如何在 node.js 中使用 .then()? [英] How to use .then() in node.js?

问题描述

我是 node.js 的完全初学者.我刚刚读到我们可以使用 .then() 函数以特定顺序执行多个函数.我打算这样写代码:

I'm a complete beginner in node.js. I've just read we can use .then() function for executing several functions in particular order. I was going to write the code this way:

function one(){
  console.log("one")
}
function two(){
  console.log("two")
}
function three(){
  console.log("three")
}
one().then(two()).then(three())

但我收到此错误:

TypeError: Cannot read property 'then' of undefined
at Object.<anonymous> (C:\chat\test.js:10:6)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)
at run (bootstrap_node.js:389:7)
at startup (bootstrap_node.js:149:9)
at bootstrap_node.js:502:3

推荐答案

.then 是 Promise 上存在的一个方法，是一种代码同步机制.您的代码不是异步的，因此您不需要使用承诺.你可以打电话

.then is a method that exists on Promises and is a mechanism for code synchronization. Your code is not asynchronous, so you wouldn't need to use promises. You can just call

one();
two();
three();

如果您的代码执行异步操作，那么您可以使用 promise 和 .then.异步操作包括读/写文件、http 请求、计时器等.

If your code does something asynchronous, then you can use promises and .then. Asynchronous operations are things like reading/writing files, http requests, timers, and many more.

举个例子，我们可以使用内置的Promise来创建我们自己的异步操作:

Just as an example, we can use the built in Promise to create our own asynchronous operations:

我不建议您正常执行此操作.我们只是以它为例.在大多数情况下，您可以调用已经为您返回承诺的函数.

function one() {
  return new Promise(resolve => {
    console.log("one");
    resolve();
  });
}

function two() {
  return new Promise(resolve => {
    console.log("two");
    resolve();
  });
}

function three(){
   console.log("three")
}

one().then(() => two()).then(() => three());

还要注意，当你使用.then时，你需要传递一个回调.two() 立即调用 two 函数，所以它和 () => 不一样.二().

Also note that when you use .then, you need to pass a callback. two() calls the two function immediately, so it's not the same as () => two().

接下来，您通常可以使用 async/await 而不是 .then 我认为这使您的代码在大多数情况下更容易推理.

Next, you can often use async/await instead of .then which I think makes your code easier to reason about in most cases.

async function run() {
  await one();
  await two();
  three();
}
run();

这与重写为使用 await 而不是 .then 的第二个示例相同.您可以将 await 之后的所有内容视为链接到 await 之后的表达式的 .then 内部.

This is the same as the second example rewritten to use await instead of .then. You can think of everything after await as being inside of a .then chained to the expression after await.

最后，您应该通过将 .catch 链接到 Promise 或在 中使用普通的 try/catch 来处理错误异步函数.

Finally, you should handle errors by either chaining .catch to the promises or using the normal try/catch inside of async functions.

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