In the Promise of Javascript, why can't the error be passed to the next chain `.catch()`?

Question

Following is an example:

var promise = new Promise(function(resolve, reject) {
  throw new Error('test');
});
promise.catch(function(error) {
  console.log(error + ' 1 ');
  return error
}).catch(function(error) {
  console.log(error + ' 2 ');
  return error
})

The result for the codes are:

Error: test 1 

As can be seen, the second catch call doesn't work. Does that mean Promise cannot handle error using the chain syntax of catch? Is there a way to pass the current error to the next catch() call?

Answer 1

Returning the error will not make it hit the next catch. You need to throw the error again:

promise.catch(function(error) {
  console.log(error + ' 1 ');
  throw error
}).catch(function(error) {
  console.log(error + ' 2 ');
  return error
})

-> Error: test 1 
-> Error: test 2 

Answer 2

You have to throw the error again, as opposed to just returning an error object:

var promise = new Promise(function(resolve, reject) {
  throw new Error('test');
});
promise.catch(function(error) {
  console.log(error + ' 1 ');
  throw error
}).catch(function(error) {
  console.log(error + ' 2 ');
  return error
})

js has no internal notion of "this is an error object so throw it" - if there had been a then() clause following your first catch(), operation would have been deferred to that, like so:

var promise = new Promise(function(resolve, reject) {
  throw new Error('test');
});
promise.catch(function(error) {
  console.log(error + ' 1 ');
  return error
}).then(function(error) {
  console.log(error + ' 2 ');
  return error
})

Hope this helps!