overEach - apply a function over each key of an object, or just get all its keys safely

overEach.js

var pojo = {
  foo: 'bar',
  baz: 'qux'
};

var ownKeys = function () {
  return Object.keys(this).filter(function(k) {
    return this.hasOwnProperty(k);
  }, this);
};

// overEach(fn: (key: String, value: Object, context: Object, tuple: {<String>: <Object>}), [Object]) : Array
// overEach(): [<String>, ...]
var overEach = function (fn, context) {
  var keys = this.ownKeys();
  if (! fn) return keys;
  
  var arity = fn.length;

  return keys.map(function(k) {
    var tuple = {};
    tuple[k] = this[k];
    var args = [k, this[k], this, tuple];
    return fn.apply(context || this, args.slice(0, arity));
  }, this);
};

Object.prototype.overEach = overEach;
pojo.overEach = overEach;

// apply a function
var mojo = 'mojo';
var tuples = pojo.overEach(function (k, v, o, t) {
  console.log(`${this}: ${k} => ${v}`);
  return t;
}, mojo).map(function (o) {
  return o.overEach(function(k, v) {
    console.log(`nested obj: ${k} => ${v}`);
    return [k, v];
  });
});
console.log(tuples);

// just give me the keys
console.log(pojo.overEach());

I can't remember why I wrote this.