How to check if a given string key exists in Enum

Question

I have an enum defined like this

export enum someEnum {
    None = \'\',
    value1 = \'value1\',
    value2 = \'value2\',
         


        

Answer 1

Typescript enums are a little annoying because they have two sides - the atom used to name a value, and the actual value. These each have a different way to check them.

Let's use an example enum that represents actions a content blocker can take. We'll get a kebab-case value from our API but want to use a camelCase value in TS:

enum ActionType {
  block = "block",
  cssDisplayNone = "css-display-none",
  ignorePreviousRules = "ignore-previous-rules"
}

Now if we wanted to check if it was valid to say ActionType.cssDisplayNone in our code, we can check that with the in operator. However, if we have a value from an API and we want to see if the value we got is an ActionType, that won't work!

const canBlockCss = 'cssDisplayNone' in ActionType; // Returns true
const isValidAction = 'css-display-none' in ActionType; // Returns false!

In this case, we need to write a type guard:

function isActionType(test: any): test is ActionType {
    return (Object.values(ActionType).indexOf(test) !== -1);
}

const isValidAction = isActionType('css-display-none') // Returns true

This has the added bonus that if you have a variable of unknown type and pass it to a type guard, the return value will be included in Typescript's understanding of the variable, allowing you to cast it at the same time as you check it.