javascript

CHAPTER 4 ■ CONTROLLING FLOW
98
statement following its boolean expression. In the following if statement, run(miles); is the one child
statement permitted by JavaScript:
if (timeToRun === true) run(miles);
Oftentimes this will not do, and JavaScript knows this. If you bundle several child statements in a
pair of curly braces, JavaScript will look the other way and view the bundle, referred to as a block, as one
child statement. So if I want JavaScript to run three child statements whenever it’s time to run, which is
to say timeToRun contains true, then I can bundle those statements in a block. JavaScript will be happy as
a clam, and I get to run in shoes rather than barefoot:
if (timeToRun === true) {
lace(shoes);
run(miles);
shower();
}
Note that the block of child statements is not followed by a semicolon. However, the child
statements within the block are.
Writing an if Condition
Oftentimes, you will want JavaScript to run a path if circumstances permit but otherwise do nothing and
move on. if conditional statements will be your bread and butter for this type of decision. To write one
of these statements, simply type the keyword if, followed by an expression in parentheses, and then a
path in the form of a child statement or block. In the event that the expression does not return a
boolean, JavaScript will convert the value to a boolean by passing it to Boolean(), which we explored in
Chapter 2. So if the expression returns any value other than undefined, null, "", 0, NaN, or false,
JavaScript has a green light to run the path.
Therefore, a JavaScript interpreter views an if condition like so:
if (Boolean(expression)) path
But you write it like this:
if (expression) path
Open firebug.html in Firefox and then press F12 to enable Firebug. If you’re just joining us, flip back
to the Preface for details on how to do this.
For any fast running I do, I tend to wear the Nike Mayfly, which weighs just four ounces. By
comparison, most running shoes weigh three or four times as much. However, the downside to the
Mayfly’s minimalist design is that its cushioning goes dead after just 100 kilometers.
Let’s create a mayfly object containing two methods that may query a secret variable named tally,
which will contain a tally of kilometers run in a pair of Mayfly shoes. mayfly.addToTally() adds its
parameter (named km) to tally only if km is safe for addition—that is to say, if km is of the number
datatype but not the special numbers NaN or Infinity. The other method, mayfly.kmLeftToLive(), will
return a message indicating how many kilometers of cushioning the Mayfly has left only if tally is less
than 100.
So in Firebug, enter the following code, and click Run. Doing so creates a closure so that tally may
be queried only by addToTally() and kmLeftToLive(). Closures are covered in Chapter 6, so just nod
knowingly for now. Anyway, just focus on the two if conditions.
var mayfly = function () {
var tally = 0;
return {