Use `keyof typeof ` for enums

[qix]

Enum types in TypeScript are not actually just the enum values. The types land up being restricted to string internally rather than the actual values. This makes the types a little stricter.

[dangcuuson]

Hi qix,

Could you explain more about your case?

From my understanding, I think you mean that TypeScript treats enum as equivalent to string.

However, in my projects I have to use the enum values to assign to a variable with corresponding enum type.

E.g:

enum Color {
    RED = 'RED',
    GREEN = 'GREEN',
    BLUE = 'BLUE'
}

const color1: Color = Color.RED; // this one is ok
const color2: Color = 'RED'; // TypeScript complains in this case

however, compare enum value with string is ok

switch (color1) {
    case 'RED': // fine
    case Color.R:  // fine
    case 'YELLOW': { // TypeScript complains in this case
        break;
    }
    
    default: {
        break;
    }
}

I'm not sure whether if it's due to some different in tsconfig.json file or not.

The reason why I need to clarify this is because there is a catch here: keyof typeof enum refers to the keys of that enum, not the actual value. Therefore, it only works when the enum's keys are the same as its values. E.g:

// note that the enum keys were changed to R/G/B instead of RED/GREEN/BLUE
enum Color {
    R = 'RED',
    G = 'GREEN',
    B = 'BLUE'
}

const color1: Color = Color.R;
console.log(color1); // RED

const color2: keyof typeof Color = 'R'; // assign 'RED' will make TypeScript complains
console.log(color2); // R

This catch is not a problem in this library since the enum keys and values are always the same. However, it could lead to some troubles in the future :) Maybe we could wrap your implementation in a config option?