在Promise中,使用catch和第二个参数之间的区别是什么？

Question

这两个陈述之间究竟有什么区别？

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


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

Answer 1

除了.catch(fn)作为捷径之外.then(null, fn),你的例子中的不同之处在于

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 
*/ })

而第二个是

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)
  */ }
);

Answer 2

.catch(foo) 等于 .then(undefined, foo)

但是您的代码示例之间存在差异：

funcThatReturnsAPromise()
  .then(() => { /* success case of funcThatReturnsAPromise */ })
  .catch(() => { /* both fail case of funcThatReturnsAPromise 
                     and fail case of "then" function */ });


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