如何在 for 循环中使用 await/async

Question

Why the code print this way??

await can not be used under loop?

>>>>> START
1

The expect result is '>>>>> START 1 2 3 >>>>> END'

let arr = [1, 2, 3]
async function print() {
  for (let n of arr) {
    await new Promise(
      resolve => {
        setTimeout(() => {
          console.log(n);
          resolve;
        }, 1000)
      }
    );
  }
}

async function main() {
  console.log(">>>>> START");
  await print();
  console.log(">>>>> END");
}

main()

Answer 1

Simple typo, you are not calling the resolve function. You need to actually invoke it, otherwise your promise will never resolve and your code will never continue (note the addition of parentheses after resolve in your print function):

async function print() {
  for (let n of arr) {
    await new Promise(
      resolve => {
        setTimeout(() => {
          console.log(n);
          resolve(); // invoke the function
        }, 1000)
      }
    );
  }
}

Also, each console.log call prints on a new line so the output won't be exactly what you expect, instead it'll be more like:

>>>>> START
1
2
3
>>>>> END