<p>`Lang` contains JavaScript language utilities and extensions that are used in the YUI library.

```

var Y = YUI();

// true, an array literal is an array

Y.Lang.isArray([1, 2]);

// false, an object literal is not an array

Y.Lang.isArray({"one": "two"});

// however, when declared as an array, it is true

function() {

var a = new Array();

a["one"] = "two";

return Y.Lang.isArray(a);

}();

// false, a collection of elements is like an array, but isn't

Y.Lang.isArray(document.getElementsByTagName("body"));

// true, false is a boolean

Y.Lang.isBoolean(false);

// false, 1 and the string "true" are not booleans

Y.Lang.isBoolean(1);

Y.Lang.isBoolean("true");

// null is null, but false, undefined and "" are not

Y.Lang.isNull(null); // true

Y.Lang.isNull(undefined); // false

Y.Lang.isNull(""); // false

// a function is a function, but an object is not

Y.Lang.isFunction(function(){}); // true

Y.Lang.isFunction({foo: "bar"}); // false

// true, ints and floats are numbers

Y.Lang.isNumber(0);

Y.Lang.isNumber(123.123);

// false, strings that can be cast to numbers aren't really numbers

Y.Lang.isNumber("123.123");

// false, undefined numbers and infinity are not numbers we want to use

Y.Lang.isNumber(1/0);

// true, objects, functions, and arrays are objects

Y.Lang.isObject({});

Y.Lang.isObject(function(){});

Y.Lang.isObject([1,2]);

// false, primitives are not objects

Y.Lang.isObject(1);

Y.Lang.isObject(true);

Y.Lang.isObject("{}");

// strings

Y.Lang.isString("{}"); // true

Y.Lang.isString({foo: "bar"}); // false

Y.Lang.isString(123); // false

Y.Lang.isString(true); // false

// undefined is undefined, but null and false are not

Y.Lang.isUndefined(undefined); // true

Y.Lang.isUndefined(false); // false

Y.Lang.isUndefined(null); // false

```