For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/array/keyBy.md.
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    keyBy es-toolkit

    Returns a new object with array elements converted to key-value pairs using a key-generating function.

    const result = keyBy(arr, getKeyFromItem);

    Usage

    keyBy(arr, getKeyFromItem)

    Use keyBy when you want to create a key-indexed object for quick lookup of each element in an array. By providing a function that generates a unique key from each element, it creates an object where you can access elements using that key. If multiple elements generate the same key, the last element is used.

    import { keyBy } from 'es-toolkit/array';

// Use object's id property as key
const users = [
  { id: 1, name: 'john' },
  { id: 2, name: 'jane' },
  { id: 3, name: 'bob' },
];
keyBy(users, user => user.id);
// Returns: {
//   1: { id: 1, name: 'john' },
//   2: { id: 2, name: 'jane' },
//   3: { id: 3, name: 'bob' }
// }

// Use string property as key
const products = [
  { category: 'fruit', name: 'apple' },
  { category: 'fruit', name: 'banana' },
  { category: 'vegetable', name: 'carrot' },
];
keyBy(products, item => item.category);
// Returns: {
//   fruit: { category: 'fruit', name: 'banana' }, // Last fruit element
//   vegetable: { category: 'vegetable', name: 'carrot' }
// }

    Complex key generation logic can also be used.

    import { keyBy } from 'es-toolkit/array';

// Combine multiple properties to create a key
const orders = [
  { date: '2023-01-01', customerId: 1, amount: 100 },
  { date: '2023-01-01', customerId: 2, amount: 200 },
  { date: '2023-01-02', customerId: 1, amount: 150 },
];
keyBy(orders, order => `${order.date}-${order.customerId}`);
// Returns: {
//   '2023-01-01-1': { date: '2023-01-01', customerId: 1, amount: 100 },
//   '2023-01-01-2': { date: '2023-01-01', customerId: 2, amount: 200 },
//   '2023-01-02-1': { date: '2023-01-02', customerId: 1, amount: 150 }
// }

    Parameters

    arrreadonly T[]required

    The array to convert to an object.

    getKeyFromItem(item: T, index: number, array: T[]) => Krequired

    A function that generates a key from each element, its index, and the array.

    Returns

    Returns an object with the generated keys as property names and the corresponding elements as values. If multiple elements generate the same key, the last element is used as the value.

    Record<K, T>

    Examples

    // Using index parameter
const items = ['a', 'b', 'c'];
const result = keyBy(items, (item, index) => index);
// Returns: { 0: 'a', 1: 'b', 2: 'c' }

// Using array parameter
const numbers = [10, 20, 30];
const result2 = keyBy(numbers, (item, index, arr) => (item > arr[0] ? 'large' : 'small'));
// Returns: { small: 10, large: 30 }
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.