indexing optional attributes fails when indexing with a key that is keyof interface

[sezanzeb]

Bug Report

🔎 Search Terms

• TS2532
• keyof
• undefined
• interface
• optional
• Object is possibly 'undefined'

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.0.0-dev.20230221#code/JYOwLgpgTgZghgYwgAhgezcg3gWAFDKHIBGcUA-AFzIgCuAtsdPgL774y0gJjBogk4ALwAU+IsgCOtAB7V0aADTiiMYBAA2AE2oBrCAE80MVBnwBKagDc0wLdhWFgJkdJkBtNZq0BdZAEIAXkDkLi0INRAILXMHAgkiBH4AZzQNCAA6DTQAc1dZT3VtPwBqZABGc0dkNjxaoA

💻 Code

interface foo {
    bar?: number
}

function baz(
    qux: foo,
    field: keyof foo
): void {
    if (qux[field] !== undefined) {
        console.log(qux[field] + 1)
    }
}

🙁 Actual behavior

Object is possibly 'undefined' error

🙂 Expected behavior

To work just like if field is set to 'bar' instead of keyof foo

interface foo {
    bar?: number
}

function baz(
    qux: foo
): void {
    const field = 'bar';
    if (qux[field] !== undefined) {
        console.log(qux[field] + 1)
    }
}

[MartinJohns]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal. Just store it in a local const variable.

[sezanzeb]

That works, thanks!

interface foo {
    bar?: number
}

function baz(
    qux: foo,
    field_: keyof foo
): void {
    const field = field_;

    if (qux[field] !== undefined) {
        console.log(qux[field] + 1)
    }
}

[sezanzeb]

This does not work if there are multiple optional fields:

https://www.typescriptlang.org/play?ts=5.0.0-dev.20230221#code/JYOwLgpgTgZghgYwgAhgezcg3gWAFDKHIBGcUAjAPwBcyIArgLbHT5ElkBMNdTLU+AL758MeiARhgaEBwBeACjZEAjvQAetdGgA0ywjGAQANgBMA+rQDWEAJ5oYqDPgCUtAG5pgp7PuQIZAGcwVCMzZABeUJMLAG4RAiJgRwU1dQBtQxiAXWQAQgio8VMIQxAIUxdfRPZCAJBAtGMIADpjNABzVI1MsNNcgGpkchc-YTxBIA

interface foo {
    bar1?: number
    bar2?: number
}

function baz(
    qux: foo,
    field_: keyof foo
): void {
    const field = field_;

    if (qux[field] !== undefined) {
        console.log(qux[field] + 1)
    }
}

same error, "Object is possibly 'undefined'"

[sezanzeb]

Oh, you were probably talking about storing qux[field] in a const. That works for the second case as well:

interface foo {
    bar1?: number
    bar2?: number
}

function baz(
    qux: foo,
    field: keyof foo
): void {
    const blub = qux[field];
    if (blub !== undefined) {
        console.log(blub + 1)
    }
}

[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.