In TypeScript, How to cast boolean to number, like 0 or 1

You can convert anything to boolean and then to a number by using +!!:

const action: string = actions[+!!isPlay]

This can be useful when for example you want at least two out of three conditions to hold, or exactly one to hold:

const ok = (+!!something)  + (+!!somethingelse) + (+!!thirdthing) > 1
const ok = (+!!something)  + (+!!somethingelse) + (+!!thirdthing) === 1

You can't just cast it, the problem is at runtime not only at compile time.

You have a few ways of doing that:

let action: string = actions[isPlay ? 1 : 0];
let action: string = actions[+isPlay];
let action: string = actions[Number(isPlay)];

Those should be fine with both the compiler and in runtime.