Type narrowed too much about typescript

[fatcerberus]

@craigphicks Yes, IIFEs are special-cased: since the compiler can readily see from the syntax that the function will always be called immediately (and exactly once), it's treated the same as if the code was written inline.

[MartinJohns]

Another duplicate of #9998.

[fjolliton]

Is there anything to prevent to narrow such types? (a configuration option or a directive to put somewhere)

[MartinJohns]

No. It's all written in that issue. You can use a type assertion to prevent the initial narrowing: 42 as undefined | number.

[craigphicks]

Note: Typescript is aware that the value of a can change in a synchronous IIFE

const test = () => {
    let a: undefined | number = undefined;
    // Typescript is aware that the value of `a` can change in a synchronous IIFE
    (() => { a = 1; })();
    a; // number
};
test();

[typescript-bot]

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