I am trying to assign a value from an object of type TestInterface to another object of type TestInterface by accessing it with a dynamic key of type keyof TestInterface. However typescript seems to forget that this key is the same on both instances and therefore throws an error.

Since this sentence is hard to read, here is an example:

interface TestInterface {
    testKey?: NestesOptInterface;
    optKey?: string;
}

interface NestesOptInterface {
    key1?: string;
    key2?: string
}

const myObj : TestInterface= {

}

const myObj2 : TestInterface = {
    testKey: {
        key1: "test",
    }
}

const attributes : string[] = ['testKey', 'optKey']

const myTestKey: keyof TestInterface = attributes[0] as keyof TestInterface

/**Type 'string | NestesOptInterface | undefined' is not assignable to type '(NestesOptInterface & string) | undefined'.
 * Type 'NestesOptInterface' is not assignable to type 'NestesOptInterface & string'.
 * Type 'NestesOptInterface' is not assignable to type 'string'.(2322)**/
myObj[myTestKey] = myObj2[myTestKey]

//does work (obviously)
myObj['testKey'] = myObj2['testKey']

The tricky part is that testKey in the assignment of myTestKey should actually be any value of type string.

I was wondering whether this is a bug or how I can do this properly? Here is the Link to the Typescript Playground.