Can you create a TypeScript type that is restricted to having only one property?

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Yes, it is possible to create a TypeScript type that is restricted to having only one property using the Record type. Here is an example:

type SinglePropertyType<K extends string, V> = Record<K, V> & { [key: string]: never };

In this example, K is the key of the single property and V is the value type.

The Record<K, V> type constraint ensures that the type has only one property with the specified key and value type. The second constraint { [key: string]: never } ensures that there are no other properties in the type.

Here is an example usage of the SinglePropertyType:

// This type is restricted to having only one property "name" of type string
type Person = SinglePropertyType<'name', string>;

// This is valid
const person: Person = { name: 'Alice' };

// This is invalid because it has more than one property
const person2: Person = { name: 'Bob', age: 30 };