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Listing 117 Undeclared Variables
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myVariable = "Hello!"; // missing var keyword alert (myVariable) // alerts "Hello!" function myMethod() { myOtherVariable = "hi!"; // missing var keyword } myMethod(); alert (myOtherVariable); // alerts "hi!";
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null and undefined null and undefined are two reserved words that deserve special attention null is a reserved word that means no value and is used to point a vari-
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able to no data so that memory can be reused Unlike the NET languages, however, null (or Nothing in VB) is not the default value of a newly
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Strings
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declared variable Rather, undefined is the default value of a newly declared variable undefined is a primitive value and a type Both styles of undefined are supported by all modern browsers, but they serve different purposes As a primitive value, undefined refers to a newly declared variable s default value As a type, undefined refers to a variable that has never been declared and is accessed using the typeof keyword Listing 118 demonstrates the different usages of null and undefined
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Listing 118 Comparing null and the undefined Value
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var x = null; var y; alert alert alert alert (x); // alerts 'null' (y); // alerts 'undefined'-this is a value (typeof(z)); // alerts-'undefined' this is a type (y == typeof(z)); // alerts 'false'
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Comparing a null valued variable and an undefined valued variable will evaluate to true when using the nonstrict comparison and evaluate to false when using the strict comparison, as shown in Listing 119
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Listing 119 Comparing null to Itself
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var x; alert (x == null); // alerts 'true' alert (x === null); // alerts 'false'
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This typeof comparison is using undefined as a primitive value If we were to use undefined as a type, using the keyword typeof to return us a type, comparing to null will evaluate to false, using either strict or nonstrict Listing 120 demonstrates this
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Listing 120 Comparing Using the undefined Type
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var x; alert (typeof(x) == null); // alerts 'false' alert (typeof(x) === null); // alerts 'false'
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As shown in the Function Arguments section later in the chapter, undefined is useful for determining whether an argument was passed into a function
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1: Programming with JavaScript typeof typeof returns a string based on the data type of its operand The operand
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can be a variable, string, object, or keyword typeof is most commonly used to determine whether a variable has been declared and/or assigned to by testing it against the undefined type we described earlier Listing 121 demonstrates this type of usage
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Listing 121 Using typeof to Test Variable Declaration and Assignment
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var x; alert (typeof(x)); // alerts "undefined" function abc (param1) { if (typeof(param1) === "undefined") { alert ("param1 was not supplied"); } } // execute abc, but leave out the parameter abc();
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Using typeof on variables can produce some interesting results Table 13 displays the results of executing typeof on variables of the stated types
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Table 13 typeof Evaluations Statement alert (typeof(new Array())); alert (typeof(new Object ())); alert (typeof(new Date())); alert (typeof(new String("hi!"))); alert (typeof (33)); alert (typeof (true)); alert (typeof ("hi!")); alert (typeof (null)); alert (typeof (undefined)); Output "object" "object" "object" "object" "number" "boolean" "string" "object" "undefined"
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Notice how the null, Array, and Date types return "object" as their data types This might seem a bit odd at rst because we expect the method call to return something more representative of the actual object, such as "Array" or "Date" This is a quirk of the typeof function It returns only the base type of the operand Because all variables at their base types are fundamentally either a primitive data type, an object, or unde ned, those are the only types of values that typeof returns
Function Arguments
You can explicitly de ne a function s arguments using the following common pattern:
function fn(var1, var2)
However, explicit de nition isn t required because method arguments are always supplied to the function in a special local arguments variable This local variable is accessible once inside the function through the arguments keyword Each function has its own local arguments variable, including functions that are contained within other functions The arguments local variable acts like a quasi-array but without any of the array methods such as join or split It has two properties: length and callee length refers to the number of entries in the array, and callee refers to currently executing function