使用 TypeScript 提取缩小的对象类型

Question

I have a type MyType that is equivalent to the following type definition:

type MyTypeEquivalent = { foo: 'one', value: number } | { foo: 'two', value: string }

How can I extract the type where foo is 'one' , ie:

type DesiredExtractedType = { foo: 'one', value: number }

I am aware that I can do it programmatically, ie:

if (obj.foo === 'one') {
  obj.value // <-- type will be number
}

But I need the actual type definition, not the TS type inference.

Answer 1

You can use MyType & {foo: 'one'} to achieve this. You're telling TS that foo is 'one' , it can infer the type of the rest of the object from that.