choice

alternatives

up till now, all the code we've written was always executed.
at first thought, this may sound perfectly desirable, but it can be limiting. we're forced to follow the same course of action each time through the program. if we could specify blocks of code that wouldn't always execute, that would allow us alternatives. we could do some things only when we felt like it—when the conditions were right.

if, then; else

so first we need to be able to specify conditions, statements that evaluate to be true, or false. remember these are boolean expressions, and we use them already in our loops to check if we should continue looping:

while (x >= 5)
for (var y=0; y < x; y++)

boolean values

we can store boolean values in variables, either directly, or as the result of evaluating a statement or calling a function:

var expensive:Boolean = false;
var outrageous:Boolean = true;
var y:Boolean = (5 < 28);
var n:Boolean = (1+1 == 3);
var stillPlaying:Boolean = inBounds();

logical AND, OR

in logic, or when speaking, it's natural to say things like:
“the price is cheap AND the style is outrageous.”, or
“the color was green OR brown.”
in ecmascript, we have corresponding operators for logical AND– &&, and logical OR– ||

(price == cheap) && (style == outrageous)
(color == green) || (color == brown)

if.. (then)

conditional statements are composed using the keyword if, a boolean expression in parentheses, and code to execute when the conditional expression evaluates to true inside curly braces { }

so, to only purchase the coolest items, and do nothing otherwise:

if (outrageous && !expensive) { buy(); }

or, to pick over the not-so-tasty strawberries:

if (green || brown) { discard(); }

remember that the code inside the curly braces will be executed only in the event that the boolean expression is true; at any other time execution of the program will skip right past, ignoring what's inside the conditional statement.

..else

if we always want to take some action, we can add the keyword else. this allows us to act on the alternative, when the if expression is false

if (outrageous && !expensive) { buy(); }
else { goToNewStore(); }

now some code will be executed every time this program is run, but which code it is depends on the values of the two boolean variables outrageous and expensive.

switch

if we have several possible conditions to check, we can string together several if..else clauses:

var choice:Number = Math.floor(Math.random() * 3);
if (choice == 0) {
    // first option
}
else if (choice == 1) {
// second option
}
else if (choice == 2) {
    // third option
}

or we could also use a switch statement:

var choice:Number = Math.floor(Math.random() * 3);
switch (choice) {
    case (0) :
    // first option
    break;
    
    case (1) :
    // second option
    break;
    
    case (2) :
    // third option
    break;
}