Typescript: type safety for interface properties in a function declaration

Question

Let's say we have this simple example:

interface Steps {
  stepOne?: boolean;
  stepTwo?: boolean;
  stepThree?: boolean;
}

let steps: Steps = {};

function markStepDone (step: ???) {
  steps[step] = true;
}


markStepDone('anything');

How can I prevent it from allowing to pass 'anything' to this function and allow only ['stepOne', 'stepTwo', 'stepThree']?

I also tried to do it with enum, but turned out that you cannot use enum as an index signature...

Answer 1

What you're looking for is the keyof operator, which is being implemented this week (yes, really). It will look like this once it's ready:

function markStepDone (step: keyof Steps) {
  steps[step] = true;
}

An early PR with a different name (keysof) is here: https://github.com/Microsoft/TypeScript/pull/10425

In the meantime, string is a rough approximation, or the hand-written type "stepOne" | "stepTwo" | "stepThree" will give you the exact behavior of keyof Steps