如何中断Promise的链式调用

Promise的then用起来很方便，通过链式调用可以让代码看起来更清晰。

let p = new Promise((resolve, reject) => {
  resolve('step1');
});

p.then(data => {
  console.log(data);
  return 'step2';
}).then(data => {
  console.log(data);
  return 'step3';
}).then(data => {
  console.log(data);
  return 'step4';
}).catch(reason => {
  console.log(reason);
}).finally(() => {
  console.log('finished.');
});

对应的结果是：

"step1"
"step2"
"step3"
"finished."  /*return 'step4'后面没有继续处理了，所以不会打印*/

可是，如果我们在处理step2的时候，因为条件满足了，后面的步骤不需要执行，这时候就需要去中断后续的调用链。

方法一：通过抛出一个异常来终止

let needBreak = true;
let p = new Promise((resolve, reject) => {
  resolve('step1');
});

p.then(data => {
  console.log(data);
  return 'step2';
}).then(data => {
  console.log(data);
  if (needBreak) {
    throw "we need break";
  }
  return 'step3';
}).then(data => {
  console.log(data);
  return 'step4';
}).catch(reason => {
  console.log('got error:', reason);
}).finally(() => {
  console.log('finished.');
});

这时候的输出就成了这样：

step1
step2
got error: we need break
finished.

方法二：通过reject来中断

let needBreak = true;
let p = new Promise((resolve, reject) => {
  resolve('step1');
});

p.then(data => {
  console.log(data);
  return 'step2';
}).then(data => {
  console.log(data);
  if (needBreak) {
    return Promise.reject('break without exception.');
  }
  return 'step3';
}).then(data => {
  console.log(data);
  return 'step4';
}).catch(reason => {
  console.log(reason);
}).finally(() => {
  console.log('finished.');
});

输出结果：

step1
step2
break without exception.
finished.