JS: How to convert from string to enum to map key?

Question

Snippet:

enum Color {
    RED = 0,
    GREEN
}

const myMap = new Map<Color, string>([
    [Color.GREEN, "foo"],
    [Color.RED, "bar"]
])


const red_s = "RED"
const myColor: Color = red_s as unknown as Color
console.log(`myColor ${myColor}`)

const mapVal = myMap.get(myColor)
if (mapVal) {
    console.log(`mapVal ${mapVal}`)
}
else {
    console.log(`no mapVal found`)
}

Why is the enum myColor not found in myMap?

How can I use the enum as a key for the map?

Color.GREEN is 1 and Color.RED is 0, they're not the strings "GREEN" and "RED". You'll see that if you log these values or your Map.Color[Color.RED] is "RED" — Nick Parsons
– Nick Parsons, Commented Dec 1, 2022 at 10:58
@NickParsons: I do not pass a string to .get(). The type of myColor is Color. And the map is declared with keys of type Color. How can I make this work? — Juergen
– Juergen, Commented Dec 1, 2022 at 11:05
Sean's answer explains what's going on well and how it can be fixed — Nick Parsons
– Nick Parsons, Commented Dec 1, 2022 at 11:10

Sean Anglim · Accepted Answer · 2022-12-01 11:07:33Z

3

The as keyword only tells the compiler to treat a variable as a certain type. It does not actually change the underlying type. If you run a typeof check, you will see myColor is a string, not Color:

const red_s = "RED"
const myColor: Color = red_s as unknown as Color
console.log(`${typeof myColor}`) // Prints string

To set red_s as an enum, you need to do:

const red_s: Color = Color["RED"];

edited Dec 1, 2022 at 11:07

answered Dec 1, 2022 at 11:06

Sean Anglim

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2 Comments

Juergen Over a year ago

Thank you. It is really confusing if we declare myColor with type Color, but really it is not.

Sean Anglim Over a year ago

No problem. The compiler would have thrown an error when you initially tried to cast red_s as Color. But then you added the as unknown to get around the error. This should generally be avoided!

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