在 Promise 中，使用 catch 和 then 的第二个参数有什么区别?

问题描述

这两种说法到底有什么区别?

What exactly is the difference between these two statements?

funcThatReturnsAPromise()
  .then(() => { /* success */ })
  .catch(() => { /* fail */ });


funcThatReturnsAPromise()
  .then(() => { /* success */ }, () => { /* fail */ });

推荐答案

除了 .catch(fn) 作为 .then(null, fn) 的快捷方式之外，你的例子的不同之处在于

Besides .catch(fn) being a shortcut for .then(null, fn), the difference in your examples is that

funcThatReturnsAPromise()
  .then(() => { /* success */ })
  .catch(() => { /* fail */ });

// is equivalent to

const p1 = funcThatReturnsAPromise()
const p2 = p1.then(() => { /* success */ })
const p3 = p2.catch(() => { /* 
   executed if p1 is rejected
   executed if p2 is rejected 
*/ })

虽然第二个是

funcThatReturnsAPromise()
  .then(() => { /* success */ }, () => { /* fail */ });

// equivalent to

const p1 = funcThatReturnsAPromise()
const p2 = p1.then(
  () => { /* success */ },
  () => { /*
     executed if p1 is rejected
     (p2 will be actually resolved by the result of this function only when p1 is rejected)
  */ }
);

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