Typescript conditional types make room for the inclusion of null values

Consider this scenario involving conditional types:

type MyType = 'Number' | 'String';

interface Test<T extends MyType> {
  bar: T extends 'Number' ? 25 : '25'
}

When attempting the following assignment:

const test: Test<'Number'> = {bar: null}

No errors are reported, indicating valid TypeScript syntax. However, hovering over bar displays a tooltip stating

(property) Test<"Number">.bar: 25
, suggesting that TypeScript interprets the type correctly.

Why does the assignment not raise an error then? What steps can be taken to address this?

Answer №1

To activate the compiler option, simply use either --strict or --strictNullChecks.

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