nerr

Properly constructed base class for error objects

npm install nerr
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nerr

Using custom errors is tricky in JS. Unfortunately, it is not enough to just inherit your class from Error. You need to do really strange things to provide stack trace with your error objects.

nerr fixes this problem by providing ErrorBase class. By inheriting it you are getting correct error implementation with stack trace included.

ErrorBase features:

  • Inherited from Error, all your error objects will be instances of Error
  • Fully Error-compatible, mimics all it's properties, code working with Error will also work with classes inherited from ErrorBase
    • but if Object.defineProperties is not available, you will need to use functions instead of stack and message properties, see below
  • message is getter, not just string property, you can override getMessage() to construct message you need using any of your error object properties

How to use

  • Inherit your error class from ErrorBase
  • Don't forget to call superclass constructor in your constructor
  • Override prototype.name by setting it to your class name
  • Override getMessage() to provide error message

Note, that ErrorBase hasn't message argument that Error has. However, you can provide it in your error class yourself, as well as any other arguments and properties you may need. But be careful with property names. For example, inaccurate overriding of message property (which is a getter) can break functionality.

Usage example

Defining custom error class:

var inherits = require('util').inherits;
var ErrorBase = require('nerr').ErrorBase;

var MyError = function (code) {
    ErrorBase.call(this);
    this.code = code;
};
inherits(MyError, ErrorBase);

MyError.prototype.name = 'MyError';

MyError.prototype.getMessage = function () {
    return 'Error code: ' + this.code;
};

Using it:

try {
    throw new MyError(500);
}
catch(err) {
    // prints error name, message and stack trace
    console.log('Stack trace:\n', err.stack);

    console.log();

    // prints error name and message
    console.log('Error:', err.toString());

    console.log();

    console.log('Error name:', err.name);
    console.log('Error message:', err.message);
}

How it works

ErrorBase constructor calls Error.captureStackTrace(obj, func) which provides an object instance with a stack getter (func is used to appropriately truncate captured stack trace, excluding error construction function).

stack getter returns concatenation of obj.toString() and a captured stack trace. Obtained value is cached, so sequential getter calls will return the same value even if obj.toString() result is changed.

Compatibility

Error.captureStackTrace() is undocumented feature of V8 (at least), so it's behavior can be changed.

If captureStackTrace() is not available, ErrorBase uses new Error() to get stack trace. In this case it cannot exclude error construction part of trace.

If Object.defineProperties is not available, message and stack getters will not be exposed, use getMessage() and getStackTrace functions instead.

License

MIT

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