CODEWITHSUNDEEP
javascriptnode.jsq
Isaac Dontje Lindell
Isaac Dontje Lindell

Reputation: 3446

How to propagate nested error in Q .then() callback

I feel like this should be easy, but I've been struggling for a while on it.

I have code that looks like this (obviously this is a simplified example).

getPromise(param1, param2)
.then(function (results) {
    // do something that causes an error:
    var err1 = new Error()
    throw err1;
})
.then(function (results) {
    getAnotherPromise(param1)
    .then(function (res) {
        // do something else that might cause an error:
        var err2 = new Error()
        throw err2;
    })
})
.then(function (results) {
   // deal with the results of above code
})
.fail(function (error) {
   // handle error
})

The issue is that err2 never causes the .fail handler to be called. err1 is dealt with as expected, but err2 just disappears. I've tried adding another .fail handler after the .then responsible for generating err2 that just re-throws err2, but that doesn't change anything.

How can I write one error handler that deals with both err1 and err2?

Upvotes: 1

Views: 711

Answers (1)

Leonid Beschastny
Leonid Beschastny

Reputation: 51480

Q can't handle nested promise unless you return it. So, your code should look like this:

getPromise(param1, param2).then(function (results) {
  // do something that causes an error:
  throw new Error('error 1');
}).then(function (results) {
  return getAnotherPromise(param1).then(function (res) {
    // do something else that might cause an error:
    throw new Error('error 2');
  })
}).then(function (results) {
  // deal with the results of above code
}).fail(function (error) {
  // handle error
})

Actually, promise values are propogated as well as errors, so you may write your code in the following way:

getPromise(param1, param2).then(function (results) {
  // do something that causes an error:
  throw new Error('error 1');
}).then(function (results) {
  return getAnotherPromise(param1)
}).then(function (res) {
  // do something else that might cause an error:
  throw new Error('error 2');
}).then(function (results) {
  // deal with the results of above code
}).fail(function (error) {
  // handle error
})

Upvotes: 3

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