Issue with assignment in TypeScript: strings and string enums are not interchangeable

Question

Issue with assignment in TypeScript: strings and string enums are not interchangeable

I have been struggling to find adequate information in the Typescript handbook on how to correctly utilize string enums.
Here is my example:

enum ICE_CREAM {
    strawberry = "STRAWBERRY",
    vanilla = "VANILLA"
}

type TORDER = {
    greetings: string,
    flavor: ICE_CREAM
}

const mockData: TORDER = {
    greetings: "Hello",
    flavor: "VANILLA",
}

This results in an error message stating:

'"VANILLA"' is not assignable to type 'ICE_CREAM'.(2322)

My goal is to ensure that the value of the "flavor" key in the data sent by my backend matches one of the values defined in my ICE_CREAM string enum. Can you point out what I may be doing incorrectly?

typescript enums

Answer 1

Answer №1

When it comes to assigning a string to a property that is of type enum, there are limitations in place for this purpose. You have to reference the enum itself to assign one of its values.

flavor: ICE_CREAM.vanilla

However, there is an alternative approach using string union types. This method allows you to maintain strong typing while still working with strings as needed.

type ICE_CREAM = 'STRAWBERRY' | 'VANILLA';

type TORDER = {
    greetings: string,
    flavor: ICE_CREAM
}

const mockData: TORDER = {
    greetings: "Hello",
    flavor: "VANILLA", // No errors here
}

const badMockData: TORDER = {
  greetings: "Hello",
  flavor: "vanilla", // Error: Flavor can only be 'STRAWBERRY' or 'VANILLA'
}

Answer 2

When it comes to assigning a string to a property that is of type enum, there are limitations in place for this purpose. You have to reference the enum itself to assign one of its values.

flavor: ICE_CREAM.vanilla

However, there is an alternative approach using string union types. This method allows you to maintain strong typing while still working with strings as needed.

type ICE_CREAM = 'STRAWBERRY' | 'VANILLA';

type TORDER = {
    greetings: string,
    flavor: ICE_CREAM
}

const mockData: TORDER = {
    greetings: "Hello",
    flavor: "VANILLA", // No errors here
}

const badMockData: TORDER = {
  greetings: "Hello",
  flavor: "vanilla", // Error: Flavor can only be 'STRAWBERRY' or 'VANILLA'
}

Answer 3

Answer №2

At first glance, it may seem logical to utilize the original string because they appear identical, right?

However, in terms of semantics, they are actually quite different. Consider this enum as an illustration:

enum SILLY_CODER {
    ten = "TEN",
    twenty = "TWENTY"
}

Imagine a programmer using this enum throughout their extensive codebase only to realize later on that there was a mistake! The initial definition was supposed to be:

enum SILLY_CODER {
    ten = "TWENTY",
    twenty = "TEN"
}

Oh dear! Does this mean every line of code needs to be rewritten? Not at all - just modify the enum. This practice proves particularly beneficial when developing APIs and encountering errors in constant data being returned.

Without this capability, instead of automatic updates, there would be type mismatches requiring manual correction wherever errors are spotted.

Hence, the decision was made for this specific reason, demonstrating why it is deemed the "correct" choice for achieving the desired outcome.

Answer 4

At first glance, it may seem logical to utilize the original string because they appear identical, right?

However, in terms of semantics, they are actually quite different. Consider this enum as an illustration:

enum SILLY_CODER {
    ten = "TEN",
    twenty = "TWENTY"
}

Imagine a programmer using this enum throughout their extensive codebase only to realize later on that there was a mistake! The initial definition was supposed to be:

enum SILLY_CODER {
    ten = "TWENTY",
    twenty = "TEN"
}

Oh dear! Does this mean every line of code needs to be rewritten? Not at all - just modify the enum. This practice proves particularly beneficial when developing APIs and encountering errors in constant data being returned.

Without this capability, instead of automatic updates, there would be type mismatches requiring manual correction wherever errors are spotted.

Hence, the decision was made for this specific reason, demonstrating why it is deemed the "correct" choice for achieving the desired outcome.

Issue with assignment in TypeScript: strings and string enums are not interchangeable

Answer №1

Answer №2

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