Restrict field type based on another field value

Question

Given the following:

type ActionType = 
    'action1' |
    'action2' |
    'action3';

interface Action {
    type: ActionType;
    value?: number | Date;
}

Is there a way in typescript to restrict the type of value based on the value of type ?

For example, if type equals action1 or action2 , I want the value field to be of type number but if type is action3 then I want the value to be of type Date .

It's probably not possibly but just checking..

Answer 1

Your best chance would probably be to use discriminated unions:

type Action = {
    type: 'action1' | 'action2';
    value: number;
} | {
    type: 'action3';
    value: Date;
}