A safeguard for specific values in union types

Question

A safeguard for specific values in union types

In TypeScript, I have defined two types:

type PublicMethods = 'Time' | 'Assets' | 'AssetPairs';
type PrivateMethods = 'Balance' | 'TradeBalance';

I am looking to streamline the usage of the api function for both types, but with different behaviors for each. Here is what I have tried:

public api = (method: PublicMethods | PrivateMethods, params: any) => {

  // How can I implement a type guard here?
  if(method instanceof PublicMethods) { // 😱 This doesn't work!
    // ...
  }

}

I also attempted overloading the function:

public api(method: PublicMethods | PrivateMethods, params: any, callback: Function);

public api(method: PublicMethods, params: any, callback: Function) {
   // ...implementation
}

public api(method: PrivateMethods, params: any, callback: Function) {
   // ...implementation
}

Neither approach worked as expected. Any ideas on how to achieve this?

typescript

Answer 1

Answer №1

If you want to create a custom type guard, here's how you can do it:

const isPublicMethod = (value: any): value is PublicMethod => {
  return ['Time', 'Assets', 'AssetPairs'].includes(value);
};

You can then incorporate this type guard into your API integration function like this:

public api = (method: PublicMethods | PrivateMethods, params: any) => {
  if(isPublicMethod(method)) {
    ...
  }
}

Just a heads up! Make sure to update the PublicMethod type guard when making changes to the PublicMethod type in TypeScript.

For better management, consider using this alternative method:

const PUBLIC_METHODS = ['Time', 'Assets', 'AssetPairs'] as const;

type PublicMethod = typeof PUBLIC_METHODS[number];

const isPublicMethod = (value: any): value is PublicMethod => {
  return PUBLIC_METHODS.includes(value);
};

Answer 2

If you want to create a custom type guard, here's how you can do it:

const isPublicMethod = (value: any): value is PublicMethod => {
  return ['Time', 'Assets', 'AssetPairs'].includes(value);
};

You can then incorporate this type guard into your API integration function like this:

public api = (method: PublicMethods | PrivateMethods, params: any) => {
  if(isPublicMethod(method)) {
    ...
  }
}

Just a heads up! Make sure to update the PublicMethod type guard when making changes to the PublicMethod type in TypeScript.

For better management, consider using this alternative method:

const PUBLIC_METHODS = ['Time', 'Assets', 'AssetPairs'] as const;

type PublicMethod = typeof PUBLIC_METHODS[number];

const isPublicMethod = (value: any): value is PublicMethod => {
  return PUBLIC_METHODS.includes(value);
};

Answer 3

Answer №2

When defining types based on a constant array, you have the flexibility to use the name PublicMethods both as a type and a variable that references the union of strings and arrays.

const PublicMethods = ['Time', 'Assets', 'AssetPairs'] as const;
const PrivateMethods = ['Balance', 'TradeBalance'] as const;

type PublicMethods = typeof PublicMethods[number];
type PrivateMethods = typeof PrivateMethods[number];

By implementing a user-defined type guard using a type predicate, you can ensure that the value is narrowed down to PublicMethods, the type:

function isPublicMethod(value: string): value is PublicMethods {
    // An assertion is necessary to avoid an error stating that type 'string' is not assignable to types '"Time" | "Assets" | "AssetPairs"'
    return PublicMethods.includes(value as any);
}

This method allows you to specifically identify and narrow down values to the type PublicMethods by verifying if the array includes the specified value:

function doStuff(method: PrivateMethods | PublicMethods) {
    if (isPublicMethod(method)) {
        method
        // This now represents PublicMethods
    }
}

Feel free to experiment with this concept in the Playground.

Answer 4

When defining types based on a constant array, you have the flexibility to use the name PublicMethods both as a type and a variable that references the union of strings and arrays.

const PublicMethods = ['Time', 'Assets', 'AssetPairs'] as const;
const PrivateMethods = ['Balance', 'TradeBalance'] as const;

type PublicMethods = typeof PublicMethods[number];
type PrivateMethods = typeof PrivateMethods[number];

By implementing a user-defined type guard using a type predicate, you can ensure that the value is narrowed down to PublicMethods, the type:

function isPublicMethod(value: string): value is PublicMethods {
    // An assertion is necessary to avoid an error stating that type 'string' is not assignable to types '"Time" | "Assets" | "AssetPairs"'
    return PublicMethods.includes(value as any);
}

This method allows you to specifically identify and narrow down values to the type PublicMethods by verifying if the array includes the specified value:

function doStuff(method: PrivateMethods | PublicMethods) {
    if (isPublicMethod(method)) {
        method
        // This now represents PublicMethods
    }
}

Feel free to experiment with this concept in the Playground.

A safeguard for specific values in union types

Answer №1

Answer №2

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