TypeScript incorrectly infers logical operation result type with empty string

🔎 Search Terms

string logical disjunction type inference string logical inference type infer empty string logical operation type

⏯ Playground Link

Playground link with relevant code

💻 Code

const str: string | undefined = ''

const test = (str || undefined) && {some: 'thing'}

🙁 Actual behavior

Inferred type of operation is

"" | {
    some: string;
} | undefined

🙂 Expected behavior

Inferred type of operation is

{
    some: string;
} | undefined

because (str || undefined) prevents empty string to be returned

Start with the linked TypeScript Playground and the code sample using (str || undefined) && {some: 'thing'}. Trace the logical-operation type inference that produces the unexpected "" union member; done means the inferred type is only { some: string } | undefined, with coverage for this example.