assignWith (Lodash compatibility)

Implementing custom logic is recommended

This assignWith function is relatively slow due to complex customizer function processing.

Instead, use Object.assign and implement custom logic directly.

Assigns properties from source objects to a target object using a customizer function.

const result = assignWith(target, source1, source2, customizer);

Reference

assignWith(object, ...sources, customizer)

Use assignWith when you want to customize how properties are assigned. The customizer function determines the final value for each property.

import { assignWith } from 'es-toolkit/compat';

// Basic usage - assign only when undefined
const target = { a: 1, b: undefined };
const source = { b: 2, c: 3 };
const result = assignWith(target, source, (objValue, srcValue) => {
  return objValue === undefined ? srcValue : objValue;
});
// Returns: { a: 1, b: 2, c: 3 }

// Array merging
const target2 = { users: ['alice'] };
const source2 = { users: ['bob', 'charlie'] };
const result2 = assignWith(target2, source2, (objValue, srcValue) => {
  if (Array.isArray(objValue)) {
    return objValue.concat(srcValue);
  }
});
// Returns: { users: ['alice', 'bob', 'charlie'] }

// Multiple sources with customizer
const target3 = { a: 1 };
const result3 = assignWith(target3, { b: 2 }, { c: 3 }, (objValue, srcValue) => {
  return objValue === undefined ? srcValue : objValue;
});
// Returns: { a: 1, b: 2, c: 3 }

Parameters

• object (any): The target object to which properties will be assigned.
• ...sources (any[]): The source objects from which properties will be copied.
• customizer (function): A function that determines the value to assign. In the form (objValue, srcValue, key, object, source) => any.

Returns

(any): Returns the target object with values determined by the customizer function.