Flow type definition (libdef) for Json, and ToJson which drops non-serializable props

Json.flow.js

// @flow
// Flow type definition for Json (lower-case so as not to collide with JSON)
// ToJson function accepts any JS variable and sets any non-serializable
// vars or properties on objects to null
// The author has found this quite useful for logging complex objects safely

export type JsonPrimitive = null | string | number | boolean;

export type JsonObject = { [string]: Json };

export type JsonArray = Json[];

export type Json = JsonPrimitive | JsonObject | JsonArray;

export function isPrimitive(value: any): boolean {
    return value === null || ['string', 'number', 'boolean'].includes(value);
}

export function ArrayToJson(array: any[]): JsonArray {
    return array.reduce(
        (json: JsonArray, value: any): JsonArray => {
            return Array.isArray(json)
                ? [...json, ToJson(value)]
                : [ToJson(value)];
        },
    );
}

export function ObjectToJson(object: Object): JsonObject {
    return Object.keys(object).reduce(
        (json: JsonObject, key: string): JsonObject => {
            return typeof json === 'object'
                    && !Array.isArray(json)
                ? { ...json, [key]: ToJson(object[key]) }
                : { [key]: ToJson(object[key]) };
        },
        {},
    );
}

export function ToJson(value: any): Json {
    return isPrimitive(value) ? value
        : Array.isArray(value) ? ArrayToJson(value)
        : typeof value === 'object' ? ObjectToJson(value)
        : null;
}