docs/reference/compat/object/assignIn.md
::: warning Use Object.assign instead
This assignIn function operates slowly due to additional processing for copying inherited properties and value comparison logic.
Use the faster and more modern Object.assign instead.
:::
Assigns all properties (including inherited properties) from source objects to a target object.
const result = assignIn(target, ...sources);
assignIn(target, ...sources)Use assignIn when you want to copy both own and inherited properties from source objects to a target object. Unlike assign, it includes properties from the prototype chain.
import { assignIn } from 'es-toolkit/compat';
// Basic usage
const target = { a: 1, b: 2 };
const source = { b: 3, c: 4 };
const result = assignIn(target, source);
// Result: { a: 1, b: 3, c: 4 }
console.log(target === result); // true (target object is modified)
// Merging multiple source objects
const target2 = { a: 1 };
const source1 = { b: 2 };
const source2 = { c: 3 };
assignIn(target2, source1, source2);
// Result: { a: 1, b: 2, c: 3 }
// Copying inherited properties too
function Parent() {}
Parent.prototype.inherited = 'inheritedValue';
const child = Object.create(Parent.prototype);
child.own = 'ownValue';
const target3 = {};
assignIn(target3, child);
// Result: { own: 'ownValue', inherited: 'inheritedValue' }
// Also copies array index properties and length
const arr = [1, 2, 3];
arr.customProp = 'custom';
const target4 = {};
assignIn(target4, arr);
// Result: { '0': 1, '1': 2, '2': 3, customProp: 'custom' }
Unlike assign, this function copies inherited properties as well. It also has an optimization that doesn't overwrite if values are the same.
target (any): The target object to which properties will be copied....sources (any[]): The source objects from which properties will be copied. Both own and inherited properties are copied.(any): Returns the modified target object. The target object itself is modified and returned.