# Error handling

As we saw earlier, `then` takes two arguments, one for success, one for failure (or fulfill and reject, in promises-speak):

```javascript
get('story.json')
.then(function(response) {
    console.log("Success!", response);
}, function(error) {
    console.log("Failed!", error);
});
```

## .fail, .catch

Some librairies implement the method `.fail` (or `.catch`) which is equivalent to `.then(undefined, func)`, for example:

```javascript
get('story.json')
.then(function(response) {
    return JSON.parse(response);
})
.then(function(response) {
    console.log("Success!", response);
})
.fail(function(error) {
    console.log("Failed!", error);
});
```

## Rejection order

The following codes are not equivalent:

```javascript
get('story.json')
.then(function(response) {
  console.log("Success!", response);
}, function(error) {
  console.log("Failed!", error);
});
```

and

```javascript
get('story.json')
.then(function(response) {
  console.log("Success!", response);
})
.fail(function(error) {
  console.log("Failed!", error);
});
```

The difference is subtle, but extremely useful. Promise rejections skip forward to the next `then` with a rejection callback (or `fail`, since it's equivalent). With `then(func1, func2)`, `func1` or `func2` will be called, never both. But with `then(func1).fail(func2)`, both will be called if `func1` rejects, as they're separate steps in the chain.

## Javascript Exceptions

Rejections happen when a promise is explicitly rejected, but also implicitly if an error is thrown in the constructor callback. Let's take a look at the previous example:

```javascript
// Step1: get the json content
get('story.json')

// Step2: parse the json content
.then(function(response) {
    return JSON.parse(response);
})

// Step3: show the parsed results
.then(function(response) {
    console.log("Success!", response);
})

// Handle errors
.fail(function(error) {
    console.log("Failed!", error);
});
```

The Javascript method [JSON.parse](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse) can throw an exception (`SyntaxError`) if the string is not valid JSON. This exception (if occurs) will be handled by the `.fail`.

Basically, the `.then(func)` is encapsulating the call to `func` with a `try { func(resolved) } catch (e) { // return rejected promise }`.