Conditional fields based on value of another field in interface in Typescript

Question

As the title says, I'm trying to create an interface that will have required fields but only if another field is of a certain value.

For example :

const schema = {
    str: { type: 'string' },
    nbr: { type: 'number' },
    bool: { type: 'boolean' },
    date: { type: 'date' },
    strs: { type: ['string'] },
    obj: { type: 'object' },
} as ISchema;

I want this code to tell me that the field obj is missing a property because the value of type is 'object'.

I succeeded to do so with this code :

interface SchemaOptionsObject {
    type: 'object' | ['object'] ;
    properties: ISchema;
}
interface SchemaOptionsString {
    type: 'string' | ['string'] ;
}
interface SchemaOptionsNumber {
    type: 'number' | ['number'] ;
}
interface SchemaOptionsBoolean {
    type: 'boolean' | ['boolean'];
}
interface SchemaOptionsDate {
    type: 'date' | ['date'] ;
}

type SchemaOptions = SchemaOptionsString | SchemaOptionsNumber | SchemaOptionsBoolean | SchemaOptionsDate | SchemaOptionsObject;

export interface ISchema {
    [key: string]: SchemaOptions;
}

But this solution is way too repetitive. I tried to factorise it and ended up with a problem :

export type SchemaAllowedTypes = 'string' | 'number' | 'boolean' | 'date' | 'object';

type SchemaOptionsObject<T extends SchemaAllowedTypes> =
    T extends 'object' ?
        { properties: ISchema } :
        {};

type SchemaOptions<T extends SchemaAllowedTypes> = {
    type: T | T[];
} & SchemaOptionsObject<T>;

export interface ISchema {
    [key: string]: SchemaOptions<SchemaAllowedTypes>;
}

I know it doesn't work because of the T extends 'object' but I have no idea on how to check the value of T, is there a keyword that could do that ?

Am I doing this the wrong way ?

Thanks for your help !

Answer 1

How about this?

export type SchemaAllowedTypes = 'string' | 'number' | 'boolean' | 'date' | 'object';

interface SchemaOptionsGeneric<T extends SchemaAllowedTypes>{
    type: T | [T] ;
}
interface SchemaOptionsObject extends SchemaOptionsGeneric<"object">{
    properties: ISchema;
}
type SchemaOptions = SchemaOptionsGeneric<"string"|"number"|"boolean"|"date"> | SchemaOptionsObject

export interface ISchema {
    [key: string]: SchemaOptions;
}