Checking if object[prop] is not undefined does not eliminate undefined type from object[prop] (type guard)

[mckravchyk]

Bug Report

🔎 Search Terms

type guard object property

🕗 Version & Regression Information

I'm using 4.2.4 in production, I don't know if this bug has occurred in prior versions.

I have tested in the playground that the bug also occurs in 4.3.x and 4.4 beta

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about it.

⏯ Playground Link

Playground link with relevant code

💻 Code

const kTableColumns = ['id', 'title', 'icon'];

type TableData = {
    id: string
    title: string
    icon: string
}

type Metadata = {
    url: string
}

type UpdateProps = Partial<Omit<TableData, 'id'>> & Partial<Metadata>;

function update(props: UpdateProps) {
    const tableUpdate: Array<{ column: string, value: string }> = [];

    kTableColumns.forEach((column) => {
        if (typeof props[column as keyof UpdateProps] !== 'undefined') {
            // Explicitly checked that props[columns] is not undefined, yet I have to
            // use non-null assertion operator to satisfy the compiler.
            // Error: string | undefined is not assignable to type string
            tableUpdate.push({ column, value: props[column as keyof UpdateProps]}) // Only ! can fix it.
            // Besdies this, the `as keyof UpdateProps` should be redundant after
            // checking that the value is not undefined.
        }
    });
}

🙁 Actual behavior

After validating that prop[columns] is not undefined, the compiler still treats prop[columns]as possiblyundefined`.

A non-null assertion operator at the end of the value is a workaround, but it feels like it should not be there.

🙂 Expected behavior

The type of prop[columns] inside the if clause should not be undefined.

Additionally, using column as keyof UpdateProps should not be required after validating that column is a valid key of the object (though I understand if there are valid reasons against it, I'm mostly concerned about the first issue).

[RyanCavanaugh]

Duplicate #44899 , others

[typescript-bot]

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