javascript exception object format

Question

By default, Node.js throws the following exception when a file is not found.

{ [Error: ENOENT, no such file or directory \'InvalidFile\']
  errno: 34,
  code: \'         


        

Answer 1

I'm answering subquestion #2. The accepted answer, though excellent, seems to omit it.

On Node, the [Error: ...] element can be accessed as .stack on the exception object. It's a string, which you can then parse with regular expressions to get, for instance, the module and line number.

For instance, you could parse the first stack frame like this:

...
} catch (e) {
    var msg, file, line, col;
    [msg,file,col] = e.stack.match(/\((.*):(\d+)\)/);
    if (file) {
        [,file,line] = file.match(/(.*):(\d+)/);
    }
}

Answer 2

Well, it's not entirely code. It's based on JavaScript's literals syntax, but is just a representation of a the object that's generated from util.inspect() (or a similar internal function).

The square brackets mention the type of Error before its message. And, the rest is a list of enumerable properties and their values that were added to it.

To create it yourself:

var error = new Error("ENOENT, no such file or directory 'InvalidFile'");
error.errno = 34;
error.code = 'ENOENT';
error.path = 'InvalidFile';
error.syscall = 'open'

console.log(error);               // uses `util.inspect()`
console.log(util.inspect(error)); // or use it directly

console.log(error.message); // "ENOENT, no such ..."
console.log(Object.prototype.toString.call(error)); // "[object Error]"

And, for a larger sample of the format, try logging some built in modules:

console.log(console);

{ log: [Function],
  info: [Function],
  warn: [Function],
  error: [Function],
  dir: [Function],
  time: [Function],
  timeEnd: [Function],
  trace: [Function],
  assert: [Function],
  Console: [Function: Console] }