Data in a JavaScript program can be of several different types. In the language, there are a few built-in types: number, boolean, string. Additionally, variables can have the value of a function or any of various kinds of an object. A variable that has never been set to a value is considered undefined.
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// A number data type document.writeln(typeof 3.14 + "<br />"); // A boolean data type document.writeln(typeof true + "<br />"); // A string data type document.writeln(typeof "XoaX.net" + "<br /><br />"); // An undefined variable document.writeln(typeof Y + "<br /><br />"); // A function var fnF = function(dX) { return 2.0*dX;} document.writeln(typeof fnF + "<br /><br />"); // Different kinds of objects // An array var iaA = [2,3,6,9,2]; document.writeln(typeof iaA + "<br />"); // The null value document.writeln(typeof null + "<br />"); // A programmer-defined and initialized object document.writeln(typeof {dX:5.0, dY:3.0} + "<br />");
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