Decisions and Loops - Beginning JavaScript - page 81

Let's take a look at the structure of the while statement, as
illustrated in Figure 3-12.

Condition—keep looping while this
condition is still true

You can see that the while loop has fewer parts to it
than the for loop. The while loop consists of a condition
which, if it evaluates to true , causes the block of code
inside the curly braces to execute once; then the condition
is re‐evaluated. If it's still true , the code is executed again,
the condition is re‐evaluated, and so on until the condition
evaluates to false .

while ( degCent != 100)
{

// some code

}

One thing to watch out for is that if the condition is false to
start with, the while loop never executes. For example:

Code looped through

var degCent = 100;

figure 3-12

while (degCent != 100) {
// some code
}

Here, the loop will run if degCent does not equal 100 . However, because degCent is 100 , the
condition is false , and the code never executes.

In practice you would normally expect the loop to execute once; whether it executes again will
depend on what the code inside the loop has done to variables involved in the loop condition. For
example:

var degCent = [];
degFahren = [34, 123, 212];
var loopCounter = 0;
while (loopCounter < 3) {
degCent[loopCounter] = 5/9 * (degFahren[loopCounter] - 32);
loopCounter++;
}

The loop will execute so long as loopCounter is less than 3 . It's the code inside the loop
( loopCounter++; ) that increments loopCounter and will eventually cause loopCounter < 3 to be
false so that the loop stops. Execution will then continue on the first line after the closing brace of
the while statement.

Something to watch out for is the infinite loop —a loop that will never end. Suppose you forgot to
include the loopCounter++; line in the code. Leaving this line out would mean that loopCounter
will remain at 0 , so the condition (loopCounter < 3) will always be true , and the loop will
continue until the user gets bored and cross, and shuts down her browser. However, it is an easy
mistake to make, and one that JavaScript won't warn you about.

It's not just missing lines that can cause infinite loops, but also mistakes inside the loop's code. For
example:

var testVariable = 0;
while (testVariable <= 10) {