如何让 TypeScript 根据返回值识别函数参数的类型？

Question

function isType(
    value: any,
    type: 'undefined'|'object'|'boolean'|'number'|'bigint'|'string'|'symbol'|'function'
): boolean {
    return typeof value === type;
}

function getDefaultIfUndefined<T>(value: T|undefined, defaultValue: T): T
{
    return isType(value, 'undefined') ? defaultValue : value;
}

TypeScript complains that the return type of getDefaultIfUndefined() is T|undefined not just T. But it will always be T because the ternary ensures "defaultValue" will be returned if "value" is undefined.

TypeScript has no problem when it is written like this though:

function getDefaultIfUndefined<T>(value: T|undefined, defaultValue: T): T
{
    return typeof value === 'undefined' ? defaultValue : value;
}

Answer 1

I have it! That wasn't easy, look:

interface TypeMap {
    undefined: undefined;
    object: object;
    boolean: boolean;
    number: number;
    bigint: bigint;
    string: string;
    symbol: symbol;
    function: (...args: any[]) => any;
}
function isType<T extends keyof TypeMap>(
    value: any,
    type: T 
): value is TypeMap[T] {
    return typeof value === type;
}