Let’s say I have an options variable and I want to set some default value.
What’s is the benefit / drawback of these two alternatives?
Using object spread
options = {...optionsDefault, ...options};
Or using Object.assign
options = Object.assign({}, optionsDefault, options);
This is the commit that made me wonder.
This isn't necessarily exhaustive.
options = {...optionsDefault, ...options};
If authoring code for execution in environments without native support, you may be able to just compile this syntax (as opposed to using a polyfill). (With Babel, for example.)
Less verbose.
When this answer was originally written, this was a proposal, not standardized. When using proposals consider what you'd do if you write code with it now and it doesn't get standardized or changes as it moves toward standardization. This has since been standardized in ES2018.
Literal, not dynamic.
Object.assign()options = Object.assign({}, optionsDefault, options);
Standardized.
Dynamic. Example:
var sources = [{a: "A"}, {b: "B"}, {c: "C"}];
options = Object.assign.apply(Object, [{}].concat(sources));
// or
options = Object.assign({}, ...sources);
This is the commit that made me wonder.
That's not directly related to what you're asking. That code wasn't using Object.assign(), it was using user code (object-assign) that does the same thing. They appear to be compiling that code with Babel (and bundling it with Webpack), which is what I was talking about: the syntax you can just compile. They apparently preferred that to having to include object-assign as a dependency that would go into their build.
For reference object rest/spread is finalised in ECMAScript 2018 as a stage 4. The proposal can be found here.
For the most part object assign and spread work the same way, the key difference is that spread defines properties, whilst Object.assign() sets them. This means Object.assign() triggers setters.
It's worth remembering that other than this, object rest/spread 1:1 maps to Object.assign() and acts differently to array (iterable) spread. For example, when spreading an array null values are spread. However using object spread null values are silently spread to nothing.
Array (Iterable) Spread Example
const x = [1, 2, null , 3];
const y = [...x, 4, 5];
const z = null;
console.log(y); // [1, 2, null, 3, 4, 5];
console.log([...z]); // TypeError
Object Spread Example
const x = null;
const y = {a: 1, b: 2};
const z = {...x, ...y};
console.log(z); //{a: 1, b: 2}
This is consistent with how Object.assign() would work, both silently exclude the null value with no error.
const x = null;
const y = {a: 1, b: 2};
const z = Object.assign({}, x, y);
console.log(z); //{a: 1, b: 2}
I think one big difference between the spread operator and Object.assign that doesn't seem to be mentioned in the current answers is that the spread operator will not copy the the source object’s prototype to the target object. If you want to add properties to an object and you don't want to change what instance it is of, then you will have to use Object.assign.
Edit: I've actually realised that my example is misleading. The spread operator desugars to Object.assign with the first parameter set to an empty object. In my code example below, I put error as the first parameter of the Object.assign call so the two are not equivalent. The first parameter of Object.assign is actually modified and then returned which is why it retains its prototype. I have added another example below:
const error = new Error();
error instanceof Error // true
const errorExtendedUsingSpread = {
...error,
...{
someValue: true
}
};
errorExtendedUsingSpread instanceof Error; // false
// What the spread operator desugars into
const errorExtendedUsingImmutableObjectAssign = Object.assign({}, error, {
someValue: true
});
errorExtendedUsingImmutableObjectAssign instanceof Error; // false
// The error object is modified and returned here so it keeps its prototypes
const errorExtendedUsingAssign = Object.assign(error, {
someValue: true
});
errorExtendedUsingAssign instanceof Error; // trueSee also: https://github.com/tc39/proposal-object-rest-spread/blob/master/Spread.md
There's a huge difference between the two, with very serious consequences. The most upvoted questions do not even touch this, and the information about object spread being a proposal is not relevant in 2022 anymore.
Object.assign changes the object in-place, while the spread operator (...) creates a brand new object, and this will break object reference equality.First, let's see the effect, and then I'll give a real-world example of how important it is to understand this fundamental difference.
