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# Type Checking & Validation
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Comprehensive type checking utilities for runtime validation and type guards. Includes checks for all JavaScript primitive types, built-in objects, DOM elements, and web API types. Essential for robust error handling and input validation.
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## Capabilities
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### Primitive Type Checks
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Type checking functions for JavaScript primitive types with TypeScript type guards.
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```javascript { .api }
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/**
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 * Check if value is an array
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 * @param val - Value to check
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 * @returns True if value is array
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 */
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function isArray(val: any): val is any[];
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/**
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 * Check if value is a string
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 * @param val - Value to check
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 * @returns True if value is string
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 */
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function isString(val: any): val is string;
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/**
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 * Check if value is a number (excluding NaN)
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 * @param val - Value to check
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 * @returns True if value is number
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 */
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function isNumber(val: any): val is number;
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/**
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 * Check if value is a boolean
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 * @param val - Value to check
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 * @returns True if value is boolean
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 */
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function isBoolean(val: any): val is boolean;
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/**
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 * Check if value is a function
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 * @param val - Value to check
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 * @returns True if value is function
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 */
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function isFunction(val: any): val is Function;
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/**
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 * Check if value is a symbol
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 * @param val - Value to check
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 * @returns True if value is symbol
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 */
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function isSymbol(val: any): val is symbol;
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```
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### Object Type Checks
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Type checking for various object types and complex structures.
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```javascript { .api }
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/**
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 * Check if value is an object (excluding null and arrays)
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 * @param val - Value to check
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 * @returns True if value is object
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 */
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function isObject(val: any): val is object;
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/**
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 * Check if value is a plain object (created by {} or new Object())
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 * @param val - Value to check
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 * @returns True if value is plain object
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 */
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function isPlainObject(val: any): val is object;
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/**
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 * Check if value is a Date object
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 * @param val - Value to check
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 * @returns True if value is Date
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 */
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function isDate(val: any): val is Date;
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/**
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 * Check if value is a RegExp object
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 * @param val - Value to check
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 * @returns True if value is RegExp
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 */
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function isRegExp(val: any): val is RegExp;
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/**
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 * Check if value is an Error object
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 * @param val - Value to check
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 * @returns True if value is Error
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 */
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function isError(val: any): val is Error;
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/**
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 * Check if value is a TypeError object
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 * @param val - Value to check
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 * @returns True if value is TypeError
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 */
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function isTypeError(val: any): val is TypeError;
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```
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### Collection Type Checks
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Type checking for ES6+ collection types.
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```javascript { .api }
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/**
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 * Check if value is a Map object
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 * @param val - Value to check
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 * @returns True if value is Map
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 */
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function isMap(val: any): val is Map<any, any>;
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/**
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 * Check if value is a WeakMap object
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 * @param val - Value to check
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 * @returns True if value is WeakMap
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 */
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function isWeakMap(val: any): val is WeakMap<any, any>;
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/**
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 * Check if value is a Set object
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 * @param val - Value to check
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 * @returns True if value is Set
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 */
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function isSet(val: any): val is Set<any>;
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/**
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 * Check if value is a WeakSet object
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 * @param val - Value to check
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 * @returns True if value is WeakSet
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 */
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function isWeakSet(val: any): val is WeakSet<any>;
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```
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### Null/Undefined Checks
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Specialized checks for null, undefined, and related states.
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```javascript { .api }
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/**
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 * Check if value is null
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 * @param val - Value to check
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 * @returns True if value is null
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 */
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function isNull(val: any): val is null;
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/**
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 * Check if value is undefined
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 * @param val - Value to check
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 * @returns True if value is undefined
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 */
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function isUndefined(val: any): val is undefined;
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/**
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 * Check if value is NaN
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 * @param val - Value to check
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 * @returns True if value is NaN
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 */
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function isNaN(val: any): boolean;
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/**
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 * Check if value is null or undefined
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 * @param val1 - First value
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 * @param val2 - Second value (optional)
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 * @returns True if both values are null/undefined or equal
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 */
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function eqNull(val1: any, val2?: any): boolean;
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```
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### Number Type Checks
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Specialized number type checking including finite, integer, and float checks.
