Javascript函数始终返回未定义

Question

I have this vue2 code:

checkUser2() {
    var returnValue;
    axios.get(`api/users/CheckUsername/${this.username}`)
          .then(response => {
              returnValue = response.data.error === 0;
          }, errorCallBack => {
              returnValue = false;
          });
    return returnValue;
}

I call it with:

var a = this.checkUser2();
console.log(a);

and it always returns undefined. What am I doing wrong?

Answer 1

If you want to return the value from this method, then you will have to return the promise from the function and use a callback to capture the result on success.

checkUser2() {
    return axios.get(`api/users/CheckUsername/${this.username}`)
      .then(response => response.data.error === 0, errorCallBack => false);
}

And when you call it:

this.checkUser2().then(value => console.log(value))

Answer 2

我对vue2一无所知，但我确实知道axios在axios工作，因此，由于您在promise之外返回returnValue ，因此您的函数正在返回returnValue的当前值，该值是undefined 。

Answer 3

It is because you are returning the value before the request was made. You need to make a Promise, and when it resolves, return the value.

checkUser2() {
  return new Promise((resolve, reject) => {

    var returnValue;
    axios.get(`api/users/CheckUsername/${this.username}`)
    .then(response => {
      returnValue = response.data.error === 0;
      resolve(returnValue);
    }, errorCallBack => {
      returnValue = false;
      reject(returnValue);
    });
  });
}

Then you just need to call it like this to get the value:

this.checkUser2().then(val => {
 a = val;
})

Hope this helps you.