Object.assign()
The Object.assign()
static method
copies all enumerable
own properties from one or more
source objects to a target object. It returns the modified target
object.
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Syntax
Object.assign(target)
Object.assign(target, source1)
Object.assign(target, source1, source2)
Object.assign(target, source1, source2, /* …, */ sourceN)
Parameters
Return value
The target object.
Description
Properties in the target object are overwritten by properties in the sources if they have the same key. Later sources' properties overwrite earlier ones.
The Object.assign()
method only copies enumerable and
own properties from a source object to a target object. It uses
[[Get]]
on the source and [[Set]]
on the target, so it will
invoke getters and setters. Therefore it
assigns properties, versus copying or defining new properties. This may make it
unsuitable for merging new properties into a prototype if the merge sources contain
getters.
For copying property definitions (including their enumerability) into prototypes, use
Object.getOwnPropertyDescriptor()
and
Object.defineProperty()
instead.
Both String
and Symbol
properties are copied.
In case of an error, for example if a property is non-writable, a
TypeError
is raised, and the target
object is
changed if any properties are added before the error is raised.
Examples
Cloning an object
const obj = { a: 1 };
const copy = Object.assign({}, obj);
console.log(copy); // { a: 1 }
Warning for Deep Clone
For deep cloning, we need to use alternatives like structuredClone()
, because Object.assign()
copies property values.
If the source value is a reference to an object, it only copies the reference value.
const obj1 = { a: 0, b: { c: 0 } };
const obj2 = Object.assign({}, obj1);
console.log(obj2); // { a: 0, b: { c: 0 } }
obj1.a = 1;
console.log(obj1); // { a: 1, b: { c: 0 } }
console.log(obj2); // { a: 0, b: { c: 0 } }
obj2.a = 2;
console.log(obj1); // { a: 1, b: { c: 0 } }
console.log(obj2); // { a: 2, b: { c: 0 } }
obj2.b.c = 3;
console.log(obj1); // { a: 1, b: { c: 3 } }
console.log(obj2); // { a: 2, b: { c: 3 } }
// Deep Clone
const obj3 = { a: 0, b: { c: 0 } };
const obj4 = structuredClone(obj3);
obj3.a = 4;
obj3.b.c = 4;
console.log(obj4); // { a: 0, b: { c: 0 } }
Merging objects
const o1 = { a: 1 };
const o2 = { b: 2 };
const o3 = { c: 3 };
const obj = Object.assign(o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }
console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.
Merging objects with same properties
const o1 = { a: 1, b: 1, c: 1 };
const o2 = { b: 2, c: 2 };
const o3 = { c: 3 };
const obj = Object.assign({}, o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }
The properties are overwritten by other objects that have the same properties later in the parameters order.
Copying symbol-typed properties
const o1 = { a: 1 };
const o2 = { [Symbol("foo")]: 2 };
const obj = Object.assign({}, o1, o2);
console.log(obj); // { a : 1, [Symbol("foo")]: 2 } (cf. bug 1207182 on Firefox)
Object.getOwnPropertySymbols(obj); // [Symbol(foo)]
Properties on the prototype chain and non-enumerable properties cannot be copied
const obj = Object.create(
// foo is on obj's prototype chain.
{ foo: 1 },
{
bar: {
value: 2, // bar is a non-enumerable property.
},
baz: {
value: 3,
enumerable: true, // baz is an own enumerable property.
},
},
);
const copy = Object.assign({}, obj);
console.log(copy); // { baz: 3 }
Primitives will be wrapped to objects
const v1 = "abc";
const v2 = true;
const v3 = 10;
const v4 = Symbol("foo");
const obj = Object.assign({}, v1, null, v2, undefined, v3, v4);
// Primitives will be wrapped, null and undefined will be ignored.
// Note, only string wrappers can have own enumerable properties.
console.log(obj); // { "0": "a", "1": "b", "2": "c" }
Exceptions will interrupt the ongoing copying task
const target = Object.defineProperty({}, "foo", {
value: 1,
writable: false,
}); // target.foo is a read-only property
Object.assign(target, { bar: 2 }, { foo2: 3, foo: 3, foo3: 3 }, { baz: 4 });
// TypeError: "foo" is read-only
// The Exception is thrown when assigning target.foo
console.log(target.bar); // 2, the first source was copied successfully.
console.log(target.foo2); // 3, the first property of the second source was copied successfully.
console.log(target.foo); // 1, exception is thrown here.
console.log(target.foo3); // undefined, assign method has finished, foo3 will not be copied.
console.log(target.baz); // undefined, the third source will not be copied either.
Copying accessors
const obj = {
foo: 1,
get bar() {
return 2;
},
};
let copy = Object.assign({}, obj);
console.log(copy);
// { foo: 1, bar: 2 }
// The value of copy.bar is obj.bar's getter's return value.
// This is an assign function that copies full descriptors
function completeAssign(target, ...sources) {
sources.forEach((source) => {
const descriptors = Object.keys(source).reduce((descriptors, key) => {
descriptors[key] = Object.getOwnPropertyDescriptor(source, key);
return descriptors;
}, {});
// By default, Object.assign copies enumerable Symbols, too
Object.getOwnPropertySymbols(source).forEach((sym) => {
const descriptor = Object.getOwnPropertyDescriptor(source, sym);
if (descriptor.enumerable) {
descriptors[sym] = descriptor;
}
});
Object.defineProperties(target, descriptors);
});
return target;
}
copy = completeAssign({}, obj);
console.log(copy);
// { foo:1, get bar() { return 2 } }
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.assign |
Browser compatibility
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