Typescript access and assign object property by dynamic key

Question

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 .

Answer 1

I managed to find a not-too-dirty workaround that works for my use case. See example below:

//solution
myObj = {
    ...myObj,
    [myTestKey]: myObj2[myTestKey]
}

Note: myObj is now let instead of const

Typescript Playground

Answer 2

You need to add type as -

const myTestKey: keyof TestInterface = 'testKey';

Playground