等待仅在异步功能中有效

Z时代
2024-01-10
分类：问答

我写了这段代码 lib/helper.js

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

然后我尝试在另一个文件中使用它

 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

“等待仅在异步功能中有效”

有什么问题

回答：

错误不是指myfunction而是start。

async function start() {
   ....
   const result = await helper.myfunction('test', 'test');
}

// 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来告诉你一个已知的反模式的使用await方法：return await。

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');
})();

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');
})();