JavaScript Statements

 

JavaScript Statements

This is part six of the JavaScript tutorial. If you missed the previous lessons, make sure you complete them prior to viewing this JavaScript Statements tutorial.

In JavaScript we use statements that tell the script to check to see if something is true – or present – or not. Common JavaScript statements are if, for, else, and else if. These statements are very important to JavaScript, and you will be using them a lot when you write your own JavaScript.

Let’s first look at the ‘if’ statement. We use if to check for equalities. The if statement always includes the parentheses () after it, and what we are checking for goes inside the parentheses. We also use curly brackets {}with the if statement. The code that we want to run is inside the curly brackets.

 
So, what the script does is it looks to see if the condition that we have specified in the parentheses exists, and if it does, it runs the code inside the curly brackets that follow it. If the condition does not exist, it skips the code inside the curly brackets. Here is an example:

var a = 1;
var b = 2;
var c = 3;
if ((a != 0) || ( (b >= 2) && !(c > 9) ))
{
document.write("This is right.");
}

Now, you see on the if statement that we’ve used some characters: !, | |, &&, and >. These characters all mean something.

The ! character stands for NOT. In our example, it means if a is not equal to 0.

The | | characters means OR and the > character means greater than. Our example now reads if a is not equal to 0 or b is not greater than 2.

The && Characters mean AND. The statement now reads if a is not equal to 0 or b is greater than 2 and it is not true that c is greater than 9. Now, if this statement is true, or the condition exists, the code inside the curly brackets will run, and the words This is right will be printed on the web page. Using the ! character before a statement changes the meaning of the statement to mean that it is NOT true. In this example, c is greater than 9 is NOT true.

Pay special attention to how parentheses are used in the example. This is very important, and if it isn’t done correctly, the script will not run.

Next is the else statement. Else would be used if the if statement is not true. If you will have more than two possible conditions, you would use if first, followed by else if, followed by else. You can have as many else if statements as you need.

var a = 0;
if (a > 0)
{ do something };

else if { do something else };

else { do this };

 
The JavaScript reads from top to bottom. If the if statement is true, it runs the code in the curly brackets, and ignores the else if and else statement that follows.

The for statement is used to perform a loop. This means that the script will run the same piece of code for however many times you specify. Here is an example:

for (var a=0; a<10; a++)
{ document.write("Test" + a); }

This statement says that as long as a is less than 10, the code inside the curly brackets should run. Furthermore, each time the code is run, it should be increased by 1. This is indicated with the double plus signs. You could also have it decrease by 1 each time by using a double minus sign, —

 
This concludes the JavaScript Statements lesson. In the next lesson, we will focus on JavaScript Functions and Methods.