Bind a function, with a reference to its target

bind.js

/* Raynos' function.bind module, edited to add __target__ property */
/* Original: https://github.com/Raynos/function-bind/blob/master/index.js */

var ERROR_MESSAGE = 'Function.prototype.bind called on incompatible ';
var slice = Array.prototype.slice;
var toStr = Object.prototype.toString;
var funcType = '[object Function]';

module.exports = bind;

function bind(that) {
    var target = this;
    if (typeof target !== 'function' || toStr.call(target) !== funcType) {
        throw new TypeError(ERROR_MESSAGE + target);
    }
    var args = slice.call(arguments, 1);

    var binder = function () {
        if (this instanceof bound) {
            var result = target.apply(
                this,
                args.concat(slice.call(arguments))
            );
            if (Object(result) === result) {
                return result;
            }
            return this;
        } else {
            return target.apply(
                that,
                args.concat(slice.call(arguments))
            );
        }
    };

    var boundLength = Math.max(0, target.length - args.length);
    var boundArgs = [];
    for (var i = 0; i < boundLength; i++) {
        boundArgs.push("$" + i);
    }

    var bound = Function("binder", "return function (" + boundArgs.join(",") + "){return binder.apply(this,arguments)}")(binder);

    if (target.prototype) {
        function Empty() {}
        Empty.prototype = target.prototype;
        bound.prototype = new Empty();
        Empty.prototype = null;
    }

    bound.__target__ = target; // This is the only line I added - Colin

    return bound;
}

index.js

var bind = require('bind');

function plus(x) {
    return this + x;
}

plus.call(1, 2); // > 3

var twoPlus = bind.call(plus, 2);

twoPlus(3) // > 5

twoPlus.__target__; // "function plus(x) { ... }"

twoPlus.__target__.call(3, 4); // 7