Thoughts

Approach

Code

type CharArray<T extends string> = T extends `${infer First}${infer Rest}`
	? [First, ...CharArray<Rest>]
	: [];

type FirstUniqueCharIndex<
	T extends string,
	$chars = CharArray<T>,
	$index extends number[] = [],
	$ignore extends string = "",
> = $chars extends [infer First extends string, ...infer Rest]
	? First extends Rest[number]
		? FirstUniqueCharIndex<T, Rest, [...$index, 1], $ignore | First>
		: First extends $ignore
			? FirstUniqueCharIndex<T, Rest, [...$index, 1], $ignore>
			: $index["length"]
	: -1;

type a = FirstUniqueCharIndex<"leetcode">;
//   ^?
type b = FirstUniqueCharIndex<"loveleetcode">;
//   ^?
type c = FirstUniqueCharIndex<"">;
//   ^?
type d = FirstUniqueCharIndex<"aabb">;
//   ^?