Consider the following scenario:
interface Steps {
stepOne?: boolean;
stepTwo?: boolean;
stepThree?: boolean;
}
let steps: Steps = {};
function markStepDone (step: ???) {
steps[step] = true;
}
markStepDone('anything');
Is there a way to restrict the input to only ['stepOne', 'stepTwo', 'stepThree'] for this function and prevent other values like 'anything'?
I attempted using an enum but encountered limitations due to being unable to use it as an index signature...
The upcoming feature you're seeking is the keyof operator, set to be rolled out this week (yes, it's happening). Once implemented, it will appear as follows:
function finishTask (task: keyof Tasks) {
tasks[task] = true;
}
An early PR utilizing a different name (keysof) can be found here: https://github.com/Microsoft/TypeScript/pull/10425
Meanwhile, using string can serve as a temporary solution, or specifying a manual type like
"taskOne" | "taskTwo" | "taskThree"keyof Tasks
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