Avoid type casting in Typescript

Question

I have the following situation:

const data = {
  bmw: 10,
  opel: 2
}

type customType = keyof typeof data

const myKey = 'bmw';

console.log(data[myKey as customType])

Doing this data[myKey as customType] i want to access the key from data . I read that using as we use type casting and this is not very safe.

Question : How to rewrite the code above avoiding the use of as operator?

Answer 1

Your code compiles perfectly well if you simply remove the typecast. Like so:

const data = {
  bmw: 10,
  opel: 2
}

type customType = keyof typeof data

const myKey = 'bmw';

console.log(data[myKey])

This is because you have defined myKey as a const variable, which means it has the type bmw (since it can never change) which is assignable the index type of 'bmw' | 'opel' 'bmw' | 'opel'

Answer 2

just define your variable as customType :

const data = {
  bmw: 10,
  opel: 2
}

type customType = keyof typeof data

const myKey: customType = 'bmw';

console.log(data[myKey])

or directly:

const myKey: keyof typeof data = 'bmw';

specifying that myKey is of type customType will ensure that you can put only bmw or opel as a value, otherwise will produce compilation error; therefore is slightly "safer" than "as".