Simplified Underscore .Each method

underscoreRemix.js

/* SIMPLIFIED UNDERSCORE .EACH method*/

var _ = function(obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
};



//context not typically neccessary, only needed if you want to pass some object (see example below)
var each = _.each = _.forEach = _.hansGruber = _.ferrisBueller = function(obj, iteration, context) {
    //check to see if array
    if (Array.isArray(obj)){
        //iterate through array
        for (var i = 0; i < obj.length; i++) {
            //call interation callback function for each element in the ojbect
            iteration.call(context, obj[i], i, obj);
        }
    //then it must be a 'legit' object in that sense
    } else {
        //iterate through ojbect to get key
        for (var key in obj)
            iteration.call(context, obj[key], key, obj);
    }
    return obj;
};


//tests
var arr = [1, 2, 4, 3];
_.hansGruber(arr, function(el) {
    console.log(el);
});

var obj = {
    year: 1918,
    newsroom: 'The New York Times',
    reason: 'For its public service in publishing in full so many official reports.'
};

_.ferrisBueller(obj, function(el) {
    console.log(el);
});


var arr = [1, 2, 3];
var myObj = {
    foo: 5,
    addFoo: function(el, i, lst) {
        console.log(el + this.foo)
    }
};

_.hansGruber(arr, myObj.addFoo, myObj);

When you're evaluating whether or not something is an array, try using Array.isArray( obj ). That's will return true or false.

I'd advise you to change iteration to iterator.