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Is this a good way to clone an object in ES6?

Googling for "javascript clone object" brings some really weird results, some of them are hopelessly outdated and some are just too complex, isn't it as easy as just:
let clone = {...original};
Is there anything wrong with this?

This is good for shallow cloning. The object spread is a standard part of ECMAScript 2018.
For deep cloning you'll need a different solution.
const clone = {...original} to shallow clone
const newobj = {...original, prop: newOne} to immutably add another prop to the original and store as a new object.

EDIT: When this answer was posted, {...obj} syntax was not available in most browsers. Nowadays, you should be fine using it (unless you need to support IE 11).
Use Object.assign.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
var obj = { a: 1 };
var copy = Object.assign({}, obj);
console.log(copy); // { a: 1 }
However, this won't make a deep clone. There is no native way of deep cloning as of yet.
EDIT: As #Mike 'Pomax' Kamermans mentioned in the comments, you can deep clone simple objects (ie. no prototypes, functions or circular references) using JSON.parse(JSON.stringify(input))

If the methods you used isn't working well with objects involving data types like Date, try this
Import _
import * as _ from 'lodash';
Deep clone object
myObjCopy = _.cloneDeep(myObj);

You can do it like this as well,
let copiedData = JSON.parse(JSON.stringify(data));

if you don't want to use json.parse(json.stringify(object)) you could create recursively key-value copies:
function copy(item){
let result = null;
if(!item) return result;
if(Array.isArray(item)){
result = [];
item.forEach(element=>{
result.push(copy(element));
});
}
else if(item instanceof Object && !(item instanceof Function)){
result = {};
for(let key in item){
if(key){
result[key] = copy(item[key]);
}
}
}
return result || item;
}
But the best way is to create a class that can return a clone of it self
class MyClass{
data = null;
constructor(values){ this.data = values }
toString(){ console.log("MyClass: "+this.data.toString(;) }
remove(id){ this.data = data.filter(d=>d.id!==id) }
clone(){ return new MyClass(this.data) }
}

Following on from the answer by #marcel I found some functions were still missing on the cloned object. e.g.
function MyObject() {
var methodAValue = null,
methodBValue = null
Object.defineProperty(this, "methodA", {
get: function() { return methodAValue; },
set: function(value) {
methodAValue = value || {};
},
enumerable: true
});
Object.defineProperty(this, "methodB", {
get: function() { return methodAValue; },
set: function(value) {
methodAValue = value || {};
}
});
}
where on MyObject I could clone methodA but methodB was excluded. This occurred because it is missing
enumerable: true
which meant it did not show up in
for(let key in item)
Instead I switched over to
Object.getOwnPropertyNames(item).forEach((key) => {
....
});
which will include non-enumerable keys.
I also found that the prototype (proto) was not cloned. For that I ended up using
if (obj.__proto__) {
copy.__proto__ = Object.assign(Object.create(Object.getPrototypeOf(obj)), obj);
}
PS: Frustrating that I could not find a built in function to do this.

structured Clone
you can Used this method
function Copy_Object(obj) { return structuredClone(obj); }

We can do that with two way:
1- First create a new object and replicate the structure of the existing one by iterating
over its properties and copying them on the primitive level.
let user = {
name: "John",
age: 30
};
let clone = {}; // the new empty object
// let's copy all user properties into it
for (let key in user) {
clone[key] = user[key];
}
// now clone is a fully independant clone
clone.name = "Pete"; // changed the data in it
alert( user.name ); // still John in the original object
2- Second we can use the method Object.assign for that
let user = { name: "John" };
let permissions1 = { canView: true };
let permissions2 = { canEdit: true };
// copies all properties from permissions1 and permissions2 into user
Object.assign(user, permissions1, permissions2);
-Another example
let user = {
name: "John",
age: 30
};
let clone = Object.assign({}, user);
It copies all properties of user into the empty object and returns it. Actually, the same as the loop, but shorter.
But Object.assign() not create a deep clone
let user = {
name: "John",
sizes: {
height: 182,
width: 50
}
};
let clone = Object.assign({}, user);
alert( user.sizes === clone.sizes ); // true, same object
// user and clone share sizes
user.sizes.width++; // change a property from one place
alert(clone.sizes.width); // 51, see the result from the other one
To fix that, we should use the cloning loop that examines each value of user[key] and, if it’s an object, then replicate its structure as well. That is called a “deep cloning”.
There’s a standard algorithm for deep cloning that handles the case above and more complex cases, called the Structured cloning algorithm.
In order not to reinvent the wheel, we can use a working implementation of it from the JavaScript library lodash the method is called _.cloneDeep(obj).

