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Yehuda Shiran November 8, 1999
Detecting Data Types
Tips: November 1999

Yehuda Shiran, Ph.D.
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The tyepof operator returns a string that identifies the data type of an expression. Its general syntax is:

typeof [(]expression[)]

As you can see, the parentheses are optional, because typeof is an operator, not a function. It returns one of the following strings:

  • "boolean"
  • "function"
  • "number"
  • "object"
  • "string"
  • "undefined"
Here's an example:

function doIt() {
  // do something
}

var a = true;
var b = doIt;
var c = 19;
var d = window.location;
var e = "Microsoft";
var f;
alert(typeof a); // "boolean"
alert(typeof b); // "function"
alert(typeof c); // "number"
alert(typeof d); // "object"
alert(typeof e); // "string"
alert(typeof f); // "undefined"


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