Why doesn't TypeScript complain about incorrect values in Partial types with computed keys?

Question

Why does TypeScript allow the following?

interface SubType {
    key: keyof MyType
}

interface MyType {
    a: string
    b: string
}

const container: SubType = { key: 'a' }

const test: Partial<MyType> = {
    [container.key]: 3
}

It correctly complains when key isn't actually a key in MyType , but it doesn't seem to care what I set the value to, even though MyType can only have string values.

Link to TS playrground

Answer 1

If the type of a computed property name is not a singleton (here the type of container.key is "a" | "b" ), then TypeScript isn't smart enough to validate the value at all. See the checkObjectLiteral function in src/compiler/checker.ts . Note that if you comment out the b: string member of MyType , so that the type of container.key becomes "a" , then the expected error appears. There's a little more information in this issue report .