Deep Objects Merge (extending)

extend.js

function extend( o, o1, o2) {
    if( !(o instanceof Object) ) o = {};

    copy(o, o1);
    if( o2 )
        copy(o, o2)

    function copy(a,b){
        // copy o2 to o
        for( var key in b )
            if( b.hasOwnProperty(key) ){
                if( isObject(b[key]) ){
                    if( !isObject(a[key]) )
                        a[key] = Object.assign({}, b[key])
                    else
                        copy(a[key], b[key])

                    continue;
                }

                if( isArray(b[key]) ){
                    a[key] = (isArray(a[key]) ? a[key] : []).concat(b[key])
                    continue;
                }

                a[key] = b[key]
            }
    }

    return o
}

function isArray(a){
    return a instanceof Array
}

function isObject(obj) {
    var type = Object.prototype.toString.call(obj).split(' ')[1].slice(0, -1);
    return obj === Object(obj) && type != 'Array' && type != 'Function' && type != 'RegExp' && type != 'HTMLUnknownElement';
}

var TEST = {a:1, b:{a:1, b:2, c:3}}

console.log(   extend({}, TEST , {a:[{foo:2}, 111], b:{a:55,  c:{a:[111]}, d:1, e:2  }})   )