如何在函数返回之前等待promise完成

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');

代码输出是:

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

你如何重写代码,输出是:

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

完成后想要发生的任何事情都将传递给您传递给它的箭头函数。

    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三个诺言都解决后才会发生的代码，因为这听起来像您想发生，那么我建议您直接在代码下方创建一个新函数，以包含将要执行的代码喜欢在解决后发生，并通过传递该函数，例如：Promise.all([p1, p2, p3]).then(newFunctionName)。以这种方式将其可视化可能会更容易，至少直到您习惯于思考它的精确工作方式为止。

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地方：

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');
});