I am attempting to specify that the payload field should be mandatory only when T is defined:
export interface Action<T = any> {
readonly type: string;
readonly payload?: T;
}
// The payload field must be included
const actionWithPayload: Action<string> = { action: 'action_name', payload: 'action payload'};
// The payload field is optional
const actionWithoutPayload: Action = { action: 'action_name' }
There are multiple ways to achieve the desired functionality. Here are a couple of options to consider:
1) Incorporate Union Type
interface Action1<T> {
readonly type: string;
readonly payload: T;
}
interface Action2 {
readonly type: string;
}
type Action<T = never> = Action1<T> | Action2
// Ensure that payload is included
const actionWithPayload: Action1<string> = { type: 'action_name', payload: 'action payload' }; // successful
const actionWithPayload2: Action1<string> = { type: 'action_name' }; // error
const actionWithoutPayload: Action = { type: 'action_name' } // successful
const actionWithoutPayload2: Action = { type: 'action_name', payload: 1 } // error
2) Utilize Conditional Type
type IsAny<T> = 0 extends (1 & T) ? true : false;
type Action<T = any> = IsAny<T> extends true ? {
readonly type: string;
} : {
readonly type: string;
readonly payload: T;
}
// Ensuring payload presence
const actionWithPayload: Action<string> = { type:'action_name', payload: 'action payload' }; // ok
const actionWithPayload2: Action<any> = { type: 'action_name', payload: 'action payload' }; //error
// Making payload optional
const actionWithoutPayload: Action = { type: 'action_name' } // successfully executed
const actionWithoutPayload2: Action = { type: 'action_name', payload: 42 }// error
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