type-samurai/src/index-of.d.ts

Advanced utility types for Typescript

import { IsEqual } from "./equal";
import { Increment } from "./increment";
import { IsStringLiteral } from "./is-string-literal";
import { IsTuple } from "./is-tuple";

type _IndexOfArray<
  T extends readonly unknown[],
  Pivot,
  Index extends number = 0
> = T extends readonly [infer First, ...infer Rest extends unknown[]]
  ? IsEqual<First, Pivot> extends true
    ? Index
    : _IndexOfArray<Rest, Pivot, Increment<Index>>
  : -1;

type _IndexOfString<
  T extends string,
  Pivot,
  Index extends number = 0
> = T extends `${infer First}${infer Rest extends string}`
  ? IsEqual<First, Pivot> extends true
    ? Index
    : _IndexOfString<Rest, Pivot, Increment<Index>>
  : -1;

/**
 * Type version of `Array.prototype.indexOf()` and `String.prototype.indexOf()`. Returns the index of the second argument in the first argument
 * @example
 * ```ts
 * // 1
 * type Case1 = IndexOf<[1, 2, 3], 2>
 * // -1
 * type Case1 = IndexOf<[1, 2, 3], 4>
 * // 1
 * type Case3 = IndexOf<'123', '2'>
 * // -1
 * type Case4 = IndexOf<'123', '4'>
 * ```
 */
export type IndexOf<
  T extends readonly unknown[] | string,
  Pivot extends T extends string ? string : unknown
> = T extends string
  ? IsStringLiteral<T> extends true
    ? _IndexOfString<T, Pivot>
    : never
  : T extends readonly unknown[]
  ? IsTuple<T> extends true
    ? _IndexOfArray<T, Pivot>
    : never
  : never;