TypeScript implementation for cloneable string iterator

cloneableStringIterator.ts

export interface StringIterator extends IterableIterator<string> {
    clone(): StringIterator 
}

export function createStringIterator(s: string, index: number = 0): StringIterator {
    return {
        next(): IteratorResult<string> {
            if (index >= s.length) {
                return {
                    done: true,
                    value: undefined,
                }
            }
            return {
                done: false,
                value: s[index++],
            }
        },
        [Symbol.iterator]() {
            return this
        },
        clone() {
            return createStringIterator(s, index)
        }
    }
}

Useful when you implement parser-like-things, and you might have to branch with multiple possibilities. Built-in iterator doesn't support proper clone. You can generalize this to other types whenever the original target s is immutable. Just change index into some closure like things.