Typescript error because nested value might be null even after I check it's not null

[eliezra236]

🔎 Search Terms

optional null error

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?target=10&pretty=false#code/JYOwLgpgTgZghgYwgAgIIFcwAsD2VkDeAUMqcsACYBcyAzmFKAOYDcRAvkUaJLIigAUc9QiTKUa9RiFZjScTLig0M2PG07dw0eEmRD6AdWDYAcugA2F0WXLU6DZm1sK1ytIrzIAPshCWLDS4EHBARAFtgAA8ICgARODA4ZABeZABtORtbcXsAcgBGACYAZjyAGiyXT3d-K0rbdgayYhzcmjzSgBYKqrJXJRpWtpyJZEK8vtImrM4AXTYiGHQQBDBgUOQmCDADMAAhAE8ASQoACjGpZgBKGj2fPwDs0hCwsGQoCFpU5EiY+MScAAdDBQOczp8QlAKNdUgA+D4QKEUIGUVIpNKUWFwb57YxmALOcQwZBnACEn1osOGbU+YHQUBAjysROmXFswBJEK+QIGeGpU0R9MZiNorJyAHoJcgACpYFAhCgoABGEAsOAA7sgNcArNq8ABrIGCqVChlMmmS6W2IG2ynNNqm6puGiU3k1E3SziNLhZOnm5mBDhEIA

💻 Code

interface Author {
    id: string;
}

interface Post {
    id: string;
    author: Author;
}

interface PostWithNull {
    id: string;
    author: Author | null;
}

const mixedData = [
    {
        id: '123',
        author: null,
    },
    {
        id: '234',
        author: {
            id: '1'
        },
    }
];

function getPostById(id: string): Post | null {
    // Simulate Drizzle response type -> DO NOT CHANGE
    const res = mixedData.find((record) => record.id === id) as PostWithNull;
    if (!res) {
        return null;
    }

    // The problematic part
    if (res.author) {
        return res; // Error: Type 'PostWithNull' is not assignable to type 'Post'.
        // The code below will work.
        // return {
        //     ...res,
        //     author: res.author
        // }
    }


    return null;
}

🙁 Actual behavior

Type 'PostWithNull' is not assignable to type 'Post'.
  Types of property 'author' are incompatible.
    Type 'Author | null' is not assignable to type 'Author'.
      Type 'null' is not assignable to type 'Author'.

🙂 Expected behavior

Since I validate author is not null, I expect the type Post to be correct, but instead I am getting an error that type is incorrect because it might be null

Additional information about the issue

No response

[MartinJohns]

This is working as intended. Checking res.author does not turn res into a different type.

Duplicate of #42384.

[eliezra236]

@MartinJohns but it literally means that it cannot be null, and later on the error specify that's the reason it cannot be returned, recasting it is literally creating the same object and then it accept it as correct return type

[MartinJohns]

My comment from the other issue:

Quoting @jack-williams:

Type guards do not propagate type narrowings to parent objects. The narrowing is only applied upon access of the narrowed property which is why the destructing function works, but the reference function does not. Narrowing the parent would involve synthesizing new types which would be expensive. More detail in this comment.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.