How to assert a value is a valid key without knowing the object keys at compile-time?

Question

In TypeScript, is there any way to declare that a variable is a keyof some Record without knowing what the keys of the Record are?

Assume an API response returns a payload of JSON. I want to define a type for the keys of the payload so I can index through the payload and know that the value will not be undefined. Is this possible or do I have to simply check for undefined every time I index the payload object?

Example:

const payload: Record<string, ValidValue> = await myApiCall()

const prop1 = 'a'
const prop2 = 'b'

if (isValidKey(prop1, payload)) {
  const value1 = payload[prop1] // the type of `value1` should evaluate to `ValidValue`
}

const value2 = payload[prop2] // the type of `value2` should evaluate to `ValidValue | undefined`

Answer 1

I got it to work using a type guard function as well, but not returning key is keyof MyMap. Instead, I had a type for the payload object of Record<string, ValidValue | undefined> and then the type guard returns payload is Record<K, ValidValue> where K is the type of the key param.

You can play with it on TS Playground.

// Type alias for convenience
type Payload = Record<string, ValidValue | undefined>;

const payload: Payload = await myApiCall()

function isValidKey<K extends string>(key: K, payload: Payload): payload is Record<K, ValidValue> {
  return key in payload;
}

const prop1 = 'a'
const prop2 = 'b'

if (isValidKey(prop1, payload)) {
  const value1 = payload[prop1] // the type of `value1` should evaluate to `ValidValue` ✅
}

const value2 = payload[prop2] // the type of `value2` should evaluate to `ValidValue | undefined` ✅