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Function: index()

index<A>(idx): Lens<readonly A[], A | undefined>

Defined in: lens/indexLens/index.ts:49

Creates a Lens focusing on a specific index in an array.

Allows immutable updates to array elements at a specific position. If the index is out of bounds, get returns undefined and set has no effect.

Type Parameters​

A​

A

Parameters​

idx​

number

The array index to focus on

Returns​

Lens<readonly A[], A | undefined>

A Lens focusing on the element at the specified index

Examples​

const firstLens = index<number>(0)
const thirdLens = index<number>(2)

const numbers = [1, 2, 3, 4, 5]

firstLens.get(numbers) // 1
thirdLens.get(numbers) // 3

firstLens.set(10)(numbers) // [10, 2, 3, 4, 5]
thirdLens.set(30)(numbers) // [1, 2, 30, 4, 5]
// With objects in arrays
interface Todo {
  id: number
  text: string
  done: boolean
}

const todos: Todo[] = [
  { id: 1, text: 'Buy milk', done: false },
  { id: 2, text: 'Walk dog', done: true }
]

const secondTodoLens = index<Todo>(1)
secondTodoLens.get(todos) // { id: 2, text: 'Walk dog', done: true }

See​

  • prop - For object property access
  • path - For nested property access