Chapter 2: How to code a JavaScript application

This chapter presents a subset of JavaScript and DOM scripting that will soon have you writing significant applications. If you don't have any programming experience, this chapter also makes a great aptitude test. If you read it and can do the exercises at the end of the chapter, you're ready for the rest of this book.

How to code numeric and string expressions

An expression can be as simple as a single value or it can be a series of operations that result in a single value. In figure 2-8, you can see the operators for coding both numeric and string expressions. If you've programmed in another language, these are probably similar to what you've been using. In particular, the first four arithmetic operators are common to most programming languages.

Most modern languages also have a modulus, or mod, operator that calculates the remainder when the left value is divided by the right value. In the example for this operator, 13 % 4 means the remainder of 13 / 4. Then, since 13 / 4 is 3 with a remainder of 1, 1 is the result of the expression.

One of the common uses of the mod operator is determining if a number is even or odd. For instance, any number % 2 will return 0 if the number is even and 1 if the number is odd. Another use is determining if a year is a leap year. If, for example, year % 4 returns 0, then year is divisible by 4.

In contrast to the first five operators in this figure, the increment and decrement operators add or subtract one from a variable. If you aren't already familiar with these operators, you'll see them illustrated later in this book.

When an expression includes two or more operators, the order of precedence determines which operators are applied first. This order is summarized in the table in this figure. For instance, all multiplication and division operations are done from left to right before any addition and subtraction operations are done.

To override this order, though, you can use parentheses. Then, the expressions in the innermost sets of parentheses are done first, followed by the expressions in the next sets of parentheses, and so on. This is typical of all programming languages, and the examples in this figure show how this works.

The one operator that you can use with strings is the concatenation operator. This operator joins two strings with no additional characters between them.

Since the plus sign is used for both addition and concatenation, JavaScript has to look at the data type of the values to determine whether it should add or concatenate the values. If both values are numbers, JavaScript adds them together. If both values are strings, JavaScript concatenates them. And if one value is a string and the other is a number, JavaScript converts the number to a string and then concatenates the two values.

Common arithmetic operators

Operator	Description	Example	Result
+	Addition	5 + 7	12
-	Subtraction	5 - 12	-7
*	Multiplication	6 * 7	42
/	Division	13 / 4	3.25
%	Modulus	13 % 4	1
++	Increment	counter++	adds 1 to counter
--	Decrement	counter--	subtracts 1 from counter

The order of precedence for arithmetic expressions

Order	Operators	Direction	Description
1	++	Left to right	Increment operator
2	--	Left to right	Decrement operator
3	* / %	Left to right	Multiplication, division, modulus
4	+ -	Left to right	Addition, subtraction

Examples of precedence and the use of parentheses

3 + 4 * 5 // Result is 23 since the multiplication is done first
(3 + 4) * 5 // Result is 35 since the addition is done first
13 % 4 + 9 // Result is 10 since the modulus is done first
13 % (4 + 9) // Result is 0 since the addition is done first

The concatenation operator for strings

Operator	Description	Example	Result
+	Concatenation	"Ray " + "Harris" "Months: " + 120	"Ray Harris" "Months: 120"

Description

• To code an arithmetic expression, you can use the arithmetic operators to operate on two or more values.
• An arithmetic expression is evaluated based on the order of precedence of the operators.
• To override the order of precedence, you can use parentheses.
• To concatenate two or more strings, you can use the + operator.
• Since the plus sign can be used for both numeric and string expressions, JavaScript looks at the data types to determine which operation to perform. If both values are numbers,
  JavaScript adds. If both values are strings, JavaScript concatenates. If one value is a number and one is a string, JavaScript converts the number to a string and concatenates.

Figure 2-8 How to code numeric and string expressions