Using the Lodash setWith Customizer Function

Lodash's _.setWith method extends the functionality of _.set by accepting a customizer callback to control how intermediate objects along a specified path are produced. This article explains how the customizer function works, its arguments, and how to use it to guide path generation, such as forcing plain object creation instead of default arrays or instantiating specific data structures.

Understanding the customizer Signature

The _.setWith method follows this syntax:

_.setWith(object, path, value, [customizer])

When traversing or creating paths, _.setWith invokes the customizer function for each unresolved segment along the path. The callback accepts up to three arguments:

• nsValue: The current value of the resolved path segment.
• key: The key or index of the current path segment.
• nsObject: The parent object where the property is being assigned.

If the customizer returns undefined, _.setWith falls back to its default behavior: creating an array if the next segment is a number or numeric string, and creating a plain object otherwise. If the customizer returns any other value, that returned value becomes the intermediate container at that point in the path.

Preventing Default Array Creation

By default, Lodash creates arrays when intermediate path segments are integers. A common use case for the customizer is to prevent this and force plain object creation:

const _ = require('lodash');

const object = {};

// Without customizer: creates an array at key '0'
// _.set(object, '[0][1]', 'a'); // => { '0': [ <1 empty item>, 'a' ] }

// With customizer: force plain objects for numeric keys
_.setWith(object, '[0][1]', 'value', (nsValue) => {
  return _.isObject(nsValue) ? nsValue : {};
});

console.log(object);
// Output: { '0': { '1': 'value' } }

In this implementation, the customizer checks if nsValue is already an object. If it is not, it returns a new object {} instead of allowing Lodash to generate an array.

Using Custom Constructors and Prototypes

You can also use the customizer to inject custom class instances or specific prototypes into the hierarchy:

class CustomNode {
  constructor() {
    this.isCustom = true;
  }
}

const tree = {};

_.setWith(tree, 'branch.leaf', 'data', (nsValue) => {
  return _.isObject(nsValue) ? nsValue : new CustomNode();
});

console.log(tree.branch instanceof CustomNode); 
// Output: true
console.log(tree.branch.leaf); 
// Output: 'data'

Selective Customization Using Path Keys

Because the key argument is provided, intermediate objects can be created conditionally based on property names:

const config = {};

_.setWith(config, 'settings.list.0', 'item', (nsValue, key) => {
  if (key === 'list') {
    return [];
  }
  return _.isObject(nsValue) ? nsValue : {};
});

console.log(Array.isArray(config.settings.list)); 
// Output: true

By returning defined values selectively, the customizer function gives complete control over intermediate container initialization while preserving Lodash's safe path traversal.