I have an object, x. I'd like to copy it as object y, such that changes to y do not modify x. I realized that copying objects derived from built-in JavaScript objects will result in extra, unwanted properties. This isn't a problem, since I'm copying one of my own, literal-constructed objects.
How do I correctly clone a JavaScript object?
转载于:https://stackoverflow.com/questions/728360/how-do-i-correctly-clone-a-javascript-object
From this article: How to copy arrays and objects in Javascript by Brian Huisman:
Object.prototype.clone = function() {
var newObj = (this instanceof Array) ? [] : {};
for (var i in this) {
if (i == 'clone') continue;
if (this[i] && typeof this[i] == "object") {
newObj[i] = this[i].clone();
} else newObj[i] = this[i]
} return newObj;
};
Here is a function you can use.
function clone(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = clone(obj[key]);
return temp;
}
A.Levy's answer is almost complete, here is my little contribution: there is a way how to handle recursive references, see this line
if(this[attr]==this) copy[attr] = copy;
If the object is XML DOM element, we must use cloneNode instead
if(this.cloneNode) return this.cloneNode(true);
Inspired by A.Levy's exhaustive study and Calvin's prototyping approach, I offer this solution:
Object.prototype.clone = function() {
if(this.cloneNode) return this.cloneNode(true);
var copy = this instanceof Array ? [] : {};
for(var attr in this) {
if(typeof this[attr] == "function" || this[attr]==null || !this[attr].clone)
copy[attr] = this[attr];
else if(this[attr]==this) copy[attr] = copy;
else copy[attr] = this[attr].clone();
}
return copy;
}
Date.prototype.clone = function() {
var copy = new Date();
copy.setTime(this.getTime());
return copy;
}
Number.prototype.clone =
Boolean.prototype.clone =
String.prototype.clone = function() {
return this;
}
See also Andy Burke's note in the answers.
Using Lodash:
var y = _.clone(x, true);
If you're okay with a shallow copy, the underscore.js library has a clone method.
y = _.clone(x);
or you can extend it like
copiedObject = _.extend({},originalObject);
You can clone an object and remove any reference from the previous one using a single line of code. Simply do:
var obj1 = { text: 'moo1' };
var obj2 = Object.create(obj1); // Creates a new clone without references
obj2.text = 'moo2'; // Only updates obj2's text property
console.log(obj1, obj2); // Outputs: obj1: {text:'moo1'}, obj2: {text:'moo2'}
For browsers / engines that do not currently support Object.create you can use this polyfill:
// Polyfill Object.create if it does not exist
if (!Object.create) {
Object.create = function (o) {
var F = function () {};
F.prototype = o;
return new F();
};
}
One particularly inelegant solution is to use JSON encoding to make deep copies of objects that do not have member methods. The methodology is to JSON encode your target object, then by decoding it, you get the copy you are looking for. You can decode as many times as you want to make as many copies as you need.
Of course, functions do not belong in JSON, so this only works for objects without member methods.
This methodology was perfect for my use case, since I'm storing JSON blobs in a key-value store, and when they are exposed as objects in a JavaScript API, each object actually contains a copy of the original state of the object so we can calculate the delta after the caller has mutated the exposed object.
var object1 = {key:"value"};
var object2 = object1;
object2 = JSON.stringify(object1);
object2 = JSON.parse(object2);
object2.key = "a change";
console.log(object1);// returns value
If you do not use functions within your object, a very simple one liner can be the following:
var cloneOfA = JSON.parse(JSON.stringify(a));
This works for all kind of objects containing objects, arrays, strings, booleans and numbers.
See also this article about the structured clone algorithm of browsers which is used when posting messages to and from a worker. It also contains a function for deep cloning.
With jQuery, you can shallow copy with extend:
var copiedObject = jQuery.extend({}, originalObject)
subsequent changes to the copiedObject will not affect the originalObject, and vice versa.
Or to make a deep copy:
var copiedObject = jQuery.extend(true, {}, originalObject)
There are many answers, but none that mentions Object.create from ECMAScript 5, which admittedly does not give you an exact copy, but sets the source as the prototype of the new object.
Thus, this is not an exact answer to the question, but it is a one-line solution and thus elegant. And it works best for 2 cases:
Example:
var foo = { a : 1 };
var bar = Object.create(foo);
foo.a; // 1
bar.a; // 1
foo.a = 2;
bar.a; // 2 - prototype changed
bar.a = 3;
foo.a; // Still 2, since setting bar.a makes it an "own" property
Why do I consider this solution to be superior? It's native, thus no looping, no recursion. However, older browsers will need a polyfill.
There are several issues with most solutions on the internet. So I decided to make a follow-up, which includes, why the accepted answer shouldn't be accepted.
I want to deep-copy a Javascript Object with all of its children and their children and so on. But since I'm not kind of a normal developer, my Object has normal properties, circular structures and even nested objects.
So let's create a circular structure and a nested object first.
function Circ() {
this.me = this;
}
function Nested(y) {
this.y = y;
}
Let's bring everything together in an Object named a.
var a = {
x: 'a',
circ: new Circ(),
nested: new Nested('a')
};
Next, we want to copy a into a variable named b and mutate it.
var b = a;
b.x = 'b';
b.nested.y = 'b';
You know what happened here because if not you wouldn't even land on this great question.
console.log(a, b);
a --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
Now let's find a solution.
The first attempt I tried was using JSON.
var b = JSON.parse( JSON.stringify( a ) );
b.x = 'b';
b.nested.y = 'b';
Don't waste too much time on it, you'll get TypeError: Converting circular structure to JSON.
