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...

Ryan Cavanaugh · Accepted Answer · 2016-10-25 14:48:35Z

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

Thank you! Until keyof is out I ended up doing something like this: type AllSteps = 'stepOne' | 'stepTwo' | 'stepThree'; interface Steps { [index: string]: boolean; } function markStepDone (step: AllSteps) {

Collectives™ on Stack Overflow

Typescript: type safety for interface properties in a function declaration

1 Answer 1

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Related