Question 222506 in Javascript Question posted in
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Javascript – Type checking for null

I just wrote method in service and faced that types saying me that I may return null while I already made checking for null in if block

    public async getUserById(id: string): Promise<UserEntity> {
        const user = this.userRepository.getById(id); // returns <UserEntity | null>

        if (!user) { // checking for null
            throw new NotFoundUser(`Not found user`);
        }

         // Type 'UserEntity | null' is not assignable to type 'UserEntity'.
         // Type 'null' is not assignable to type 'UserEntity'.
        return user;
    }

I expected for throw exception in case if user variable is null and return UserEntity if is not null;

If I’m typing double exclamation mark there then problem is gone.

if (!!user) { // checking for null
            throw new NotFoundUser(`Not found user`);
}

But if I’ll type !!null in console, then it will return false, so in this case I’ll never get into throwing exception.
Why is such behavior?

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3

Answers


  1. Chosen as BEST ANSWER
    0

    Problem was with not placed await for async function. But that type issue definitely drove me away from real problem


  2. 0

    because !! similar to Boolean, in this line you do something like Boolean(null) so you get false because null in boolean is false you can check for null with user === null

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  3. 0

    Maybe try the following:

    public async getUserById(id: string): NonNullable<Promise<NonNullable <UserEntity>>> {}

    I’m not sure but I have a feeling that static hints can help type inference.

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