First, let's use Object.assign:
// Let's create a new object, that contains a child object;
const parentObject = { childObject: { hello: 'world '} };
// Let's get a reference to the child object;
const childObject = parentObject.childObject;
// Let's change the child object using Object.assign, adding a new `foo` key with `bar` value;
Object.assign(parentObject.childObject, { foo: 'bar' });
// childObject is still the same object in memory, it was changed IN PLACE.
parentObject.childObject === childObject
// true
Now the same exercise with the spread operator:
// Let's create a new object, that contains a child object;
const parentObject = { childObject: { hello: 'world '} };
// Let's get a reference to the child object;
const childObject = parentObject.childObject;
// Let's change the child object using the spread operator;
parentObject.childObject = {
...parentObject.childObject,
foo: 'bar',
}
// They are not the same object in memory anymore!
parentObject.childObject === childObject;
// false
It's easy to see what is going on, because on the parentObject.childObject = {...} we are cleary assigning the value of the childObject key in parentObject to a brand new object literal, and the fact it's being composed by the old childObject content's is irrelevant. It's a new object.
And if you assume this is irrelevant in practice, let me show a real world scenario of how important it is to understand this.
In a very large Vue.js application, we started noticing a lot of sluggishness when typing the name of the customer in an input field.
After a lot of debugging, we found out that each char typed in that input triggered a hole bunch of computed properties to re-evaluate.
This wasn't anticipated, since the customer's name wasn't used at all in those computeds functions. Only other customer data (like age, sex) was being used. What was goin on? Why was vue re-evaluating all those computed functions when the customer's name changed?
Well, we had a Vuex store that did this:
mutations: {
setCustomer(state, payload) {
// payload being { name: 'Bob' }
state.customer = { ...state.customer, ...payload };
}
And our computed were like this:
veryExpensiveComputed() {
const customerAge = this.$store.state.customer.age;
}
So, voilá! When the customer name changed, the Vuex mutation was actually changing it to a new object entirely; and since the computed relied on that object to get the customer age, Vue counted on that very specific object instance as a dependency, and when it was changed to a new object (failing the === object equality test), Vue decided it was time to re-run the computed function.
The fix? Use Object.assign to not discard the previous object, but to change it in place ...
mutations: {
setCustomer(state, payload) {
// payload being same as above: { name: 'Bob' }
Object.assign(state.customer, payload);
}
BTW, if you are in Vue2, you shouldn't use Object.assign because Vue 2 can't track those object changes directly, but the same logic applies, just using Vue.set instead of Object.assign:
mutations: {
setCustomer(state, payload) {
Object.keys(payload).forEach(key => {
Vue.set(state.customer, key, payload[key])
})
}
NOTE: Spread is NOT just syntactic sugar around Object.assign. They operate much differently behind the scenes.
Object.assign applies setters to a new object, Spread does not. In addition, the object must be iterable.
Copy Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other owners of the object.
Use it for creating a shallow copy of the object good practice to always set immutable properties to copy - because mutable versions can be passed into immutable properties, copy will ensure that you'll always be dealing with an immutable object
Assign Assign is somewhat the opposite to copy. Assign will generate a setter which assigns the value to the instance variable directly, rather than copying or retaining it. When calling the getter of an assign property, it returns a reference to the actual data.
I'd like to summarize status of the "spread object merge" ES feature, in browsers, and in the ecosystem via tools.
var x = { a: 1, b: 2 };
var y = { c: 3, d: 4, a: 5 };
var z = {...x, ...y};
console.log(z); // { a: 5, b: 2, c: 3, d: 4 }
Again: At time of writing this sample works without transpilation in Chrome (60+), Firefox Developer Edition (preview of Firefox 60), and Node (8.7+).
I'm writing this 2.5 years after the original question. But I had the very same question, and this is where Google sent me. I am a slave to SO's mission to improve the long tail.
Since this is an expansion of "array spread" syntax I found it very hard to google, and difficult to find in compatibility tables. The closest I could find is Kangax "property spread", but that test doesn't have two spreads in the same expression (not a merge). Also, the name in the proposals/drafts/browser status pages all use "property spread", but it looks to me like that was a "first principal" the community arrived at after the proposals to use spread syntax for "object merge". (Which might explain why it is so hard to google.) So I document my finding here so others can view, update, and compile links about this specific feature. I hope it catches on. Please help spread the news of it landing in the spec and in browsers.
Lastly, I would have added this info as a comment, but I couldn't edit them without breaking the authors' original intent. Specifically, I can't edit @ChillyPenguin's comment without it losing his intent to correct @RichardSchulte. But years later Richard turned out to be right (in my opinion). So I write this answer instead, hoping it will gain traction on the old answers eventually (might take years, but that's what the long tail effect is all about, after all).