Let's have a look at the accepted answer.
function cloneSO(obj) {
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = cloneSO(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
var copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = cloneSO(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
Looks good, heh? It's a recursive copy of the object and handles other types as well, like Date, but that wasn't a requirement.
var b = cloneSO(a);
b.x = 'b';
b.nested.y = 'b';
Recursion and circular structures doesn't work well together... RangeError: Maximum call stack size exceeded
After arguing with my co-worker, my boss asked us what happened, and he found a simple solution after some googling. It's called Object.create.
var b = Object.create(a);
b.x = 'b';
b.nested.y = 'b';
This solution was added to Javascript some time ago and even handles circular structure.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
... and you see, it didn't work with the nested structure inside.
There's a polyfill for Object.create in the older browser just like the IE 8. It's something like recommended by Mozilla, and of course, it's not perfect and results in the same problem as the native solution.
function F() {};
function clonePF(o) {
F.prototype = o;
return new F();
}
var b = clonePF(a);
b.x = 'b';
b.nested.y = 'b';
I've put F outside the scope so we can have a look at what instanceof tells us.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> F {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> true
Same problem as the native solution, but a little bit worse output.
When digging around, I found a similar question (In Javascript, when performing a deep copy, how do I avoid a cycle, due to a property being "this"?) to this one, but with a way better solution.
function cloneDR(o) {
const gdcc = "__getDeepCircularCopy__";
if (o !== Object(o)) {
return o; // primitive value
}
var set = gdcc in o,
cache = o[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
o[gdcc] = function() { return result; }; // overwrite
if (o instanceof Array) {
result = [];
for (var i=0; i<o.length; i++) {
result[i] = cloneDR(o[i]);
}
} else {
result = {};
for (var prop in o)
if (prop != gdcc)
result[prop] = cloneDR(o[prop]);
else if (set)
result[prop] = cloneDR(cache);
}
if (set) {
o[gdcc] = cache; // reset
} else {
delete o[gdcc]; // unset again
}
return result;
}
var b = cloneDR(a);
b.x = 'b';
b.nested.y = 'b';
And let's have a look at the output...
console.log(a, b);
a --> Object {
x: "a",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "a"
}
}
b --> Object {
x: "b",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> false
The requirements are matched, but there are still some smaller issues, including changing the instance of nested and circ to Object.
The structure of trees that share a leaf won't be copied, they will become two independent leaves:
[Object] [Object]
/ \ / \
/ \ / \
|/_ _\| |/_ _\|
[Object] [Object] ===> [Object] [Object]
\ / | |
\ / | |
_\| |/_ \|/ \|/
[Object] [Object] [Object]
The last solution using recursion and a cache, may not be the best, but it's a real deep-copy of the object. It handles simple properties, circular structures and nested object, but it will mess up the instance of them while cloning.
For those using AngularJS, there is also direct method for cloning or extending of the objects in this library.
var destination = angular.copy(source);
or
angular.copy(source, destination);
More in angular.copy documentation...
In ECMAScript 6 there is Object.assign method, which copies values of all enumerable own properties from one object to another. For example:
var x = {myProp: "value"};
var y = Object.assign({}, x);
But be aware that nested objects are still copied as reference.
Interested in cloning simple objects :
JSON.parse(JSON.stringify(json_original));
Source : How to copy JavaScript object to new variable NOT by reference?
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need.
var clone = Object.assign({}, obj);
The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}
You can simply use a spread property to copy an object without references. But be careful (see comments), the 'copy' is just on the lowest object/array level. Nested properties are still references!
Complete clone:
let x = {a: 'value1'}
let x2 = {...x}
// => mutate without references:
x2.a = 'value2'
console.log(x.a) // => 'value1'
Clone with references on second level:
const y = {a: {b: 'value3'}}
const y2 = {...y}
// => nested object is still a references:
y2.a.b = 'value4'
console.log(y.a.b) // => 'value4'
JavaScript actually does not support deep clones natively. Use an utility function. For example Ramda:
Per MDN:
Object.assign({}, a)JSON.parse(JSON.stringify(a))There is no need for external libraries but you need to check browser compatibility first.
New answer to an old question! If you have the pleasure of having using ECMAScript 2016 (ES6) with Spread Syntax, it's easy.
keepMeTheSame = {first: "Me!", second: "You!"};
cloned = {...keepMeTheSame}
This provides a clean method for a shallow copy of an object. Making a deep copy, meaning makign a new copy of every value in every recursively nested object, requires on of the heavier solutions above.
JavaScript keeps evolving.
In ES-6 you can simply use Object.assign(...). Ex:
let obj = {person: 'Thor Odinson'};
let clone = Object.assign({}, obj);
A good reference is here: https://googlechrome.github.io/samples/object-assign-es6/
let clone = Object.assign( Object.create( Object.getPrototypeOf(obj)), obj)
ES6 solution if you want to (shallow) clone a class instance and not just a property object.
OK, imagine you have this object below and you want to clone it:
let obj = {a:1, b:2, c:3}; //ES6
or
var obj = {a:1, b:2, c:3}; //ES5
The answer is mainly depeneds on which ECMAscript you using, in ES6+, you can simply use Object.assign to do the clone:
let cloned = Object.assign({}, obj); //new {a:1, b:2, c:3};
or using spread operator like this:
let cloned = {...obj}; //new {a:1, b:2, c:3};
But if you using ES5, you can use few methods, but the JSON.stringify, just make sure you not using for a big chunk of data to copy, but it could be one line handy way in many cases, something like this:
let cloned = JSON.parse(JSON.stringify(obj));
//new {a:1, b:2, c:3};, can be handy, but avoid using on big chunk of data over and over