => { await delay(time); row.value = row.value + to_add; console.log("done with " + row.type); return true; } const two = async() => { obj.rows.forEach(async(row) => { switch (row.type) { case "A": console.log("A detected"); promises.push(alter_value(row, 1, 1000)) promises.push(alter_value(row, 2, 1800)) break; case "B": console.log("B detected"); promises.push(alter_value(row, 5, 1400)) break; case "C": console.log("C detected"); promises.push(alter_value(row, 200, 2400)) break; } }); return promises; } const one = async() => { console.log("before"); Promise.all(two()).then(console.log("after")); } one();

Answer 1

I see at least two issues with your code, explaining the result you're getting:

1. Your two function should not be async . async functions return an implicit Promise . Here you just want to return an array of Promise s that you already construct yourself, so a normal function is what you need.
2. .then(console.log("after")) will execute the console.log right away: then() expects a function to execute later, so you have to change it to .then(() => console.log("after")) .

This becomes:

 const obj = { "rows": [{ "type": "A", "value": 0 }, { "type": "B", "value": 0 }, { "type": "C", "value": 0 }] }; function delay(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } const alter_value = async (row, to_add, time) => { await delay(time); row.value = row.value + to_add; console.log("done with " + row.type); return true; }; const two = () => { const promises = []; obj.rows.forEach(async(row) => { switch (row.type) { case "A": console.log("A detected"); promises.push(alter_value(row, 1, 1000)); promises.push(alter_value(row, 2, 1800)); break; case "B": console.log("B detected"); promises.push(alter_value(row, 5, 1400)); break; case "C": console.log("C detected"); promises.push(alter_value(row, 200, 2400)); break; } }); return promises; }; const one = async () => { console.log('before'); Promise.all(two()).then(() => console.log('after')); }; one();

Note that, as an alternative to .then() , you can simply use await on Promise.all as well, to make your code more consistent:

await Promise.all(two());
console.log('after');

Answer 2

It looks like the OP requests synchronous behavior.

Demo Outline

• function modVal(i, value, time) parameters are derived from the async function syncro() parameter: const sync an array of objects. Each object contains the index of obj.rows , the value at that index obj.rows[i].value , and the time for the timeout in the Promise of modVal() .

• Parameters and Arguments: const sync = [{r: 0, v: 1, t: 1000}, ...];
  seq.r: obj.rows[ number ]
  seq.v: obj.rows[seq.r].value += number
  seq.t: ...resolve(obj.rows[i].value += value), number );

• sync array is iterated by a for...of loop. On each iteration await modVal() is called synchronously.

 let obj = { "rows": [{ "type": "A", "value": 0 }, { "type": "B", "value": 0 }, { "type": "C", "value": 0 }] } const sync = [{ r: 0, v: 1, t: 1000 }, { r: 0, v: 2, t: 1800 }, { r: 1, v: 5, t: 1400 }, { r: 2, v: 200, t: 2400 }]; const syncro = async(sync) => { const modVal = (i, value, time) => { return new Promise(resolve => { setTimeout(() => resolve(obj.rows[i].value += value), time); }); } for (let seq of sync) { await modVal(seq.r, seq.v, seq.t); console.log(JSON.stringify(obj.rows)); } } syncro(sync);

 .as-console-row.as-console-row::after { content:''; padding:0; margin:0; border:0; width:0; }