Use `Function.prototype.bind` to debug `Function.prototype.bind`.

README.md

Bindception

Use Function.prototype.bind to debug Function.prototype.bind.

The Problem

Debugging functions created with Function.prototype.bind is not easy. Calling toString on a bound function gives only 'function () { [native code] }', and the details of the binding are buried inside and unavailable.

With only a reference to a bound function, these questions can't be answered:

• What bound arguments does the function carry?
• What is the this object of the function?
• What is the original function that was bound?

Answering these questions often requires deep knowledge of the entire system, or infinite patience stepping through the debugger.

A Solution

Functions created with this debug version of Function.prototype.bind will carry binding metadata on the target property, which is a hash object with the following keys:

• arguments - An array of bound arguments, if any
• fn - The original function
• thisObj - The bound this object, if any

No changes to existing calls to Function.prototype.bind are necessary, no changes to existing functionality are made, and answering the three questions posed above is now much easier.

debugBind.js

(function () {
  var origBind = Function.prototype.bind;

  Function.prototype.bind = function () {
    var boundFn = origBind.apply(this, arguments);

    boundFn.target = {
      arguments: [].slice.call(arguments, 1),
      fn: this,
      thisObj: arguments[0]
    };

    return boundFn;
  };
}());

example.js

var fn = console.log.bind(console, 'bound');

fn('hello');
//=> "bound hello"

fn.target.arguments;
//=> ['bound']

fn.target.fn === console.log;
//=> true

fn.target.thisObj === console;
//=> true