How to Use Lodash keyBy to Create an Object Dictionary

This article explains how the _.keyBy function in the Lodash JavaScript library transforms an array of items into an object dictionary. You will learn the underlying mechanics of the method, how to define lookup keys using property names or custom functions, how the function handles duplicate keys, and why this pattern is preferred for optimizing data lookups in JavaScript applications.

What is _.keyBy?

The _.keyBy function is a utility in Lodash that constructs an object composed of keys generated from the results of running each element of an input collection through an iteratee. The corresponding value of each key is the original item from the array.

The signature is:

_.keyBy(collection, [iteratee=_.identity])

• collection: The array or collection to iterate over.
• iteratee: The criterion used to produce the keys. This can be a string property name or a custom callback function.

How _.keyBy Works Step-by-Step

When you pass an array to _.keyBy, Lodash executes the following process:

1. Initializes an empty object: Lodash creates a new, empty dictionary object ({}) to store the results.
2. Iterates over the collection: It traverses each element in the array from left to right.
3. Executes the iteratee: For every element, it resolves the key:
   • If the iteratee is a string, it extracts the property corresponding to that string name (e.g., 'id').
   • If the iteratee is a function, it passes the current element to the function and uses the returned value as the key.
4. Assigns the key-value pair: It sets accumulator[generatedKey] = currentElement.
5. Returns the dictionary: Once the iteration is complete, it returns the populated object.

Example 1: Grouping by a Property Name

The most common use case is transforming a list of objects with unique identifiers into a lookup dictionary using a property string:

const users = [
  { id: 'u101', name: 'Alice', role: 'Admin' },
  { id: 'u102', name: 'Bob', role: 'Developer' },
  { id: 'u103', name: 'Charlie', role: 'Designer' }
];

const usersById = _.keyBy(users, 'id');

Output:

{
  'u101': { id: 'u101', name: 'Alice', role: 'Admin' },
  'u102': { id: 'u102', name: 'Bob', role: 'Developer' },
  'u103': { id: 'u103', name: 'Charlie', role: 'Designer' }
}

Now, instead of using users.find(u => u.id === 'u102') (an \(O(n)\) search), you can access the user instantly using usersById['u102'] (an \(O(1)\) lookup).

Example 2: Using a Function Iteratee

If the desired key requires transformation or computation across multiple properties, you can supply a function:

const products = [
  { code: 'app', version: 1, name: 'Alpha' },
  { code: 'app', version: 2, name: 'Beta' }
];

const productsByCompositeKey = _.keyBy(products, item => `${item.code}_v${item.version}`);

Output:

{
  'app_v1': { code: 'app', version: 1, name: 'Alpha' },
  'app_v2': { code: 'app', version: 2, name: 'Beta' }
}

Handling Duplicate Keys

Because object keys must be unique, if multiple elements generate the same key, _.keyBy adopts a last-write-wins strategy. The element appearing later in the array will overwrite any earlier elements that share the same key:

const records = [
  { category: 'news', title: 'Post 1' },
  { category: 'news', title: 'Post 2' }
];

const result = _.keyBy(records, 'category');
// Result: { news: { category: 'news', title: 'Post 2' } }

If your objective is to preserve all items with matching keys, use _.groupBy instead, which groups duplicate matches into an array under each key.