Decisions and Loops - Beginning JavaScript - page 67

The easiest way to work out what multiple conditions are doing is to split them up into smaller
pieces and then evaluate the combined result. In this example you have entered the value 30 ,
which has been stored in the variable myAge . You'll substitute this value into the conditions to see how
they work.
Here's the first if statement:
if (myAge >= 0 && myAge <= 10) {
document.write("myAge is between 0 and 10<br />");
}
The first if statement is asking the question, “Is myAge between 0 and 10?” You'll take the LHS of
the condition first, substituting your particular value for myAge . The LHS asks, “Is 30 greater than or
equal to 0?” The answer is true . The question posed by the RHS condition is “Is 30 less than or equal
to 10?” The answer is false . These two halves of the condition are joined using && , which indicates
the AND operator. Using the AND results table shown earlier, you can see that if LHS is true and RHS
is false , you have an overall result of false . So the end result of the condition for the if statement is
false , and the code inside the braces won't execute.
Let's move on to the second if statement:
if ( !(myAge >= 0 && myAge <= 10) ) {
document.write("myAge is NOT between 0 and 10<br />");
}
The second if statement is posing the question, “Is myAge not between 0 and 10?” Its condition is
similar to that of the first if statement, but with one small difference: You have enclosed the condition
inside parentheses and put the NOT operator ( ! ) in front.
The part of the condition inside the parentheses is evaluated and, as before, produces the same result—
false . However, the NOT operator reverses the result and makes it true . Because the if statement's
condition is true , the code inside the braces will execute this time, causing a document.write() to
write a response to the page.
What about the third if statement?
if ( myAge >= 80 || myAge <= 10 ) {
document.write("myAge is 80 or above OR 10 or below<br />");
}
The third if statement asks, “Is myAge greater than or equal to 80, or less than or equal to 10?” Taking
the LHS condition first—“Is 30 greater than or equal to 80?”—the answer is false . The answer to the
RHS condition—“Is 30 less than or equal to 10?”—is again false . These two halves of the condition
are combined using | | , which indicates the OR operator. Looking at the OR result table earlier in this
section, you see that false OR false produces a result of false . So again the if statement's condition
evaluates to false , and the code within the curly braces does not execute.
The final if statement is a little more complex:
if ( (myAge >= 30 && myAge <= 39) || (myAge >= 80 && myAge <= 89) ) {
document.write("myAge is between 30 and 39 or myAge is between 80 and 89");
}