Question

基本承诺问题：

console.log('Promise START');

function makeFullJSON(time) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, time, [time]);
  })
}

var p1 = makeFullJSON(1000);
var p2 = makeFullJSON(500);
var p3 = makeFullJSON(750);

p1.then(array => {
  console.log('Promise 1 complete', array);
});

p2.then(array => {
  console.log('Promise 2 complete', array);
});

p3.then(array => {
  console.log('Promise 3 complete', array);
});

Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
  console.log('Array of resolved values:', arrayOfAllResolvedValues);
});

console.log('Promise END');

代码输出为：

Promise START
Promise END
Promise 2 complete [ 500 ]
Promise 3 complete [ 750 ]
Promise 1 complete [ 1000 ]
Array of resolved values: [ [ 1000 ], [ 500 ], [ 750 ] ]

如何重写代码，输出为：

Promise START
Promise 2 complete [ 500 ]
Promise 3 complete [ 750 ]
Promise 1 complete [ 1000 ]
Array of resolved values: [ [ 1000 ], [ 500 ], [ 750 ] ]
Promise END

Answer 1

完成后您想要发生的任何事情都会转移到您传递给的箭头功能。

    console.log('Promise START');

    function makeFullJSON(time) {
       return new Promise((resolve, reject) => {
       setTimeout(resolve, time, [time]); 
    })}

    var p1 = makeFullJSON(1000);
    var p2 = makeFullJSON(500);
    var p3 = makeFullJSON(750);

    p1.then(array => {
        console.log('Promise 1 complete', array);});

    p2.then(array => {
        console.log('Promise 2 complete', array););

    p3.then(array => {
        console.log('Promise 3 complete', array);});

    Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
        console.log('Array of resolved values:', arrayOfAllResolvedValues);

        console.log('Promise END');
    });

为了放弃立即执行程序，并开始编写仅在所有3个promises解决之后才会发生的代码，因为它听起来你想要发生，那么我建议在代码的正下方创建一个新函数，以包含代码您希望在解决后发生这种情况，并将该函数传递给：Promise.all([p1, p2, p3]).then(newFunctionName)。您可能更容易以这种方式对其进行可视化，至少在您习惯于思考它是如何正常工作之前。

Answer 2

首先修复语法错误。然后将console.log移动到整个过程结束的位置：

console.log('Promise START');

function makeFullJSON(time) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, time, [time]);
  })}

var p1 = makeFullJSON(1000);
var p2 = makeFullJSON(500);
var p3 = makeFullJSON(750);

p1.then(array => {
  console.log('Promise 1 complete', array);});

p2.then(array => {
  console.log('Promise 2 complete', array);});  // fixed syntax error here

p3.then(array => {
  console.log('Promise 3 complete', array);});

Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
  console.log('Array of resolved values:', arrayOfAllResolvedValues);
  console.log('Promise END');
});

Answer 3

如果放置console.log('Promise END')的明显答案没有浮动你的船，为什么不呢？

//didnt change anything at all above here
Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
    console.log('Array of resolved values:', arrayOfAllResolvedValues);
}).then(() => {
    console.log('Promise END');
});

如果要在完成所有承诺后对操作进行排序，则必须按照打印数组值时的then()顺序进行排序

Answer 4

使用async / await：

(async () => {
  console.log('Promise START');

  function makeFullJSON(time) {
    return new Promise((resolve, reject) => {
    setTimeout(resolve, time, [time]); 
  })}

  var p1 = makeFullJSON(1000);
  var p2 = makeFullJSON(500);
  var p3 = makeFullJSON(750);

  p1.then(array => {
    console.log('Promise 1 complete', array);});

  p2.then(array => {
    console.log('Promise 2 complete', array);});

  p3.then(array => {
    console.log('Promise 3 complete', array);});

  console.log('Array of resolved values:', await Promise.all([p1, p2, p3]));

  console.log('Promise END');
})();

如何在函数返回之前等待promise完成

4 个答案: