Javascript：如何从Promise for循环中返回值？

Question

在此示例中，如何返回sum的值？

function slowFunction(par) {
      sum = 0
      for (let i = 0, p = Promise.resolve(); i < 5; i++) {
        p = p.then(_ => new Promise(resolve =>
          setTimeout(function () {
            sum += i ;
            console.log(i);
            resolve();
          }, Math.random() * 1000)
        ));
      }
    }

Answer 1

我不确定您在寻找什么，但这将是兑现承诺的正确方法：

function slowFunction(par) {
    return new Promise(resolve => {
        var sum = 0
        for (let i = 0; i < 5; i++) {
            setTimeout(function () {
                sum += i ;
                resolve(sum);
            }, Math.random() * 1000)
        }
    })   
}

async function main() {
    const i = await slowFunction()
    console.log(i) 
}

main()

不使用asyc / await：

function slowFunction(par) {
    return new Promise(resolve => {
        var sum = 0
        for (let i = 0; i < 5; i++) {
            setTimeout(function () {
                sum += i ;
                resolve(sum);
            }, Math.random() * 1000)
        }
    })   
}

slowFunction().then(value => console.log(value))