I found a solution which seems to copy functions as well, correct me if this example is an error.
Attention I have not tested this method with more complex object cases, which, for example, would include methods with this for reference
Take for example the price of a breakfast, I have this price available globally but I would like to adjust it individually for a hotel room
// make an object for a booking option
var opt_resa = { breakfast_val: 900 }
// i define a function for opt_resa :
opt_resa.func = function(){ alert('i am a function'); }
// copy object in modif.opt_resa :
var modif = { opt_resa : {} }
for ( var v in opt_resa ){
modif.opt_resa[v] = opt_resa[v];
}
// test
modif.opt_resa.breakfast_val = 1500;
// old value
console.log( opt_resa.breakfast_val );
// output : 900
// modified value
console.log( modif.opt_resa.breakfast_val );
// output : 1500
// function copied
modif.opt_resa.func();
// this function works

All the methods above do not handle deep cloning of objects where it is nested to n levels. I did not check its performance over others but it is short and simple.
The first example below shows object cloning using Object.assign which clones just till first level.
var person = {
name:'saksham',
age:22,
skills: {
lang:'javascript',
experience:5
}
}
newPerson = Object.assign({},person);
newPerson.skills.lang = 'angular';
console.log(newPerson.skills.lang); //logs Angular
Using the below approach deep clones object
var person = {
name:'saksham',
age:22,
skills: {
lang:'javascript',
experience:5
}
}
anotherNewPerson = JSON.parse(JSON.stringify(person));
anotherNewPerson.skills.lang = 'angular';
console.log(person.skills.lang); //logs javascript

Javascript Object Assignment Infinite recursion

I have an issue that I am struggling to grasp. Any help would be greatly appreciated.
I have an Object, and I assign the current object state to a property on the current object.
example below:
var product = {
ropeType: 'blah',
ropePrice: 'blah',
ropeSections: {
name: 'blaah',
price: 'blaah'
},
memory: false
}
product.memory = product;
Now when I look at the product object within the console I get a inifinite recursion of Product.memory.Product.memory.Product....
screenshot below:
I know its something to do with that an object references itself, but I cannot seem to grasp the concept. Could someone explain?
The reason I am trying to do something like this is to save in local storage the current state of the object.
I hope I have made sense.

I assign the current object state to a property on the current object.
No, you created a property that referred to itself.
If you want to save the current state of the property then you need to clone the object.
If you want to create a (shallow) copy of an object then you can use:
function clone(obj) {
if(obj === null || typeof(obj) !== 'object' || 'isActiveClone' in obj)
return obj;
var temp = obj.constructor();
for(var key in obj) {
if(Object.prototype.hasOwnProperty.call(obj, key)) {
obj['isActiveClone'] = null;
temp[key] = obj[key];
delete obj['isActiveClone'];
}
}
return temp;
}
[code taken from here - and modified slightly to do a shallow copy rather than recursive deep copy]
then do:
product.memory = clone( product );
You may find you get the issues with recursion if you clone it a second time and it copies the product.memory along with the rest of the object. In that case just delete product.memory before doing subsequent clones.
Something like:
function saveCurrentState( obj ){
if ( 'memory' in obj )
delete obj.memory;
obj.memory = clone( obj );
}
Aside
If you want a deep copy then you can do:
function deepCopy(obj){
return JSON.parse(JSON.stringify(obj));
}
[As suggested here - but note the caveats it has for Date objects]

you could do your idea by clone the current product into new. We've Object.keys to get all attribute of object. So here is my idea :
product = {
ropeType: 'blah',
ropePrice: 'blah',
ropeSections: {
name: 'blaah',
price: 'blaah'
},
memory: false
};
var keys = Object.keys(product);
var newProduct = {};
keys.forEach(function(key){
if(key === 'memory') return;
newProduct[key] = product[key];
});
product.memory = newProduct;

Instead of actually storing a reference to the object, you might want to transform that object's state. Maybe by cloning it onto a new object or possibly keeping it as a JSON string (which you'll want to do if you're using localStorage).
Since you will probably want to see the current state of the object whenever you check the memory property, you should make memory a function that does that transformation.
Maybe something like this:
var product = {
ropeType: 'blah',
ropePrice: 'blah',
ropeSections: {
name: 'blaah',
price: 'blaah'
},
memory: function() {
return JSON.stringify(this);
}
}
You can then call product.memory() and get its state in JSON.

