Compile time extraction of constant string from type field

Question

Compile time extraction of constant string from type field

I am currently facing an issue with a field in my type that contains a constant string literal. My goal is to be able to reference both the type and field by name so that I can utilize this string literal throughout my code. Here is an example:

export type MyType = {
    action?: 'TestAction';
    arg0: true,
};

/////

let input: any = {
    action: 'XYZ',
};

if (input.action === MyType.action) { // This obviously does not work
    // TODO: perform TestAction
}

Is there a way to achieve this without having to separate 'TestAction' into its own const definition?

typescript

Answer 1

Answer №1

To ensure consistency, you can define the type as a constant and use it in both instances

const ActionType = "TestAction";

export type MyType = {
  action?: typeof ActionType;
  arg0: true;
};

/////

let input: any = {
  action: "XYZ",
};

if (input.action === ActionType) {
  // This won't work
  // TODO: perform TestAction
}

Alternatively, you could consider utilizing a switch statement with type coercion based on your MyType definition, like so:

export type MyType = {
    action?: 'TestAction';
    arg0: true,
};


/////

let input: any = {
    action: 'XYZ',
};

switch (input.action as MyType['action']) {
    case 'TestAction':
        // perform TestAction
        break;

    default:
        break;
}

Answer 2

To ensure consistency, you can define the type as a constant and use it in both instances

const ActionType = "TestAction";

export type MyType = {
  action?: typeof ActionType;
  arg0: true;
};

/////

let input: any = {
  action: "XYZ",
};

if (input.action === ActionType) {
  // This won't work
  // TODO: perform TestAction
}

Alternatively, you could consider utilizing a switch statement with type coercion based on your MyType definition, like so:

export type MyType = {
    action?: 'TestAction';
    arg0: true,
};


/////

let input: any = {
    action: 'XYZ',
};

switch (input.action as MyType['action']) {
    case 'TestAction':
        // perform TestAction
        break;

    default:
        break;
}

Answer 3

Answer №2

In the realm of TS land, nothing can be used at runtime, hence opting for a const definition is likely your safest bet. If altering the definition of MyType is not an option, consider implementing the following approach:

// Guarantee consistency with MyType's definition
const TEST_ACTION: MyType['action'] = 'TestAction';

Playground.

Answer 4

In the realm of TS land, nothing can be used at runtime, hence opting for a const definition is likely your safest bet. If altering the definition of MyType is not an option, consider implementing the following approach:

// Guarantee consistency with MyType's definition
const TEST_ACTION: MyType['action'] = 'TestAction';

Playground.

Compile time extraction of constant string from type field

Answer №1

Answer №2

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