await 仅在异步函数中有效 [英] await is only valid in async function

问题描述

我在 lib/helper.js

var myfunction = async function(x,y) {
   ....
   return [variableA, variableB]
}
exports.myfunction = myfunction;

然后我尝试在另一个文件中使用它

and then I tried to use it in another file

 var helper = require('./helper.js');   
 var start = function(a,b){
     ....
     const result = await helper.myfunction('test','test');
 }
 exports.start = start;

我有一个错误

await 仅在异步函数中有效"

"await is only valid in async function"

有什么问题?

推荐答案

错误不是参考 myfunction 而是 start.

The error is not refering to myfunction but to start.

async function start() {
   ....

   const result = await helper.myfunction('test', 'test');
}

<小时>

// My function
const myfunction = async function(x, y) {
  return [
    x,
    y,
  ];
}

// Start function
const start = async function(a, b) {
  const result = await myfunction('test', 'test');
  
  console.log(result);
}

// Call start
start();

我利用这个问题的机会向您提供有关使用 await 的已知反模式的建议，即:return await.

I use the opportunity of this question to advise you about an known anti pattern using await which is : return await.

错误

async function myfunction() {
  console.log('Inside of myfunction');
}

// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later

// useless async here
async function start() {
  // useless await here
  return await myfunction();
}

// Call start
(async() => {
  console.log('before start');

  await start();
  
  console.log('after start');
})();

正确

async function myfunction() {
  console.log('Inside of myfunction');
}

// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for aswell
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later

// Also point that we don't use async keyword on the function because
// we can simply returns the promise returned by myfunction
function start() {
  return myfunction();
}

// Call start
(async() => {
  console.log('before start');

  await start();
  
  console.log('after start');
})();

另外，要知道有一种特殊情况，return await 是正确且重要的:(使用 try/catch)

Also, know that there is a special case where return await is correct and important : (using try/catch)

`return await`有性能问题吗?

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