- Adding jQuery and JavaScript to a Web Page
- Accessing the DOM
- Understanding JavaScript Syntax
- Understanding JavaScript Syntax, continued
- Summary / Q&A / Workshop
This chapter is from the book
This chapter is from the book
This chapter is from the book
Understanding JavaScript Syntax
Like any other computer language, JavaScript is based on a rigid syntax where specific words mean different things to the browser as it interprets the script. This section is designed to walk you through the basics of creating variables, working with data types, and using looping and functions in JavaScript to manipulate your web pages.
Creating Variables
The first place to begin with in JavaScript is variables. Variables are a means to name data so that you can use that name to temporarily store and access data from your JavaScript files. Variables can point to simple data types, such as numbers or strings, or they can point to more complex data types, such as objects.
To define a variable in JavaScript, you must use the var keyword and then give the variable a name; for example:
var myData;
You can also assign a value to the variable in the same line. For example, the following line of code creates a variable myString and assigns it the value of "Some Text":
var myString = "Some Text";
This works as well as
var myString; myString = "Some Text";
After you have declared the variable, you can use the name to assign the variable a value and access the value of the variable. For example, the following code stores a string into the myString variable and then uses it when assigning the value to the newString variable:
var myString = "Some Text"; var newString = myString + "Some More Text";
Your variable names should describe the data that is stored in them so that it is easy to use them later in your program. The only rule for creating variable names is that they must begin with a letter, $, or _, and they cannot contain spaces. Also remember that variable names are case sensitive, so using myString is different from MyString.
Understanding JavaScript Data Types
JavaScript uses data types to determine how to handle data that is assigned to a variable. The variable type will determine what operations you can perform on the variable, such as looping or executing. The following list describes the most common types of variables that we will be working with through the book:
- String—Stores character data as a string. The character data is specified by either single or double quotes. All the data contained in the quotes will be assigned to the string variable. For example:
var myString = 'Some Text'; var anotherString = “Some Other Text";
var myInteger = 1; var cost = 1.33;
var yes = true; var no = false;
var arr = ["one", "two", "three"] var first = arr[0];
var obj = {"name":"Brad", "occupation":"Hacker", "age", "Unknown"};
var name = obj.name;
var newVar = null;
Using Operators
JavaScript operators provide the capability to alter the value of a variable. You are already familiar with the = operator because you used it several times in the book already. JavaScript provides several operators that can be grouped into two types—arithmetic and assignment.
Arithmetic Operators
Arithmetic operators are used to perform operations between variable and direct values. Table 5.1 shows a list of the arithmetic operations along with the results that get applied.
Table 5.1 Table Showing JavaScript’s Arithmetic Operators as Well as Results Based on y=4 to Begin With
|
Operator |
Description |
Example |
Resulting x |
Resulting y |
|---|---|---|---|---|
|
+ |
Addition |
x=y+5 x=y+"5" x="Four"+y+"4" |
9"49" "Four44" |
444 |
|
- |
Subtraction |
x=y-2 |
2 |
4 |
|
++ |
Increment |
x=y++ |
4 |
5 |
|
x=++y |
5 |
5 |
||
|
-- |
Decrement |
x=y-- |
4 |
3 |
|
x=--y |
3 |
3 |
||
|
* |
Multiplication |
x=y*4 |
16 |
4 |
|
/ |
Division |
x=10/y |
2.5 |
4 |
|
% |
Modulous (remainder of Division) |
x=y%3 |
1 |
4 |
Assignment Operators
Assignment operators are used to assign a value to a variable. You are probably used to the = operator, but there are several forms that allow you to manipulate the data as you assign the value. Table 5.2 shows a list of the assignment operations along with the results that get applied.
Table 5.2 JavaScript’s Assignment Operators as Well as Results Based on x=10 to Begin With
|
Operator |
Example |
Equivalent Arithmetic Operators |
Resulting x |
|---|---|---|---|
|
= |
x=5 |
x=5 |
5 |
|
+= |
x+=5 |
x=x+5 |
15 |
|
-= |
x-=5 |
x=x-5 |
5 |
|
*= |
x*=5 |
x=x*5 |
25 |
|
/= |
x/=5 |
x=x/5 |
2 |
|
%= |
x%=5 |
x=x%5 |
0 |
Applying Comparison and Conditional Operators
Conditionals are a way to apply logic to your applications so that certain code will be executed only under the correct conditions. This is done by applying comparison logic to variable values. The following sections describe the comparisons available in JavaScript and how to apply them in conditional statements.
Comparison Operators
A comparison operator evaluates two pieces of data and returns true if the evaluation is correct or false if the evaluation is not correct. Comparison operators compare the value on the left of the operator against the value on the right.
The simplest way to help you understand comparisons is to provide a list with some examples. Table 5.3 shows a list of the comparison operators along with some examples.
