gistfile1.js

// myFunc({ a: 1, b: 2, c: 3 })
function myFunc(options) {
  var default_options = {
    a: 1,
    b: 2,
    c: 3
  }

  options = options || {}

  for (var key in options) {
    if (options.hasOwnProperty(key)) {
      default_options[key] = options[key];
    }
  }
  return default_options;
}

// myFunc({ a: 1, b: 2, c: 3 })
function myFunc(options) {
  var default_options = {
    a: 1,
    b: 2,
    c: 3
  }

  return _.extend(default_options, options || {});
}

// myFunc(a = 1, b = 2, c = {})
function myFunc(a, b, c) {
  var default_args = [1, 2, {}],
    args = Array.prototype.slice.apply(arguments).concat(default_args.slice(arguments.length));

  return args;
}

Function.prototype.default_args = function() {
  var __method__ = this, __default_args__ = Array.prototype.slice.apply(arguments);

  return function() {
    return __method__.apply(this, Array.prototype.slice.apply(arguments).concat(__default_args__.slice(arguments.length)));
  }
}

var myFunc = function(a, b, c) { console.log(a, b, c); }.default_args(1, 2, {});

// If you don't want to extend the Function prototype
Function.default_args = function(__method__) {
  var __default_args__ = Array.prototype.slice.call(arguments, 1);

  return function() {
    return __method__.apply(this, Array.prototype.slice.apply(arguments).concat(__default_args__.slice(arguments.length)));
  }
}

var myFunc = Function.default_args(function(a, b, c) {
  console.log(a, b, c);
}, 1, 2, {});