使用Promise将元素推入数组

Question

When I console.log my array it remains empty, so I am failing to push the element into the array with my code. I can do it outside of a function, but do not know what I am doing wrong here with the promises. I am new to promises. Thanks for any tips:

function fetchApiData(num) {
 let arr = []; 

for(let i = 0; i < num; i++) {
  fetch('https://dog.ceo/api/breeds/image/random')
  .then(response => response.json())
  .then(responseJson => console.log(responseJson.message))
  .then(responseJson => arr.push(responseJson.message));
  console.log(arr);
  }
  console.log(arr);  
  // how do I change console.log to push value into an array? 
}

Answer 1

Your issue is the order of resolution. Namely, when your code executes, here is what happens:

0.) Arr = []

1.) Start for loop

2.) Initiate promise

3.) log arr

4.) loop finishes

5.) log arr

6.) At some undefined point in the future, a few milliseconds later, your promises resolve, one at the time, each adding the element to the array.

Try capturing the promises in an array and using

Promise.all(promiseArr).then(()=>{
    console.log(arr)
})

Promise.all will wait till all the promises in the array have finished before executing, so you'll be able to log your completed array.