JavaScript在迭代数组时等待后端响应

Question

我正在使用javascript，我希望在迭代下一项之前等待其他功能响应。

以下是所需行为：

这是我的代码：

let lista = [1,2,3,4]
console.log('Iteracion de la lista')
async function procesarLista(array){
    for(const item of array){
        console.log('-->START indice: ' + item)
        //Simulate delay (for each iteration) of backend response
        setTimeout(function(){
            console.log('....waiting.... for : ' + item );
        }, 2500);
        console.log('-->FINISH indice: ' + item)
    }
    console.log('Done');
 }
 //Execute:
 procesarLista(lista);

这是错误的结果：

Answer 1

在await循环内的每次迭代中尝试for一个Promise：

let lista = [1, 2, 3, 4]
console.log('Iteracion de la lista')
async function procesarLista(array) {
  for (const item of array) {
    await new Promise((resolve) => {
      console.log('-->START indice: ' + item)
      //Simulate delay (for each iteration) of backend response
      setTimeout(function() {
        console.log('....waiting.... for : ' + item);
        resolve();
      }, 500);
    });
    console.log('-->FINISH indice: ' + item)
  }
  console.log('Done');
}
//Execute:
procesarLista(lista);

for..of需要regenerator-runtime。如果你没有，那么你可以改用reduce，但你必须await在循环和 await内的最后一次迭代的解析记录Done之前的最终承诺：

let lista = [1, 2, 3, 4]
console.log('Iteracion de la lista')
async function procesarLista(array) {
  await new Promise ((outerResolve) => {
    array.reduce(async (lastPromise, item) => {
      await lastPromise;
      await new Promise((resolve) => {
        console.log('-->START indice: ' + item)
        //Simulate delay (for each iteration) of backend response
        setTimeout(function() {
          console.log('....waiting.... for : ' + item);
          resolve();
        }, 500);
      });
      console.log('-->FINISH indice: ' + item)
    }, Promise.resolve());
  });
  console.log('Done');
}
procesarLista(lista);

Answer 2

也许这对你有用

lista = [1, 2, 3, 4];

async function procesarItem(array, id) {
  if (id >= array.length) {
    console.log('Done');
    return;
  }
  console.log('-->START indice: ' + id);
  setTimeout(function () {
    console.log('-->FINISH indice: ' + id)
    procesarItem(array, id + 1);
  }, 2500);
  console.log('....waiting.... for : ' + id);
}

//Execute:
procesarItem(lista, 0);

Answer 3

你也可以使用回调：

let lista = [1,2,3,4]
console.log('Iteracion de la lista')
function procesarLista(array , callback){
  let isProcessDone = false;
  for(const item of array){

    //Simulate delay (for each iteration) of backend response
    setTimeout(function(){
            console.log('-->START indice: ' + item)
        console.log('....waiting.... for : ' + item );
        console.log('-->FINISH indice: ' + item)
        callback('Done')
    }, 2500);

  }
}
//Execute:
procesarLista(lista , function(result){
   // Do Something after each job is done
});

这将是输出：

Iteracion de la lista
-->START indice: 1
 ....waiting.... for : 1
-->FINISH indice: 1
-->START indice: 2
 ....waiting.... for : 2
-->FINISH indice: 2
-->START indice: 3
 ....waiting.... for : 3
-->FINISH indice: 3
-->START indice: 4
 ....waiting.... for : 4
-->FINISH indice: 4

这是一个例子：

https://jsfiddle.net/op98bpka/1/