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typeof Operator (JavaScript)

Returns a string that identifies the data type of an expression.

typeof[(]expression[)] ;

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.

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

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);

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.

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