Table 5.3 JavaScript’s Comparison Operators as Well as Results Based on x=10 to Begin With
|
Operator |
Example |
Example |
Result |
|---|---|---|---|
|
== |
Is equal to (value only) |
x==8 |
false |
|
x==10 |
true |
||
|
=== |
Both value and type are equal |
x===10 |
true |
|
x==="10" |
false |
||
|
!= |
Is not equal |
x!=5 |
true |
|
!== |
Both value and type are not equal |
x!=="10" |
true |
|
x!==10 |
false |
||
|
> |
Is greater than |
x>5 |
true |
|
>= |
Is greater than or equal to |
x>=10 |
true |
|
< |
Is less than |
x<5 |
false |
|
<= |
Is less than or equal to |
x<=10 |
true |
You can chain multiple comparisons together using logical operators. Table 5.4 shows a list of the logical operators and how to use them to chain comparisons together.
Table 5.4 JavaScript’s Comparison Operators as Well as Results Based on x=10 and y=5 to Begin With
|
Operator |
Description |
Example |
Result |
|---|---|---|---|
|
&& |
and |
(x==10 && y==5)
(x==10 && y>x) |
true false |
|
|| |
or |
(x>=10 || y>x)
(x<10 && y>x) |
true false |
|
! |
not |
!(x==y)
!(x>y) |
true false |
|
|
mix |
(x>=10 && y<x || x==y) ((x<y || x>=10) && y>=5) (!(x==y) && y>=10) |
true true false |
If
An if statement enables you to separate code execution based on the evaluation of a comparison. The syntax is shown in the following lines of code where the conditional operators are in () parentheses and the code to execute if the conditional evaluates to true is in {} brackets:
if(x==5){
do_something();
}
In addition to executing code only within the if statement block, you can specify an else block that will get executed only if the condition is false. For example:
if(x==5){
do_something();
} else {
do_something_else();
}
You can also chain if statements together. To do this, add a conditional statement along with an else statement. For example:
if(x<5){
do_something();
} else if(x<10) {
do_something_else();
} else {
do_nothing();
}
switch
Another type of conditional logic is the switch statement. The switch statement allows you to evaluate an expression once and then, based on the value, execute one of many sections of code.
The syntax for the switch statement is the following:
switch(expression){
case value:
code to execute
break;
case value2:
code to execute
break;
default:
code to execute if not value or value2.
}
This is what is happening. The switch statement will evaluate the expression entirely and get a value. The value may be a string, a number, a Boolean, or even an object. The switch value is then compared to each value specified by the case statement. If the value matches, the code in the case statement is executed. If no values match, the default code is executed.
Applying if Conditional Logic in JavaScript
To help you solidify using JavaScript conditional logic, use the following steps to build conditional logic into the JavaScript for a dynamic web page. The final version of the HTML document is shown in Listing 5.3:
- Create a source file named hour0503.html in the hour05 folder.
- Create a folder under hour05 named images.
- Copy the day.png and night.png images from the website under code/hour05/images, or substitute your own into the images folder.
- Add the usual basic elements (html, head, body).
- Add the following <script> element that gets the hour value using the Date().getHours() JavaScript code. The code uses if statements to determine the time of day and does two things: It writes a greeting onto the screen and sets the value of the timeOfDay variable.
- Now add the following switch statement that uses the value of timeOfDay to determine which image to display in the web page:
- Save the file and view it in a web browser. The output should be similar to Figure 5.3, depending on what time of day it is.
06 <script>
07 function writeIt(){
08 var hour = new Date().getHours();
09 var timeOfDay;
10 if(hour>=7 && hour<12){
11 document.write("Good Morning!");
12 timeOfDay="morning";
13 } else if(hour>=12 && hour<18) {
14 document.write("Good Day!");
15 timeOfDay="day";
16 } else {
17 document.write("Good Night!");
18 timeOfDay="night";
19 }
32 }
33 </script>
20 switch(timeOfDay){
21 case "morning":
22 case "day":
23 document.write("<img src='images/day.png' />")
24 break;
25 case "night":
26 document.write("<img src='images/night.png' />")
27 break;
28 default:
29 document.write("<img src='images/day.png' />")
30 break;
31 }
)
Figure 5.3 The function writeIt() is executed when the body loads and changes the greeting and image displayed on the web page.
Listing 5.3 Simple Example of Using Conditional Logic Inside JavaScript
01 <!DOCTYPE html>
02 <html>
03 <head>
04 <title>Hour 5-3</title>
05 <meta charset="utf-8" />
06 <script>
07 function writeIt(){
08 var hour = new Date().getHours();
09 var timeOfDay;
10 if(hour>=7 && hour<12){
11 document.write("Good Morning!");
12 timeOfDay="morning";
13 } else if(hour>=12 && hour<18) {
14 document.write("Good Day!");
15 timeOfDay="day";
16 } else {
17 document.write("Good Night!");
18 timeOfDay="night";
19 }
20 switch(timeOfDay){
21 case "morning":
22 case "day":
23 document.write("<img src='images/day.png' />")
24 break;
25 case "night":
26 document.write("<img src='images/night.png' />")
27 break;
28 default:
29 document.write("<img src='images/day.png' />")
30 break;
31 }
32 }
33 </script>
34 </head>
35 <body onload="writeIt()">
36 </body>
37 </html>