This here is the problem:
product.memory = product;
You're assigning a reference to an object to itself. JavaScript passes objects by reference, so it's never going to store a clone of itself through assignment.
If you're looking to record modifications made to the object over time, the best way would be to use an array to hold cloned copies of it (or at least the properties that've changed).
To give you the quickest example:
var Product = function(){
};
var product = new Product();
product.history = [];
product.saveState = function(){
var changes = {};
for(var i in this){
/** Prevent infinite self-referencing, and don't store this function itself. */
if(this[i] !== this.history && this[i] !== this.saveState){
changes[i] = this[i];
}
}
this.history.push(changes);
};
Obviously, there're many better ways to achieve this in JavaScript, but they require more explanation. Basically, looping through an object to store its properties is inevitably going to trip up upon the property that they're being assigned to, so a check is needed at some point to prevent self-referencing.

Ways to extend Array object in javascript

i try to extend Array object in javascript with some user friendly methods like Array.Add() instead Array.push() etc...
i implement 3 ways to do this.
unfortunetly the 3rd way is not working and i want to ask why? and how to do it work.
//------------- 1st way
Array.prototype.Add=function(element){
this.push(element);
};
var list1 = new Array();
list1.Add("Hello world");
alert(list1[0]);
//------------- 2nd way
function Array2 () {
//some other properties and methods
};
Array2.prototype = new Array;
Array2.prototype.Add = function(element){
this.push(element);
};
var list2 = new Array2;
list2.Add(123);
alert(list2[0]);
//------------- 3rd way
function Array3 () {
this.prototype = new Array;
this.Add = function(element){
this.push(element);
};
};
var list3 = new Array3;
list3.Add(456); //push is not a function
alert(list3[0]); // undefined
in 3rd way i want to extend the Array object internally Array3 class.
How to do this so not to get "push is not a function" and "undefined"?
Here i add a 4th way.
//------------- 4th way
function Array4 () {
//some other properties and methods
this.Add = function(element){
this.push(element);
};
};
Array4.prototype = new Array();
var list4 = new Array4();
list4.Add(789);
alert(list4[0]);
Here again i have to use prototype.
I hoped to avoid to use extra lines outside class constructor as Array4.prototype.
I wanted to have a compact defined class with all pieces in one place.
But i think i cant do it otherwise.

ES6
class SubArray extends Array {
last() {
return this[this.length - 1];
}
}
var sub = new SubArray(1, 2, 3);
sub // [1, 2, 3]
sub instanceof SubArray; // true
sub instanceof Array; // true
Using __proto__
(old answer, not recommended, may cause performance issues)
function SubArray() {
var arr = [ ];
arr.push.apply(arr, arguments);
arr.__proto__ = SubArray.prototype;
return arr;
}
SubArray.prototype = new Array;
Now you can add your methods to SubArray
SubArray.prototype.last = function() {
return this[this.length - 1];
};
Initialize like normal Arrays
var sub = new SubArray(1, 2, 3);
Behaves like normal Arrays
sub instanceof SubArray; // true
sub instanceof Array; // true

Method names should be lowercase. Prototype should not be modified in the constructor.
function Array3() { };
Array3.prototype = new Array;
Array3.prototype.add = Array3.prototype.push
in CoffeeScript
class Array3 extends Array
add: (item)->
#push(item)
If you don't like that syntax, and you HAVE to extend it from within the constructor,
Your only option is:
// define this once somewhere
// you can also change this to accept multiple arguments
function extend(x, y){
for(var key in y) {
if (y.hasOwnProperty(key)) {
x[key] = y[key];
}
}
return x;
}
function Array3() {
extend(this, Array.prototype);
extend(this, {
Add: function(item) {
return this.push(item)
}
});
};
You could also do this
ArrayExtenstions = {
Add: function() {
}
}
extend(ArrayExtenstions, Array.prototype);
function Array3() { }
Array3.prototype = ArrayExtenstions;
In olden days, 'prototype.js' used to have a Class.create method. You could wrap all this is a method like that
var Array3 = Class.create(Array, {
construct: function() {
},
Add: function() {
}
});
For more info on this and how to implement, look in the prototype.js source code

