Ensure that an Enum is limited to only numerical values (implement type checks for Enum)

Question

Ensure that an Enum is limited to only numerical values (implement type checks for Enum)

I am looking to enforce a Generic type to be strictly a numerical enum. This means that the values of the Enum should only be numbers. Here is an example:

export type TNumberEnum<Enum> = { [key in keyof Enum]: number };

enum sample {
    AA,
    BB,
    CC = 'cc'
}

class SampleClass<CustomEnum extends TNumberEnum<CustomEnum>> {

    constructor( private value: CustomEnum ){}
    public sampleFunction(): CustomEnum {
        return this.value;
    }
}

let a = new SampleClass<sample>( sample.CC ); // Should fail since keyof sample has string value

// Could valueOf and toString cause any issues?

Check out a demo here

When passing a custom Enum to the SampleClass, I want to ensure that the Generic Enum is of a numerical enum type. Is this achievable?

typescript

Answer 1

Answer №1

When working with Typescript, it's important to note that type checks for the Enum type are not supported. One way around this limitation is to iterate over the enum elements and create a new object based on them. In JavaScript, an Enum is essentially transformed into an object.

For instance, if we take the example of an enum:

(function (example) {
    example[example["AA"] = 0] = "AA";
    example[example["BB"] = 1] = "BB";
    example["CC"] = "cc";
})(example || (example = {}));

Executing this code would result in:

{
  0:    "AA",
  "AA": 0,
  1:    "BB",
  "BB": 1,
  "cc": "CC",
  "CC": "cc"
}

However, when iterating over these properties, distinguishing between enum keys and values may not always be straightforward.

To address this issue, you can create a new "enum" by enforcing all enum values to be numeric using the following code snippet:

const myEnum = {}
for (let i = 0; i < keys.length; i++) {
    myEnum[i] = keys[i]
}

This approach helps in clearly defining the enum values as numeric indices.

Answer 2

When working with Typescript, it's important to note that type checks for the Enum type are not supported. One way around this limitation is to iterate over the enum elements and create a new object based on them. In JavaScript, an Enum is essentially transformed into an object.

For instance, if we take the example of an enum:

(function (example) {
    example[example["AA"] = 0] = "AA";
    example[example["BB"] = 1] = "BB";
    example["CC"] = "cc";
})(example || (example = {}));

Executing this code would result in:

{
  0:    "AA",
  "AA": 0,
  1:    "BB",
  "BB": 1,
  "cc": "CC",
  "CC": "cc"
}

However, when iterating over these properties, distinguishing between enum keys and values may not always be straightforward.

To address this issue, you can create a new "enum" by enforcing all enum values to be numeric using the following code snippet:

const myEnum = {}
for (let i = 0; i < keys.length; i++) {
    myEnum[i] = keys[i]
}

This approach helps in clearly defining the enum values as numeric indices.

Ensure that an Enum is limited to only numerical values (implement type checks for Enum)

Answer №1

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