使用 then/catch 承诺结果

Question

I test ES6 Promise in react APP, but i've not result on my .then :

function addItem (value) {
  return new Promise((resolve, reject) => {
    document.getElementById('todo').innerHTML += `<li>${value}</li>`;
  });
}

addItem(value).then(() => {
  alert('then !');
}).catch((err) => {
  console.log(err)
})

My function is executed, but i've not event on my then.

Do you know where i was wrong ? Thank you ! :)

Answer 1

Promises are used for asynchronous tasks. There's no point in using them to execute synchronous code. In you example, then() is not working because you never resolve the promise. In order to resolve it, you need to call resolve() .

Answer 2

You need to call either reject or resolve in your promise.

function addItem (value) {
  return new Promise((resolve, reject) => {
    document.getElementById('todo').innerHTML += `<li>${value}</li>`;
    resolve();
  });
}

Answer 3

You need to resolve your promise, for example:

function addItem (value) {
  return new Promise((resolve, reject) => {
    document.getElementById('todo').innerHTML += `<li>${value}</li>`;
    resolve(10);
  });
}

addItem(value).then((data) => {
  alert('then !');
  console.log(data);// 10
}).catch((err) => {
  console.log(err)
})

Answer 4

You forgot to call the resolve and reject function in your Promise.

resolve is a function that you call when you're done with your action.

resolve();

The argument that you pass to resolve() is what you get back in the then function.

reject is a function that you call when there's a error with your action. If you don't have anything to reject then you can just leave it out and you won't have to call it.

reject();

the argument that you pass to reject() is what you get back in the catch function.