function 没有返回值 React

Question

I have a function where I form the current sum of elements. Getcoin is the method in my Api, and it works correctly if I console.log each element during the foreach cycle. But as the result I got nothing in the return and console.log function

getCurrentCost() {
        let currentCostArr = [];
        let sum = 0;
    
        let existingEntries = JSON.parse(localStorage.getItem("walletData"));
        if (existingEntries == null) existingEntries = [];
    
        existingEntries.forEach(el => {
          this.coincapService
            .getCoin(el.id)
            .then((element) => {
              currentCostArr.push(element.priceUsd * el.amount)
            })
        });
    
        for (let i = 0; i < currentCostArr.length; i++) {
          sum += +currentCostArr[i];
        }
        console.log('cevf c ' + sum)
        console.log('current cost ' + currentCostArr)
    
        return currentCostArr;
      }

getCoin method

 getCoin = async (id) => {
    const coin = await this.getResource(`/assets/${id}`);
    return this._transformCoin(coin);
  }

Answer 1

The problem is that you are not handling correctly the Promise that .getCoin() returns inside the .forEach() 'loop'. It's making multiple calls to getCoin but the code outside the forEach is not awaiting these multiple async calls, so the next lines will execute without awaiting the result of the Promises. This is causing confusion in execution order logic of your code, behaving different than what you are expecting. Actually the function getCurrentCost() is ending before the Promises inside the loop resolves.

One way to solve your problem is replacing the .forEach() with .map() , which will return several Promises that will be awaited will Promise.all() , behaving the way it's expected.

//don't forget to turn the function async, so you can use await keyword.
async getCurrentCost() {

  let sum = 0;  
  let existingEntries = JSON.parse(localStorage.getItem("walletData"));
  if (existingEntries == null) existingEntries = [];
    
  await Promise.all(existingEntries.map(async (el) => {
    const coin = await this.coincapService.getCoin(el.id)
    currentCostArr.push(coin.priceUsd * coin.amount)
  })
  
  return currentCostArr
}

Answer 2

Your forEach loop pushes values in you array asynchronously.

You calculate the sum before you get your array filled up with values.