.error

.error([function(any error) rejectedHandler]) -> Promise

Like .catch but instead of catching all types of exceptions, it only catches operational errors.

Note, "errors" mean errors, as in objects that are instanceof Error - not strings, numbers and so on. See a string is not an error.

It is equivalent to the following .catch pattern:

// Assumes OperationalError has been made global
function isOperationalError(e) {
    if (e == null) return false;
    return (e instanceof OperationalError) || (e.isOperational === true);
}

// Now this bit:
.catch(isOperationalError, function(e) {
    // ...
})

// Is equivalent to:

.error(function(e) {
    // ...
});

For example, if a promisified function errbacks the node-style callback with an error, that could be caught with .error. However if the node-style callback throws an error, only .catch would catch that.

In the following example you might want to handle just the SyntaxError from JSON.parse and Filesystem errors from fs but let programmer errors bubble as unhandled rejections:

var fs = Promise.promisifyAll(require("fs"));

fs.readFileAsync("myfile.json").then(JSON.parse).then(function (json) {
    console.log("Successful json")
}).catch(SyntaxError, function (e) {
    console.error("file contains invalid json");
}).error(function (e) {
    console.error("unable to read file, because: ", e.message);
});

Now, because there is no catch-all handler, if you typed console.lag (causes an error you don't expect), you will see:

Possibly unhandled TypeError: Object #<Console> has no method 'lag'
    at application.js:8:13
From previous event:
    at Object.<anonymous> (application.js:7:4)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Function.Module.runMain (module.js:490:10)
    at startup (node.js:121:16)
    at node.js:761:3

( If you don't get the above - you need to enable long stack traces )

And if the file contains invalid JSON:

file contains invalid json

And if the fs module causes an error like file not found:

unable to read file, because:  ENOENT, open 'not_there.txt'