Returns a string that identifies the data type of an expression.
Syntax
typeof[(]expression[)] ;
Remarks
The expression argument is any expression for which type information is sought.
The typeof operator returns type information as a string. There are six possible values that typeof returns: "number," "string," "boolean," "object," "function," and "undefined."
The parentheses are optional in the typeof syntax.
Example
The following example tests the data type of variables.
var index = 5;
var result = (typeof index === 'number');
// Output: true
var description = "abc";
var result = (typeof description === 'string');
// Output: true
Example
The following example tests for a data type of undefined for declared and undeclared variables.
var declared;
var result = (declared === undefined);
// Output: true
var result = (typeof declared === 'undefined');
// Output: true
var result = (typeof notDeclared === 'undefined')
// Output: true
var obj = {};
var result = (typeof obj.propNotDeclared === 'undefined');
// Output: true
// An undeclared variable cannot be used in a comparison without
// the typeof operator, so the next line generates an error.
// var result = (notDeclared === undefined);
Requirements
Supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards, Internet Explorer 9 standards, Internet Explorer 10 standards, Internet Explorer 11 standards. Also supported in Store apps (Windows 8 and Windows Phone 8.1). See Version Information.
See Also
Array.isArray Function
Object.getPrototypeOf Function
undefined Constant
Comparison Operators
Data Types
Operator Precedence
Operator Summary (JavaScript)