Type Checking and Validation

/**
 * Check if value is a string
 */
function isString(val: unknown): val is string;

/**
 * Check if value is a function
 */
function isFunction(val: unknown): val is Function;

/**
 * Check if value is a symbol
 */
function isSymbol(val: unknown): val is symbol;

/**
 * Check if value is an object (not null)
 */
function isObject(val: unknown): val is Record<any, any>;

/**
 * Check if value is a plain object
 */
function isPlainObject(val: unknown): val is object;

/**
 * Alias for Array.isArray with proper typing
 */
const isArray: typeof Array.isArray;

/**
 * Check if value is a Map
 */
function isMap(val: unknown): val is Map<any, any>;

/**
 * Check if value is a Set
 */
function isSet(val: unknown): val is Set<any>;

/**
 * Check if value is a Date object
 */
function isDate(val: unknown): val is Date;

/**
 * Check if value is a RegExp object
 */
function isRegExp(val: unknown): val is RegExp;

/**
 * Check if value is a Promise
 */
function isPromise<T = any>(val: unknown): val is Promise<T>;

/**
 * Check if object has own property (type-safe)
 * @param val - Object to check
 * @param key - Property key to check for
 * @returns Type predicate indicating key exists
 */
function hasOwn(val: object, key: string | symbol): key is keyof typeof val;

/**
 * Check if key represents an integer-like string
 * @param key - Key to validate
 * @returns True if key is integer-like ("0", "123", etc.)
 */
function isIntegerKey(key: unknown): boolean;

/**
 * Check if string is an event handler name (starts with "on" + uppercase)
 * @param key - Property name to check
 * @returns True if key follows event handler pattern
 */
function isOn(key: string): boolean;

/**
 * Check if string is a model listener (v-model update event)
 * @param key - Property name to check
 * @returns Type predicate for update event names
 */
function isModelListener(key: string): key is `onUpdate:${string}`;

/**
 * Check if property name is reserved by Vue
 * @param key - Property name to check
 * @returns True if property is reserved (key, ref, etc.)
 */
function isReservedProp(key: string): boolean;

/**
 * Check if directive name is a built-in Vue directive
 * @param key - Directive name to check
 * @returns True if directive is built-in (v-if, v-for, etc.)
 */
function isBuiltInDirective(key: string): boolean;

/**
 * Reference to Object.prototype.toString for type checking
 */
const objectToString: typeof Object.prototype.toString;

/**
 * Get the type string representation of a value
 * @param value - Value to get type string for
 * @returns Type string like "[object String]"
 */
function toTypeString(value: unknown): string;

/**
 * Extract the raw type name from a type string
 * @param value - Value to get raw type for
 * @returns Raw type name like "String", "Array", etc.
 */
function toRawType(value: unknown): string;

import { isString, isObject, hasOwn, isOn, toRawType } from "@vue/shared";

// Basic type checking with type narrowing
function processValue(value: unknown) {
  if (isString(value)) {
    // TypeScript knows value is string here
    return value.toUpperCase();
  }
  
  if (isObject(value)) {
    // TypeScript knows value is Record<any, any> here
    return Object.keys(value);
  }
}

// Safe property access
function getProperty(obj: object, key: string) {
  if (hasOwn(obj, key)) {
    // TypeScript knows key exists on obj
    return obj[key];
  }
  return undefined;
}

// Vue-specific checks
function isEventProp(key: string) {
  return isOn(key); // true for "onClick", "onMouseover", etc.
}

// Type introspection
const type = toRawType(new Date()); // "Date"
const type2 = toRawType([]); // "Array"