create-modify-objs.js

var dfs = {
    result: function(){
        return {
            test: []
        }
    },
    setup: function() {
        var test1 = this.result()
        test1.test.push('a');
        this.modify(test1)
        return test1;
    },
    modify: function(test1) {
        test1.test.push('zzz')
    }
}

var obj1 = dfs.setup()
var obj2 = dfs.setup()

obj1.test.push(1)
obj2.test.push(2)
console.log('DFS1 obj1:',obj1) // DFS1 obj1: Array[a, 1]
console.log('DFS1 obj2:',obj2) // DFS1 obj2: Array[a, 2]

// var dfs2 = {
//     result1: {
//         test1: []
//     },
//     setup: function() {
//         this.result1.test1.push('b');
//         return this.result1;
//     }
// }

// var obj3 = dfs2.setup()
// var obj4 = dfs2.setup()

// obj3.test1.push(1)
// obj4.test1.push(2)
// console.log('DFS2 obj3:',obj3) // DFS2 obj3: Array[b, 1]
// console.log('DFS2 obj4:',obj4) // DFS2 obj3: Array[b, 1, 2]