Lodash assignInWith Custom Conflict Resolution

The Lodash _.assignInWith method offers a flexible way to merge objects by copying both own and inherited enumerable string-keyed properties from source objects to a destination object, while allowing developers to resolve property collisions using a customizer function. When standard shallow copying or simple overrides fall short, this method provides complete control over how conflicting keys are evaluated, merged, or discarded.

How _.assignInWith Works

_.assignInWith extends the functionality of _.assignIn (which behaves like Object.assign, but includes inherited prototype properties) by accepting an additional callback function, traditionally named customizer.

The syntax is:

_.assignInWith(object, sources, [customizer])

• object: The destination target object.
• sources: One or more source objects containing properties to assign.
• customizer: The function invoked to produce the assigned values.

The Customizer Function

Whenever _.assignInWith iterates through a property from a source object, it invokes the customizer function before making an assignment. The customizer receives five arguments:

1. objValue: The current property value in the destination object.
2. srcValue: The property value in the source object attempting to overwrite it.
3. key: The key name of the property being assigned.
4. object: The destination object.
5. source: The source object currently being read.

By inspecting these parameters, you can intercept any key collision and determine the outcome based on custom business logic.

Resolving Conflicts with Examples

By default, without a customizer, standard assignment replaces objValue with srcValue. With _.assignInWith, returning a value from the customizer sets that returned value on the destination key.

Example: Merging Arrays Instead of Overwriting

const _ = require('lodash');

function customizer(objValue, srcValue) {
  if (Array.isArray(objValue)) {
    return objValue.concat(srcValue);
  }
}

const defaults = { tags: ['javascript'], settings: { theme: 'dark' } };
const userConfig = { tags: ['webdev'], settings: { theme: 'light' } };

const result = _.assignInWith({}, defaults, userConfig, customizer);

console.log(result.tags); 
// Output: ['javascript', 'webdev']

In this scenario, rather than userConfig.tags replacing defaults.tags, the collision is detected, and the arrays are concatenated.

Example: Conditional Overrides Based on Key

You can target specific keys directly using the key parameter:

function resolveConflicts(objValue, srcValue, key) {
  if (key === 'score') {
    // Keep the higher score during a merge
    return Math.max(objValue || 0, srcValue);
  }
}

const playerState = { name: 'Alex', score: 50 };
const checkpoint = { name: 'Alex', score: 35 };

_.assignInWith(playerState, checkpoint, resolveConflicts);

console.log(playerState.score); 
// Output: 50

The Fallback to Default Assignment

If the customizer function returns undefined, Lodash falls back to its default assignment behavior, which simply overwrites objValue with srcValue. This design eliminates the need to write exhaustive switch or if/else statements for every single property; you only need to return values for the specific conflicts you wish to handle.

Handling Inherited Properties

Because _.assignInWith traverses the prototype chain, it inspects properties inherited through prototypes on the source objects. If an inherited property on a source object conflicts with an own property on the destination object, the customizer will still fire, giving you the ability to prevent or accept prototype pollution and unintended overrides dynamically.