CODEWITHSUNDEEP
typescript
nth-chile
nth-chile

Reputation: 735

Check if string is member of Union type

To avoid type casting, I would like to learn when it's possible to safely determine the type of given values. I can do this with string enums like this:

enum Category {
    circle = "circle",
    square = "square",
}

const isObjValue = (val: any, obj: any) => Object.values(obj).includes(val)

isObjValue('circle', Category) // true

But how would I do it with string Unions?

const Category = "circle" | "square"

const isValidCategory = (string) => {
  if (/* string is in Category */) {
    return string as Category
  }
}

Still learning TS!

Upvotes: 2

Views: 2588

Answers (2)

Fred Johnson
Fred Johnson

Reputation: 2695

To avoid having to remember to always update both the type and "is" check function, you can instead use an array and have TS convert it to a union:

const ALL_CATEGORIES = ["circle", "square"] as const;
type typedCategoryList = typeof ALL_CATEGORIES;
type Category = typedCategoryList[number]; // this compiles to 'circle' | 'square'

const isCategory = (value: string) : value is Category => {
   return ALL_CATEGORIES.includes(value as Category )
}

Upvotes: 3

touchmarine
touchmarine

Reputation: 2058

If you have a dynamic value, you can create a user-defined type guard, to narrow the type of that value.

type Category = "circle" | "square"

// User-defined type guard that narrows the s from string type
// to Category type if s is either "circle" or "square".
function isCategory(s: string): s is Category {
    return s === "circle" || s === "square"
}

const a: string = "some dynamic string"
if (isCategory(a)) {
    // a type here is Category
    console.log(a, "is Category")
} else {
    // a type here is string
    console.log(a, "is not Category")
}

Upvotes: 4

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