1: Programming with JavaScript in Visual Studio .NET

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1: Programming with JavaScript
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Listing 12 Creating Functions
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var newMethod = new Function("alert ('new method');"); newMethod(); // alerts "new method" function newMethod2() { alert ("new method"); } newMethod2(); // alerts "new method"
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Because we can create an object of type Function, we can declare functions wherever we want without enclosing them inside another concept such as a class, as we normally do in class-based object-oriented programming languages The ability to do this makes the function a rst-class citizen (or object) of the language This idea has important rami cations It means that functions act as the bounding construct of the language and displace what we might consider a normal object-oriented principle, classes No keyword represents the common class idea found in most modern object-oriented programming languages such as Java or C# (The class keyword in JavaScript refers to a CSS class) Rather, as we discuss in the Object-Oriented JavaScript Programming section later in this chapter, functions act as the boundary for new types An important aspect of JavaScript functions is that they are unique only by name, not by name plus arguments as in other languages If we declare one function and then declare another function with the same name, the second function overwrites the rst one
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Primitive Data Types JavaScript has three primitive data types: boolean, number, and string It also has two special values: undefined and null (We cover null and undefined later in this section and explain the differences between undefined the value and undefined the type) Everything else is a variation of the Object type, which we cover in detail in this chapter s Objects section
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booleans
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A boolean has two possible values: true or false booleans can be created by assigning true, false, 1 (indicating true), or 0 (indicating false) to a variable, as shown in Listing 13
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Generally JavaScript
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Listing 13 Declaring boolean Variables
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var x var y var z alert = false; = true; = 1; (y === z); // alerts 'true'
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Numbers
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Numbers are always stored as 64-bit values, similar to doubles in NET Because of this single number type, division between any two numbers can produce fractional results The number type also contains a series of special values shown in Table 11 A number can be manipulated through normal mathematical and bitwise expressions, and normal order-of-operations precedence is applied (parentheses, exponents, multiplication, division, addition, and subtraction) If the current value has a decimal value and a bitwise expression is used, the number is rst converted to a 32-bit integer using rounding, the bitwise expression is applied, and then the number is converted back to a 64-bit double
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Table 11 Special Number Values Constant Definition
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NumberNaN or Nan
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Not a number Useful for determining whether a variable can be coerced into a Number type
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NumberInfinity or Infinity Represents the greatest possible value, but has no numeric value NumberMAX_VALUE Largest possible number represented within the 64 bits Smallest possible number represented within the 64 bits Represents positive in nity Represents negative in nity
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NumberMIN_VALUE
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NumberPOSITIVE_INFINITY NumberNegative_INFINITY
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1: Programming with JavaScript
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Strings
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A string is a sequence of zero or more Unicode values used to represent text They are immutable (modi cation produces a new string), and there is no separate character type that represents a string of length one Strings are created using quotation marks They can be either single (') or double (") quotation marks, but they have to be paired properly The forward slash (\) is used for escaping quotes and special characters within a string Listing 14 demonstrates some patterns used to create strings
Listing 14 Declaring String Variables
var x = "Hello!"; var y = 'Hello Again!'; var z = 'Hello, I\'m Bob';
Table 12 shows the other special characters that use the forward slash to escape them
Table 12 Special Characters Escape Sequence Output
\' \" \\ \b \t \n \r \f \ddd \xdd \udddd
Single quote ( ) Double quote ( ) Backslash (\) Backspace Horizontal tab New line character Carriage return character Form feed character Octal sequence (3 digits) Hexadecimal sequence (2 hex digits) Unicode sequence (4 hex digits)
Strings
NOTE
String Concatenation Is Expensive!
Any time you assign a string to a variable, memory is allocated from the heap to store that string This occurs because strings are immutable They cannot change after they have been assigned to a variable Therefore, take care, where possible, to avoid concatenating strings Certain techniques are available, which the Microsoft AJAX Library makes readily accessible through the SysStringBuilder type, to avoid string concatenations through the use of arrays to store string parts We strongly suggest that if you want to write string concatenation code that performs well, use the SysStringBuilder class just as you would on the server
Objects Besides variables that are primitive data types, every other variable in JavaScript is an object Functions, dates, and Document Object Model (DOM) elements, among many others, are all objects Objects are how we extend the language with our own types, write modular code, and generally make our code easier to understand We use objects extensively throughout the rest of this book, and we discuss how to use them in a classic object-oriented system later in this chapter For now, however, let s go over some object basics