typeof

Phương thức typeof trả về kiểu dữ liệu của đối tượng nào đó.

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Cú pháp

Phương thức typeof được theo sau bởi toán hạng operand:

typeof operand
typeof(operand)

Các tham số

operand

Là một đối tượng cần kiểu tra kiểu dữ liệu hoặc một biểu thức đại điện cho đối tượng.

Định nghĩa

Bảng tóm tắt bên dưới mô tả các giá trị có thể trả về của typeof. Xem thêm thông tin tại trang JavaScript data structure.

Các ví dụ

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof(42) === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Mặc dù "Not-A-Number" nhưng lại là number :)
typeof Number('1') === 'number'; // Number ép kiểu chuỗi thành kiểu number

typeof 42n === 'bigint';


// Strings
typeof '' === 'string';
typeof 'bla' === 'string';
typeof `template literal` === 'string';
typeof '1' === 'string'; // 1 là number nhưng khi nằm trong ngoặc '' sẽ thành kiểu string
typeof (typeof 1) === 'string'; // typeof 1 sẽ trả về chữ number, bạn tự hiểu được hen
typeof String(1) === 'string'; // String sẽ đổi kiểu số 1 từ number thành string

// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(1) === 'boolean'; // Boolean() will convert values based on if they're truthy or falsy
typeof !!(1) === 'boolean'; // two calls of the ! (logical NOT) operator are equivalent to Boolean()


// Symbols
typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'


// Undefined
typeof undefined === 'undefined';
typeof declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined'; 


// Objects
typeof {a: 1} === 'object';

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === 'object';

typeof new Date() === 'object';
typeof /regex/ === 'object'; // See Regular expressions section for historical results


// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean(true) === 'object'; 
typeof new Number(1) === 'object'; 
typeof new String('abc') === 'object';


// Functions
typeof function() {} === 'function';
typeof class C {} === 'function';
typeof Math.sin === 'function';

Additional information

null

// This stands since the beginning of JavaScript
typeof null === 'object';

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the "object" typeof return value. (reference)

A fix was proposed for ECMAScript (via an opt-in), but was rejected. It would have resulted in typeof null === 'null'.

Using new operator

// All constructor functions, with the exception of the Function constructor, will always be typeof 'object'
var str = new String('String');
var num = new Number(100);

typeof str; // It will return 'object'
typeof num; // It will return 'object'

var func = new Function();

typeof func; // It will return 'function'

Need for parentheses in Syntax

// Parentheses can be used for determining the data type of expressions.
var iData = 99;

typeof iData + ' Wisen'; // 'number Wisen'
typeof (iData + ' Wisen'); // 'string'

Regular expressions

Callable regular expressions were a non-standard addition in some browsers.

typeof /s/ === 'function'; // Chrome 1-12 Non-conform to ECMAScript 5.1
typeof /s/ === 'object';   // Firefox 5+  Conform to ECMAScript 5.1

Errors

Before ECMAScript 2015, typeof was always guaranteed to return a string for any operand it was supplied with. Even with undeclared identifiers, typeof will return 'undefined'. Using typeof could never generate an error.

But with the addition of block-scoped let and Statements/const using typeof on let and const variables (or using typeof on a class) in a block before they are declared will throw a ReferenceError. Block scoped variables are in a "temporal dead zone" from the start of the block until the initialization is processed, during which, it will throw an error if accessed.

typeof undeclaredVariable === 'undefined';

typeof newLetVariable; // ReferenceError
typeof newConstVariable; // ReferenceError
typeof newClass; // ReferenceError

let newLetVariable;
const newConstVariable = 'hello';
class newClass{};

Exceptions

All current browsers expose a non-standard host object document.all with type undefined.

typeof document.all === 'undefined';

Although the specification allows custom type tags for non-standard exotic objects, it requires those type tags to be different from the predefined ones. The case of document.all having type 'undefined' is classified in the web standards as a "willful violation" of the original ECMA JavaScript standard.

Specifications

Browser compatibility

IE-specific notes

On IE 6, 7, and 8 a lot of host objects are objects and not functions. For example:

typeof alert === 'object'

See also

• instanceof
• Why typeof is no longer "safe"
• document.all willful violation of the standard