As others have mentioned, at this moment of writing, Object.assign() requires a polyfill and object spread ... requires some transpiling (and perhaps a polyfill too) in order to work.
Consider this code:
// Babel wont touch this really, it will simply fail if Object.assign() is not supported in browser.
const objAss = { message: 'Hello you!' };
const newObjAss = Object.assign(objAss, { dev: true });
console.log(newObjAss);
// Babel will transpile with use to a helper function that first attempts to use Object.assign() and then falls back.
const objSpread = { message: 'Hello you!' };
const newObjSpread = {...objSpread, dev: true };
console.log(newObjSpread);
These both produce the same output.
Here is the output from Babel, to ES5:
var objAss = { message: 'Hello you!' };
var newObjAss = Object.assign(objAss, { dev: true });
console.log(newObjAss);
var _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };
var objSpread = { message: 'Hello you!' };
var newObjSpread = _extends({}, objSpread, { dev: true });
console.log(newObjSpread);
This is my understanding so far. Object.assign() is actually standardised, where as object spread ... is not yet. The only problem is browser support for the former and in future, the latter too.
Hope this helps.
The object spread operator (...) doesn't work in browsers, because it isn't part of any ES specification yet, just a proposal. The only option is to compile it with Babel (or something similar).
As you can see, it's just syntactic sugar over Object.assign({}).
As far as I can see, these are the important differences.
... for objects isn't standardized... protects you from accidentally mutating the object... will polyfill Object.assign in browsers without it... needs less code to express the same idea
I did a search and found a lot of misinformation on this.
...spread nor Object.assign is faster. It depends.Object.assign is almost always faster if side-effects/object mutation is not a consideration.Whether Object.assign or ...spread is faster, it depends on what you are trying to combine, and the runtime you are using (implementations and optimisations happen from time to time). For small objects, it does not matter.
For bigger objects, Object.assign is usually better. Especially if you do not need to care about side-effects, the speed gains come from saving time by just adding properties to the first object, rather than copying from two and creating a brand new one. See:
async function retrieveAndCombine() {
const bigPayload = await retrieveData()
const smallPayload = await retrieveData2()
// only the properties of smallPayload is iterated through
// whereas bigPayload is mutated.
return Object.assign(bigPayload, smallPayload)
}
In cases where side-effects matters, such as if an object to be combined with another is passed in as an argument.
In the example below, bigPayload will be mutated, which is a bad idea if other functions/objects outside of retrieveAndCombine depends on bigPayload. In this case, you can swap the arguments passed to Object.assign, or just use {} as the first argument to create a new object:
async function retrieveAndCombine(bigPayload) {
const smallPayload = await retrieveData2()
// this is slightly more efficient
return Object.assign(smallPayload, bigPayload)
// this is still okay assuming `smallPayload` only has a few properties
return Object.assign({}, smallPayload, bigPayload)
}
See if for yourself, try the code snippet below. Note: It takes awhile to run.
const rand = () => (Math.random() + 1).toString(36).substring(7)
function combineBigObjects() {
console.log('Please wait...creating the test objects...')
const obj = {}
const obj2 = {}
for (let i = 0; i < 100000; i++) {
const key = rand()
obj[rand()] = {
[rand()]: rand(),
[rand()]: rand(),
}
obj2[rand()] = {
[rand()]: rand(),
[rand()]: rand(),
}
}
console.log('Combine big objects using spread:')
console.time()
const result1 = {
...obj,
...obj2,
}
console.timeEnd()
console.log('Combine big objects using assign:')
console.time()
Object.assign({}, obj, obj2)
console.timeEnd()
console.log('Combine big objects using assign, but mutates first obj:')
console.time()
Object.assign(obj, obj2)
console.timeEnd()
}
combineBigObjects()
function combineSmallObjects() {
const firstObject = () => ({ [rand()]: rand() })
const secondObject = () => ({ [rand()]: rand() })
console.log('Combine small objects using spread:')
console.time()
for (let i = 0; i < 500000; i++) {
const finalObject = {
...firstObject(),
...secondObject(),
}
}
console.timeEnd()
console.log('Combine small objects using assign:')
console.time()
for (let i = 0; i < 500000; i++) {
const finalObject = Object.assign({}, firstObject(), secondObject())
}
console.timeEnd()
console.log('Combine small objects using assign, but mutates first obj:')
console.time()
for (let i = 0; i < 500000; i++) {
const finalObject = Object.assign(firstObject(), secondObject())
}
console.timeEnd()
}
combineSmallObjects()
Other answers are old, could not get a good answer.
