Example of custom types for fastest-validator. Advanced auto completion.

fastest-validator-advanced-types.ts

interface ValidationTest {
    arr: { haha: string } | { hoho: number }[];
    obj: { oops: boolean };
    str: string;
}

const schema: Validation<ValidationTest> = {
    arr: { type: 'array', items: { type: 'object', props: { haha: { type: 'string' } } } },
    obj: { type: 'object', props: { oops: { type: 'boolean' } } },
    str: { type: 'string' }
};

type Validation<T> = {
    [key in keyof T]?: PatchedRule<T[key]>;
};

type PatchedRule<T> = ValidationRuleObject | { type: 'object', props: Validation<T> } | PatchedArray<T>;

type PatchedArray<T> = { type: 'array', items: PatchedRule<T extends (infer R)[] ? R : T> };