javascript - 如何从完成的循环中返回数组

Question

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Answer 1

尝试跟随

function foo(items) {
 var result = []
 // Assume under forEach statement take 2 sec per loop
 items.forEach(function(item) {
    // Create a promise for each async function
    var promise = new Promise(function(resolve, reject) {
       setTimeout(function() {
           resolve(item + 2); // resolve the async function
       }, 2000);
     });
     result.push(promise); // push the promise into array
  }); 
  // Create one promise for all the promises and return
  return Promise.all(result) 
}
// Now, foo function returns a promise and then is called once the promise is resolved which contains the data
foo([1,2,3,4]).then((response) => console.log(response)).then(() => console.log("Done"));

因为 setTimeout 是一个 ansync 函数，即它是乱序执行的，因此，正如预期的那样，输出是一个空数组(当返回数组时，没有执行 setTimeout 函数的回调)。您可以使用 Promises用于使用异步函数。