Below example is for object literals, helps how both can complement each other, and how it cannot complement each other (therefore difference):
var obj1 = { a: 1, b: { b1: 1, b2: 'b2value', b3: 'b3value' } };
// overwrite parts of b key
var obj2 = {
b: {
...obj1.b,
b1: 2
}
};
var res2 = Object.assign({}, obj1, obj2); // b2,b3 keys still exist
document.write('res2: ', JSON.stringify (res2), '<br>');
// Output:
// res2: {"a":1,"b":{"b1":2,"b2":"b2value","b3":"b3value"}} // NOTE: b2,b3 still exists
// overwrite whole of b key
var obj3 = {
b: {
b1: 2
}
};
var res3 = Object.assign({}, obj1, obj3); // b2,b3 keys are lost
document.write('res3: ', JSON.stringify (res3), '<br>');
// Output:
// res3: {"a":1,"b":{"b1":2}} // NOTE: b2,b3 values are lost
Several more small examples here, also for array & object:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
The ways to (1) create shallow copies of objects and (2) merge multiple objects into a single object have evolved a lot between 2014 and 2018.
The approaches outlined below became available and widely used at different times. This answer provides some historical perspective and is not exhaustive.
Without any help from libraries or modern syntax, you would use for-in loops, e.g.
var mergedOptions = {}
for (var key in defaultOptions) {
mergedOptions[key] = defaultOptions[key]
}
for (var key in options) {
mergedOptions[key] = options[key]
}
options = mergedOptions
jQuery 1.0 has jQuery.extend():
options = $.extend({}, defaultOptions, options)
Underscore.js 1.0 has _.extend()
options = _.extend({}, defaultOptions, options)
2ality published an article about Object.assign() coming to ES2015
object-assign published to npm.
var objectAssign = require('object-assign')
options = objectAssign({}, defaultOptions, options)
The Object Rest/Spread Properties syntax proposed for ES2016.
Object.assign is supported by Chrome (45), Firefox (34) and Node.js (4). Polyfill is required for older runtimes though.
options = Object.assign({}, defaultOptions, options)
The Object Rest/Spread Properties proposal reaches stage 2.
The Object Rest/Spread Properties syntax did not get included in ES2017, but is usable in Chrome (60), Firefox (55), and Node.js (8.3). Some transpilation is needed for older runtimes though.
options = { ...defaultOptions, ...options }
Object.assign is necessary when the target object is a constant and you want to set multiple properties at once.
For example:
const target = { data: "Test", loading: true }
Now, suppose you need to mutate the target with all properties from a source:
const source = { data: null, loading: false, ...etc }
Object.assign(target, source) // Now target is updated
target = { ...target, ...source) // Error: cant assign to constant
Keep in mind that you are mutating the target obj, so whenever possible use Object.assign with empty target or spread to assign to a new obj.
This is now part of ES6, thus is standardized, and is also documented on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator
It's very convenient to use and makes a lot of sense alongside object destructuring.
The one remaining advantage listed above is the dynamic capabilities of Object.assign(), however this is as easy as spreading the array inside of a literal object. In the compiled babel output it uses exactly what is demonstrated with Object.assign()
So the correct answer would be to use object spread since it is now standardized, widely used (see react, redux, etc), is easy to use, and has all the features of Object.assign()
I'd like to add this simple example when you have to use Object.assign.
class SomeClass {
constructor() {
this.someValue = 'some value';
}
someMethod() {
console.log('some action');
}
}
const objectAssign = Object.assign(new SomeClass(), {});
objectAssign.someValue; // ok
objectAssign.someMethod(); // ok
const spread = {...new SomeClass()};
spread.someValue; // ok
spread.someMethod(); // there is no methods of SomeClass!
It can be not clear when you use JavaScript. But with TypeScript it is easier if you want to create instance of some class
const spread: SomeClass = {...new SomeClass()} // Error
The spread operator spread the Array into the separate arguments of a function.
let iterableObjB = [1,2,3,4]
function (...iterableObjB) //turned into
function (1,2,3,4)
We’ll create a function called identity that just returns whatever parameter we give it.
identity = (arg) => arg
And a simple array.
arr = [1, 2, 3]
If you call identity with arr, we know what’ll happen
