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


typeof [ ( ] expression [ ) ] ;


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 compared to undefined,
 // so the next line generates an error.
 //  var result = (notDeclared === undefined);


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.

See also

Other articles


  • Microsoft Developer Network: Article