Restricting array elements through union types in TypeScript

Question

Restricting array elements through union types in TypeScript

Imagine a scenario where we have an event type defined as follows:

interface Event {
  type: 'a' | 'b' | 'c';
  value: string;
}

interface App {
  elements: Event[];
}

Now, consider the following code snippet:

const app: App = {
  elements: [
    { type: 'a', value: '1' },
    { type: 'a', value: '2' },
  ],
}

The issue here is that I need to ensure that the elements array contains objects with unique types. For example, I want only one item with type 'a', one item with type 'b', and so on. How can I achieve this in TypeScript? Any guidance would be appreciated!

javascript typescript

Answer 1

Answer №1

If you find a way to adjust the solution provided here to fit your specific scenario (where your union consists of different SomeEvent types that can be derived from your current type), it could work for you. While it is quite complex and I didn't have enough time to fully implement it, there might be a possibility.

An alternative approach would be to utilize an object instead of an array, using keys based on the type:

elements: {a: { value: "1" } }

. (In case you need an array later on, you can easily retrieve it by utilizing Object.values(theApp.elements), even though iterating through objects, not just arrays, is simple.)

interface SomeEvent {
  type: 'a' | 'b' | 'c';
  value: string;
}

interface App {
  elements: Partial<{
    [key in SomeEvent["type"]]: Omit<SomeEvent, "type"> & { type: key};
  }>
}

This method associates the type with an object key, necessitating the assigned object to correspond with the same value as typd. (I opted for the name SomeEvent over Event to prevent confusion with DOM events.)

The following example demonstrates this concept:

const app1: App = {
  elements: {
    a: { type: 'a', value: '1' },
    b: { type: 'b', value: '2' },
  },
};

Whereas, the subsequent snippet showcases an intentional error:

const app2: App = {
  elements: {
    a: { type: 'a', value: '1' },
    a: { type: 'a', value: '2' }, // An object literal cannot have multiple properties with the same name in strict mode.
  },
};

Playground link

Answer 2

If you find a way to adjust the solution provided here to fit your specific scenario (where your union consists of different SomeEvent types that can be derived from your current type), it could work for you. While it is quite complex and I didn't have enough time to fully implement it, there might be a possibility.

An alternative approach would be to utilize an object instead of an array, using keys based on the type:

elements: {a: { value: "1" } }

. (In case you need an array later on, you can easily retrieve it by utilizing Object.values(theApp.elements), even though iterating through objects, not just arrays, is simple.)

interface SomeEvent {
  type: 'a' | 'b' | 'c';
  value: string;
}

interface App {
  elements: Partial<{
    [key in SomeEvent["type"]]: Omit<SomeEvent, "type"> & { type: key};
  }>
}

This method associates the type with an object key, necessitating the assigned object to correspond with the same value as typd. (I opted for the name SomeEvent over Event to prevent confusion with DOM events.)

The following example demonstrates this concept:

const app1: App = {
  elements: {
    a: { type: 'a', value: '1' },
    b: { type: 'b', value: '2' },
  },
};

Whereas, the subsequent snippet showcases an intentional error:

const app2: App = {
  elements: {
    a: { type: 'a', value: '1' },
    a: { type: 'a', value: '2' }, // An object literal cannot have multiple properties with the same name in strict mode.
  },
};

Playground link

Answer 3

Answer №2

One approach could involve creating an interface for Events with different types such as a, b, or c.

You can define an interface that specifies each element property can have one instance of each type.

interface EventFilter {
  elements: [EventA?, EventB?, EventC?];
}

A drawback of this method is that elements must be added in the exact order specified in the EventFilter interface.

interface Event {
  type: 'a' | 'b' | 'c';
  value: string;
}

interface EventA extends Event {
  type: 'a';
}

interface EventB extends Event {
  type: 'b';
}

interface EventC extends Event {
  type: 'c';
}

interface EventFilter {
  elements: [EventA?, EventB?, EventC?];
}

const app: EventFilter = {
  elements: [ 
    { type: 'a', value: '1' },
    { type: 'b', value: '1' } 
  ],
}

Unfortunately, to achieve a flexible array allowing elements to be in any order, you would need to list all permissible combinations explicitly.

interface EventFilter {
  elements: [EventA?, EventB?, EventC?] | [EventA?, EventC?, EventB?] 
            | [EventB?, EventA?, EventC?] | [EventB?, EventC?, EventA?]
            | [EventC?, EventA?, EventB?] | [EventC?, EventB?, EventA?];
}

const app: EventFilter = {
  elements: [ 
    { type: 'c', value: '1' },
    { type: 'a', value: '1' },
  ],
}

Answer 4

One approach could involve creating an interface for Events with different types such as a, b, or c.

You can define an interface that specifies each element property can have one instance of each type.

interface EventFilter {
  elements: [EventA?, EventB?, EventC?];
}

A drawback of this method is that elements must be added in the exact order specified in the EventFilter interface.

interface Event {
  type: 'a' | 'b' | 'c';
  value: string;
}

interface EventA extends Event {
  type: 'a';
}

interface EventB extends Event {
  type: 'b';
}

interface EventC extends Event {
  type: 'c';
}

interface EventFilter {
  elements: [EventA?, EventB?, EventC?];
}

const app: EventFilter = {
  elements: [ 
    { type: 'a', value: '1' },
    { type: 'b', value: '1' } 
  ],
}

Unfortunately, to achieve a flexible array allowing elements to be in any order, you would need to list all permissible combinations explicitly.

interface EventFilter {
  elements: [EventA?, EventB?, EventC?] | [EventA?, EventC?, EventB?] 
            | [EventB?, EventA?, EventC?] | [EventB?, EventC?, EventA?]
            | [EventC?, EventA?, EventB?] | [EventC?, EventB?, EventA?];
}

const app: EventFilter = {
  elements: [ 
    { type: 'c', value: '1' },
    { type: 'a', value: '1' },
  ],
}

Restricting array elements through union types in TypeScript

Answer №1

Answer №2

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