Question

我正在本地主机上运行我的小测试站点（尽管目前还没有服务器端交互）。一切正常，直到我重新启动PC。哦

现在，在运行站点时，将引发以下错误：

未捕获的SyntaxError：意外的新令牌

代码如下：

let promise1 = new Promise ((resolve, reject) => {
  resolve('string1');
})

 function starter (){
  promise1.then(new Promise (stringParam1) => {
    //if(stringParam1){
    console.log("stringParam1 is ", stringParam1);
       setTimeout(function(){
        console.log("timeout was executed")
        return stringParam1 + 'string2'
      }, 1000)
/*    }else{
      return 'fail1'
    }*/

  }).then((stringParam2) => {
    console.log("stringParam2 is ", stringParam2)
    return stringParam2 + 'string3'
  }).then((stringParam3) => {
    console.log(stringParam3)
  })
}

<!DOCTYPE html>
<html>
    <head>
        <title>PHP is Awesome!</title>
        <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.3.1.min.js"></script>
        <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.9/angular.min.js"></script>
        <link rel = "stylesheet" type = "text/css" href = "MyFirstPhP.css">
    </head>
        <body>
            <button onclick="starter()">promise</button>
            <p id = "output">testoutput</p>
            <script src="MyFirstPhP.js"></script>
        </body>
</html>

Answer 1

You are missing a parenthesis. You also never resolve a promise in your setTimeout function.

let promise1 = new Promise((resolve, reject) => {
  resolve('string1');
})

function starter() {
  promise1.then(stringParam1 => new Promise((resolve, reject) => {
    console.log("stringParam1 is ", stringParam1);
    setTimeout(function() {
      console.log("timeout was executed")
      resolve(stringParam1 + 'string2')
    }, 1000)
  })).then((stringParam2) => {
    console.log("stringParam2 is ", stringParam2)
    return stringParam2 + 'string3'
  }).then((stringParam3) => {
    console.log(stringParam3)
  })
}

starter();

Async/Await will really make this cleaner. It will reduce nesting and parentheses, and make your code easier to read and reuse.

let promise1 = async() => {
  return 'string1';
}

let promise2 = (param) => new Promise((resolve, reject) => {
  console.log("stringParam1 is ", param);
  setTimeout(() => {
    console.log("timeout was executed")
    resolve(param + 'string2')
  }, 1000)
})

let promise3 = async(param) => {
  return param + 'string3'
}

async function starter() {
  let stringParam1 = await promise1()
  let stringParam2 = await promise2(stringParam1)
  console.log("stringParam2 is ", stringParam2)
  let stringParam3 = await promise3(stringParam2)
  console.log(stringParam3)
}

starter()

Answer 2

There are a few issues, not just syntax related. First in your start function you are modifying the data that is resolved in promise inside of an async operation (setTimeout) so you can't return anything and instead need to call resolve/reject. (Your new Promise() should be returned inside of an anonymous function which is provided your value from promise. Then inside of your starter function you can modify what is returned in the subsequent then methods. Your starter function will now return a promise with the wrapped and modified value which is currently undefined since nothing is returned in your anonymous function as the argument to the last then method of you promise chain.)

const promise = new Promise((resolve, reject) => {
  resolve('string1')
})

function starter() {
  return promise.then(function(stringParam1) {
    return new Promise((resolve, reject) => {
      console.log('stringParam1 is ', stringParam1)

      setTimeout(function() {
        console.log('timeout was executed')
        resolve(stringParam1 + 'string2')
      }, 1000)
    })
  }).then((stringParam2) => {
    console.log('stringParam2 is ', stringParam2)
    return stringParam2 + 'string3'
  }).then((stringParam3) => {
    console.log(stringParam3)
  })
}

starter();

Answer 3

您的代码缺少promise1.then lambda周围的括号。这是固定代码：

let promise1 = new Promise ((resolve, reject) => {
  resolve('string1');
})

 function starter (){
  promise1.then(new Promise ((stringParam1) => {
    //if(stringParam1){
    console.log("stringParam1 is ", stringParam1);
       setTimeout(function(){
        console.log("timeout was executed")
        return stringParam1 + 'string2'
      }, 1000)
/*    }else{
      return 'fail1'
    }*/

  })).then((stringParam2) => {
    console.log("stringParam2 is ", stringParam2)
    return stringParam2 + 'string3'
  }).then((stringParam3) => {
    console.log(stringParam3)
  })
}

重新启动后为什么会出现语法错误？

3 个答案: