Promise catch behavior

问题:

it seems the following works without throwing an error:

var p = new Promise (function (resolve, reject) {
    window.setTimeout(function() {
        reject('ko');
    }, 1000);
});

p.then(function (value) { console.log(value); })
.catch(function () { console.log('catched'); });
// → 'catched'

But this throws an error:

var p = new Promise (function (resolve, reject) {
    window.setTimeout(function() {
        p.catch(function () { console.log('catched'); });
        reject('ko');
    }, 1000);
});

p.then(function (value) { console.log(value); });
// → 'catched'
// Uncaught (in promise) ko

Any wild guesses to why ?

回答1:

The .catch must be directly chained after .then. Even if you write it this way, it will still report uncaught:

var p = new Promise(function(resolve, reject) {
  window.setTimeout(function() {
    //p.catch(function () { console.log('catched'); });
    console.log(p)
    reject('ko');
  }, 1000);
});

p.then(function(value) {
  console.log(value);
});
p.catch(function() {
  console.log('catched');
});

The reason for this is that if you don't chain it like that, the .catch function doesn't receive the return value that gets generated when you call .then