Merge/Extend Objects in native JS

mergeObjects.js

/*!
 * Replace some functionality of jQuery's $.extend() method
 * with only native JavaScript. Unlike $.extend(), this
 * function will ONLY merge plain objects (not arrays).
 * https://gist.github.com/Error601/9a181a0b9f414b752c38
 */


function isObject( obj ){
    return Object.prototype.toString.call(obj) === '[object Object]';
}

function mergeObjects( /* [true ,] obj1, obj2 [, ...] */ ){

    var args = arguments,
        arg1 = args[0],
        arg2 = args[1] || {},
        len  = args.length,
        deep = false,
        i, arg, output = {};

    // first argument will be output object
    if ( isObject(arg1) ){
        output = arg1;
    }
    // unless set to true
    else if ( arg1.toString() === "true" ){
        deep = true;
        output = arg2;
    }

    // stop here and return the output object
    // if argument length is 0 or 1, or
    // if we're trying to deep merge a single object
    if ( len <= 1 || (deep && len === 2) ){
        return output;
    }

    function processObject( obj ){
        var prop;
        for ( prop in obj ){
            if ( obj.hasOwnProperty(prop) ){
                if ( deep && isObject(obj[prop]) ){
                    output[prop] = mergeObjects( deep, output[prop], obj[prop] );
                }
                else {
                    output[prop] = obj[prop];
                }
            }
        }
    }

    // loop over additional arguments
    for ( i = 1; i < len; i++ ){
        arg = args[i];
        if ( isObject(arg) ) {
            processObject(arg);
        }
    }

    return output;

}