java barcode generator example JavaScript: A Beginner s Guide in Java

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JavaScript: A Beginner s Guide
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string Placing the double quotes there without escaping them causes the string to terminate and gives an error As you can see, quotation marks can be a real pain when you need to use a large number of them within a string However, remembering to use the backslash to escape the quotes when necessary will save you quite a few headaches when you are looking for a missing quote I ve had to look for missing quotes in my code a number of times, and my head was spinning after a few of those encounters! Later in this chapter, you will see that you can add strings together, which can simplify the use of quotes for you
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Boolean
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A Boolean variable is one with a value of true or false Here are examples:
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var JohnCodes=true; var JohnIsCool=false;
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Notice that the words true and false do not need to be enclosed in quotes This is because they are reserved words, which JavaScript recognizes as Boolean values Instead of using the words true and false, JavaScript also allows you to use the number 1 for true and the number 0 for false, as shown here:
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var JohnCodes=1; var JohnIsCool=0;
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Using the number 1 is the same as using the value of true Using the number 0 is the same as using the value of false
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Boolean variables are useful when you need variables that can only have values of true and false, such as in event handlers (covered in 7)
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When we talk about the concept of a Boolean variable, the first letter of the word Boolean is capitalized (because it is derived from the name of the mathematician George Boole) However, the JavaScript reserved word boolean is written in all lowercase letters when you use the keyword in a script
Null
Null means that the variable has no value It is not a space, nor is it a zero; it is simply nothing If you need to define a variable with a value of null, use a declaration like this:
var variablename=null;
As with the Boolean variables, you do not need to enclose this value in quotation marks as you do with string values, because JavaScript recognizes null as a keyword with a predefined value (nothing) Null variables are useful when you test for input in scripts, as you ll learn in later chapters
3: Using Variables
Ask the Expert
Q: A:
Why do I need to learn about variables Couldn t I just put in the number or text I want to use right where I m going to use it You can do that; however, it will make longer scripts much harder to write, read, and debug It also makes it much more difficult to update your scripts because, in order to change that number or text, you would need to change every line where it appears When you use variables, you can modify just one line of code to change the value of a variable every place it is used As you gain more experience with JavaScript, you will see just how useful variables are Why don t I need to define the type of number I am using (such as float or integer) when I declare a numeric variable JavaScript doesn t require this, which can be a good or bad feature depending on your perspective To JavaScript, any number is just a number and can be used as a number variable Why do I need to put quotation marks around the text in a string This is done so that JavaScript knows where a string begins and ends Without it, JavaScript would be unsure what should be in a string and what should not But doesn t a semicolon end a statement Why not use that and lose the quote marks A variable declaration or any command involving strings can become more complex when the addition operator is used to add two strings and/or variables together When this happens, JavaScript needs to know when one string stops and another begins on the same line What does the backslash (\) character do, in general If the backslash is followed by a code to create a special character, the special character is rendered in its place Otherwise, the first character after a single backslash is seen as-is by JavaScript and treated as part of the string in which it resides
Q: A: Q: A: Q: A: Q: A:
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