A while ago I read the book Javascript Ninja written by John Resig, the creator of jQuery.
He proposed a way to mimic array-like methods with a plain JS object. Basically, only length is required.
var obj = {
length: 0, //only length is required to mimic an Array
add: function(elem){
Array.prototype.push.call(this, elem);
},
filter: function(callback) {
return Array.prototype.filter.call(this, callback); //or provide your own implemetation
}
};
obj.add('a');
obj.add('b');
console.log(obj.length); //2
console.log(obj[0], obj[1]); //'a', 'b'
I don't mean it's good or bad. It's an original way of doing Array operations. The benefit is that you do not extend the Array prototype.
Keep in mind that obj is a plain object, it's not an Array. Therefore obj instanceof Array will return false. Think obj as a façade.
If that code is of interest to you, read the excerpt Listing 4.10 Simulating array-like methods.

In your third example you're just creating a new property named prototype for the object Array3. When you do new Array3 which should be new Array3(), you're instantiating that object into variable list3. Therefore, the Add method won't work because this, which is the object in question, doesn't have a valid method push. Hope you understand.
Edit: Check out Understanding JavaScript Context to learn more about this.

You can also use this way in ES6:
Object.assign(Array.prototype, {
unique() {
return this.filter((value, index, array) => {
return array.indexOf(value) === index;
});
}
});
Result:
let x = [0,1,2,3,2,3];
let y = x.unique();
console.log(y); // => [0,1,2,3]

Are you trying to do something more complicated then just add an alias for "push" called "Add"?
If not, it would probably be best to avoid doing this. The reason I suggest this is a bad idea is that because Array is a builtin javascript type, modifying it will cause all scripts Array type to have your new "Add" method. The potential for name clashes with another third party are high and could cause the third party script to lose its method in favour of your one.
My general rule is to make a helper function to work on the Array's if it doesnt exist somewhere already and only extend Array if its extremely necessary.

You CANNOT extend the Array Object in JavaScript.
Instead, what you can do is define an object that will contain a list of functions that perform on the Array, and inject these functions into that Array instance and return this new Array instance. What you shouldn't do is changing the Array.prototype to include your custom functions upon the list.
Example:
function MyArray() {
var tmp_array = Object.create(Array.prototype);
tmp_array = (Array.apply(tmp_array, arguments) || tmp_array);
//Now extend tmp_array
for( var meth in MyArray.prototype )
if(MyArray.prototype.hasOwnProperty(meth))
tmp_array[meth] = MyArray.prototype[meth];
return (tmp_array);
}
//Now define the prototype chain.
MyArray.prototype = {
customFunction: function() { return "blah blah"; },
customMetaData: "Blah Blah",
}
Just a sample code, you can modify it and use however you want. But the underlying concept I recommend you to follow remains the same.

var SubArray = function() {
var arrInst = new Array(...arguments); // spread arguments object
/* Object.getPrototypeOf(arrInst) === Array.prototype */
Object.setPrototypeOf(arrInst, SubArray.prototype); //redirectionA
return arrInst; // now instanceof SubArray
};
SubArray.prototype = {
// SubArray.prototype.constructor = SubArray;
constructor: SubArray,
// methods avilable for all instances of SubArray
add: function(element){return this.push(element);},
...
};
Object.setPrototypeOf(SubArray.prototype, Array.prototype); //redirectionB
var subArr = new SubArray(1, 2);
subArr.add(3); subArr[2]; // 3
The answer is a compact workaround which works as intended in all supporting browsers.

Set of objects in javascript

I'd like to have a set of objects in Javascript. That is, a data structure that contains only unique objects.
Normally using properties is recommended, e.g. myset["key"] = true. However, I need the keys to be objects. I've read that Javascript casts property names to strings, so I guess I can't use myset[myobject] = true.
I could use an array, but I need something better than O(n) performance for adding, finding and removing items.
It needs to be able to tell objects apart by reference only, so given:
var a = {};
var b = {};
then both a and b should be able to be added, because they're separate objects.
Basically, I'm after something like C++'s std::set, that can store Javascript objects. Any ideas?

