Typescript supports Conditional Types. But when I try to set value of op as string, it gives me error, how do I check the type and assign value?

export const t = <IsMulti extends boolean = false>(): void => {
    const value = 'test';
    type S = string;
    type D = IsMulti extends true ? S[] : S;
    const op: D = value;
    console.log(op);
};

Error: Type 'string' is not assignable to type 'D'.

I have even tried adding a parameter of same type IsMulti and adding check based on it

export const t = <IsMulti extends boolean = false>(isMulti: IsMulti): void => {
    const value = 'test';
    type S = string;
    type D = IsMulti extends true ? S[] : S;
    if (!isMulti) {
        const op: D = value;
        console.log(op);
    }
};

still it gives the same error.