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```javascript { .api }
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/**
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 * Check if value is a finite number
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 * @param val - Value to check
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 * @returns True if value is finite number
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 */
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function isFinite(val: any): boolean;
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/**
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 * Check if value is an integer
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 * @param val - Value to check
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 * @returns True if value is integer
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 */
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function isInteger(val: any): boolean;
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/**
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 * Check if value is a float (non-integer number)
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 * @param val - Value to check
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 * @returns True if value is float
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 */
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function isFloat(val: any): boolean;
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```
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### DOM & Web API Checks
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Type checking for browser and DOM-related objects.
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```javascript { .api }
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/**
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 * Check if value is a DOM element
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 * @param val - Value to check
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 * @returns True if value is DOM element
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 */
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function isElement(val: any): val is Element;
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/**
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 * Check if value is a Document object
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 * @param val - Value to check
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 * @returns True if value is Document
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 */
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function isDocument(val: any): val is Document;
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/**
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 * Check if value is a Window object
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 * @param val - Value to check
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 * @returns True if value is Window
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 */
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function isWindow(val: any): val is Window;
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/**
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 * Check if value is a FormData object
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 * @param val - Value to check
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 * @returns True if value is FormData
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 */
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function isFormData(val: any): val is FormData;
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/**
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 * Check if value is an Arguments object
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 * @param val - Value to check
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 * @returns True if value is Arguments
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 */
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function isArguments(val: any): val is IArguments;
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```
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### Value State Checks
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Utilities for checking value states and emptiness.
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```javascript { .api }
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/**
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 * Check if value is empty (null, undefined, empty string, empty array, empty object)
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 * @param val - Value to check
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 * @returns True if value is considered empty
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 */
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function isEmpty(val: any): boolean;
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/**
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 * Check if date is a leap year
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 * @param date - Date to check
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 * @returns True if year is leap year
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 */
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function isLeapYear(date: Date | number): boolean;
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/**
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 * Check if date is valid
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 * @param date - Date to validate
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 * @returns True if date is valid
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 */
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function isValidDate(date: any): boolean;
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```
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### Deep Comparison
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Advanced comparison utilities for deep equality checking.
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```javascript { .api }
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/**
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 * Deep equality comparison
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 * @param val1 - First value
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 * @param val2 - Second value
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 * @returns True if values are deeply equal
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 */
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function isEqual(val1: any, val2: any): boolean;
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/**
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 * Deep equality comparison with custom comparator
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 * @param val1 - First value
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 * @param val2 - Second value
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 * @param callback - Custom comparison function
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 * @returns True if values are equal according to callback
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 */
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function isEqualWith(
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  val1: any, 
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  val2: any, 
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  callback: (value1: any, value2: any, key?: string) => boolean | undefined
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): boolean;
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/**
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 * Check if object matches source properties
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 * @param obj - Object to check
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 * @param source - Source object with properties to match
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 * @returns True if object contains all source properties with same values
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 */
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function isMatch(obj: any, source: any): boolean;
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```
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### Type Information
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Utilities for getting detailed type information.
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```javascript { .api }
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/**
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 * Get detailed type of value
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 * @param val - Value to analyze
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 * @returns Detailed type string
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 */
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function getType(val: any): string;
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```
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**Usage Examples:**
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```javascript
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import { 
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  isArray, isString, isNumber, isObject, isEmpty, 
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  isEqual, isValidDate, getType 
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} from 'xe-utils';
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// Basic type checking
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const data = ['hello', 42, { name: 'Alice' }, null];
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data.forEach(item => {
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  if (isString(item)) {
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    console.log(`String: ${item.toUpperCase()}`);
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  } else if (isNumber(item)) {
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    console.log(`Number: ${item * 2}`);
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  } else if (isObject(item)) {
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    console.log(`Object:`, item);
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  }
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});
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// Validation
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function validateUser(user) {
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  if (isEmpty(user)) {
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    throw new Error('User data is required');
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  }
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  if (!isString(user.name) || isEmpty(user.name)) {
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    throw new Error('Valid name is required');
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  }
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  if (!isNumber(user.age) || user.age < 0) {
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    throw new Error('Valid age is required');
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  }
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  return true;
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}
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// Deep comparison
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const obj1 = { user: { name: 'Alice', preferences: { theme: 'dark' } } };
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const obj2 = { user: { name: 'Alice', preferences: { theme: 'dark' } } };
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console.log(isEqual(obj1, obj2)); // true
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// Type information
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console.log(getType([])); // 'Array'
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console.log(getType(new Date())); // 'Date'
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console.log(getType(/regex/)); // 'RegExp'
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```