ES6 provides a native Set:
let s = new Set();
let a = {};
let b = {};
s.add(a);
console.log(s.has(a)); // true
console.log(s.has(b)); // false

Here's a mad suggestion ... key it on the result of JSON.stringify(object)

It's not possible for all objects, but if your object has a .toString() method implemented, it is:
var x = {toString: function(){ return 'foo'; }};
var y = {toString: function(){ return 'bar'; }};
var obj = {};
obj[x] = 'X';
obj[y] = 'Y';
console.log(obj);
// { foo: 'X', bar: 'Y' }
If you want to make this easier, make it a class:
function myObj(name){
this.name = name;
}
myObj.prototype.toString = function(){ return this.name; }
var obj = {};
obj[new myObj('foo')] = 'X';
obj[new myObj('bar')] = 'Y';

I'm answering my own question, but I came up with an alternative solution I thought was interesting and thought it would be useful to share it.
cwolves' answer gave me an idea. Providing an object's toString() method uniquely identifies the instance, properties of an object can be used to store a set of objects. Essentially, to store object x, you can use items[x.toString()] = x;. Note that the value is the object itself, so then the set of objects can be extracted by looking at all item's properties and dumping all the values in to an array.
Here's the class, which I call ObjectSet, in full. It requires objects are uniquely identified by their toString() method, which is OK for my purposes. add, remove and contains should all run in better than O(n) time - whatever javascript's property access efficiency is, which hopefully is either O(1) or O(n log n).
// Set of objects. Requires a .toString() overload to distinguish objects.
var ObjectSet = function ()
{
this.items = {};
this.item_count = 0;
};
ObjectSet.prototype.contains = function (x)
{
return this.items.hasOwnProperty(x.toString());
};
ObjectSet.prototype.add = function (x)
{
if (!this.contains(x))
{
this.items[x.toString()] = x;
this.item_count++;
}
return this;
};
ObjectSet.prototype.remove = function (x)
{
if (this.contains(x))
{
delete this.items[x.toString()];
this.item_count--;
}
return this;
};
ObjectSet.prototype.clear = function ()
{
this.items = {};
this.item_count = 0;
return this;
};
ObjectSet.prototype.isEmpty = function ()
{
return this.item_count === 0;
};
ObjectSet.prototype.count = function ()
{
return this.item_count;
};
ObjectSet.prototype.values = function ()
{
var i, ret = [];
for (i in this.items)
{
if (this.items.hasOwnProperty(i))
ret.push(this.items[i]);
}
return ret;
};

I used Map, solved my case
const objectsMap = new Map();
const placesName = [
{ place: "here", name: "stuff" },
{ place: "there", name: "morestuff" },
{ place: "there", name: "morestuff" },
];
placesName.forEach((object) => {
objectsMap.set(object.place, object);
});
console.log(objectsMap);

For what you're trying to do (sets of objects), there is no native Javascript implementation. You would have to implement this on your own. One way to do this would be to implement a hashing function for your objects. The backing data-type of the set would be an associative array, where the key of the array is the value you get from calling the object's hash function, and the value of the array is the object itself.
Of course, this doesn't address the issue that you highlighted, so you will need to take equality into account as well (implement an equals function perhaps)?
Instead of making the hash function a property of the object itself, you can have a standalone hash function that takes in an object as input and generates a hash value (presumably by iterating over its properties).
Using this method you should be able to get O(1) for insertion, searching, and removing (not counting the order of the hash function, which shouldn't be any worse than O(n), especially if you are iterating over its properties to create your hashed value).

