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promise

作者:互联网

a.then返回的是新的promise,第一个参数是resolved状态的回调函数,第二个参数是rejected状态的回调函数。

b.catch返回的也是新的promise,如果异步发生错误变成rejected状态,就会调用catch。注意:如果前面的then已经对reject做处理(then的第二个参数做了处理)了,catch就不会再处理

举个例子:

  new Promise((resolved, rejected) => {
    throw new Error('promise error');
    rejected();
  }).then(() => {
    console.log('then resolved')
  }, (error) => {
    console.log('then rejected');
    console.log(error);
  }).catch((error) => {
    console.log('catch rejected'); 
console.log(error); }) // then rejected // Error: promise error

//由此可见,then对rejected做了处理后,catch将不再处理

但是,then的回调函数中出现的错误也会被catch捕获,如下:

  new Promise((resolved, rejected) => {
    throw new Error('promise error');
    rejected();
  }).then(() => {
    console.log('then resolved')
  }, (error) => {
    console.log('then rejected');
    console.log(error);
    throw new Error('rejected then error');
  }).catch((error) => {
    console.log('catch rejected'); 
    console.log(error);
  })

// then rejected 
// Error: promise error
// catch rejected
// Error: rejected then error

标签:console,log,rejected,promise,error,catch
来源: https://www.cnblogs.com/superil/p/15720720.html