ECMAScript6 Set should behave like that:
Standard: http://www.ecma-international.org/ecma-262/6.0/#sec-set-o-p-v-throw
Unofficial ES6 cheat sheet: https://github.com/lukehoban/es6features#map--set--weakmap--weakset
Working example on Firefox 32 (but not implemented in Chromium 37):
if (Set) {
var s = new Set()
var a = {}
var b = {}
var c = {}
s.add(a)
s.add(b)
s.add(b)
assert(s.size === 2)
assert(s.has(a))
assert(s.has(b))
assert(!s.has(c))
}
This is not surprising since {} != {}: equality compares object addresses by default.
A module that implements it for browsers without support: https://github.com/medikoo/es6-set

Javascript Set's don't do deep object comparison.
Using lodash, this is a unique array with deep object comparison:
const objects = [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }, { 'x': 1, 'y': 2 }];
_.uniqWith(objects, _.isEqual);

Just typed this up, it's only briefly tested:
var Set = function Set()
{
var list = [];
var contains;
this.contains = contains = function(x) {
return list.indexOf(x) >= 0;
}
var put;
this.put = put = function(x) {
if (!contains(x))
list.push(x);
return this;
}
var remove;
this.remove = remove = function(x)
{
var idx = list.indexOf(x);
if (idx >= 0)
list.splice(idx,1);
return this;
}
var all;
this.all = all = function()
{
return list.concat();
}
return this;
}

It seems that the inner call of function works when prefixed with this.
Exemple:
var put;
this.put = put = function(x) {
if (!this.contains(x))
list.push(x);
return this;
}

Please use this code as a reference.
const fruits = [
{name: 'apple', price: 100},
{name: 'apple', price: 100},
{name: 'orange', price: 200},
{name: 'grapes', price: 300}
];
const hasFruitDuplicated = () => {
const duplicatedDeleteFruits = fruits.filter((fruit, index) =>
fruits.findIndex(item => item.name === fruit.name && item.price === fruit.price) === index
);
return duplicatedDeleteFruits;
};

Given an array of the following type:
Array<{ foo: T1, bar: T2 }>
You can build a corresponding dictionary of type:
{ [foo: T1]: Set<T2> }
The look-up for { foo: fooValue, bar: barValue } can be performed as follows:
if (fooValue in dictionary && dictionary[fooValue].has(barValue))
This way we can build what would be an ObjectSet<T1, T2>
.
If you now have three elements, you can build the following dictionary:
{ [foo: T1]: ObjectSet<T2, T3> }
and extend your ObjectSet to any number of properties by induction.
That is assuming your types can be used as index signatures.

CouchDB: Adding fields to "doc" in view

In my view I want to create a new object to return instead of the object that is passed to my map function. I would like to do some validation and add some fields to the new object. My map function for the view looks like this (there isn't a reduce):
function(doc) {
if(doc.type == "mytype") {
var newobj = null;
if( doc.someObjField ) {
newobj = doc.someObjField; //LINE 5: want a copy but get a reference
}
else {
newobj = new Object(); //field didn't exist create a new object
}
newobj.superId = doc._id; //these fields get added to the DB
newobj.superName = doc.name;
newobj.newField = doc.field;
emit(doc._id, newobj);
}
}
The problem is that when this view runs, it ends up adding some new fields to the documents in the database. I cannot see the fields when editing the doc in Futon, but they show up in every view of the document.
I think this is because because of LINE 5 creates a reference to that field of the document instead of a copy. Does this mean I have to include a clone() function in all my views to get what I want? Maybe I'm thinking about this wrong?
Update: I was using CouchDB version 1.0.0 and upgraded to version 1.0.1 and it seems that the behavior has changed. In the previous version when "doc" was modified in one view, the modification appeared in all views. In the newest version, this does not seem to be the case.

newobj = doc.someObjField does indeed make a reference and not a copy, so you'll will have to iterate over the properties of doc.someObjField and copy them over to a new object.
This is not that trivial, the following shows an example of how to make a shallow clone:
// get the class of an object, this is a lot better than messing with typeof and instanceof
function is(type, obj) {
return Object.prototype.toString.call(obj).slice(8, -1) === type;
}
function copy(val) {
if (is('Object', val)) { // shallow clone objects
var obj = {};
for (var f in val) {
if (val.hasOwnProperty(f)) {
obj[f] = val[f];
}
}
return obj;
} else {
return is('Array', val) ? val.slice() : val; // shallow clone arrays
}
}
Your code would look like this:
newobj = doc.someObjField ? copy(doc.someObjField) : {};
Now if you someObjField has objects or arrays as its properties those will yet again only be references, so you have to recursively clone them.