3: JavaScript Language Basics in Java

Generation QR in Java 3: JavaScript Language Basics

You might notice that Listing 3-1 contains two if-else statements, one nested inside the other Technically speaking, you can nest as many if-else statements as you want If you run across a situation in which you need more than one or two nested if-else statements to do the job, however, you might want to consider the switch statement (which I describe in the next section) instead The switch statement is much more efficient at testing a condition multiple times Some JavaScript programmers end each statement with a semicolon, like this:

if (a == b) { // if a is equal to b c = d; // assign the value of d to the c variable, e = f; // assign the value of f to the e variable, // and assign the string American Beauty // to the variable called favoriteMovie favoriteMovie = American Beauty ; }

Semicolons are optional in JavaScript, with one exception If you place more than one JavaScript statement on the same line, you must separate those statements with semicolons For example:

// Correct (if a bit hard to read) c = d; e = f; favoriteMovie = American Beauty ;

The switch statement provides an easy way to check an expression for a bunch of different values without resorting to a string of if-else statements Here s the syntax:

switch (expression) { case label : statement break case label : statement break default : statement }

Suppose you want to examine a value and find out whether it matches one of a number of predefined values Listing 3-2 shows how you can go about it by using the switch statement

switch (month) { case 0 : displayMonth = January break case 1 : displayMonth = February break case 2 : displayMonth = March break case 3 : displayMonth = April break case 4 : displayMonth = May break case 5 : displayMonth = June break case 6 : displayMonth = July break case 7 : displayMonth = August break case 8 : displayMonth = September break case 9 : displayMonth = October break case 10 : displayMonth = November break case 11 : displayMonth = December break default: displayMonth = INVALID }

The code shown in Listing 3-2 tests the value of the month variable If month contains the number 0, the variable displayMonth is set to January If month contains the number 1, displayMonth is set to February and so on, all the way through the 12 months of the year

The companion CD contains a date_and_time_formattedhtm file, a working copy of the script in Listing 3-2 Note that if you forget to finish each case with a break statement (and it s easy to do), the interpreter falls through, meaning that it performs all the statements that it finds until it either Finds a break Detects the end of the switch statement For instance, in Listing 3-2, if you removed all the break statements, a month value of 0 would cause displayMonth to be set not to January, as it should be, but to INVALID instead In some cases, you may want to leave out the break statement on purpose to force the JavaScript interpreter to fall through two or more cases Doing so allows you to group values easily For example, the following code treats month values of 0, 1, or 2 (which correspond to January, February, and March, respectively) the same, by assigning the value Q1 to the displayQuarter variable Months 3, 4, and 5 (April, May, and June, respectively) are treated the same, by assigning the value Q2 to the displayQuarter variable; and so on

switch (monthId) { case 0: case 1: case 2: displayQuarter break; case 3: case 4: case 5: displayQuarter break; case 6: case 7: case 8: displayQuarter break; case 9: case 10: case 11: displayQuarter break; }

= Q1 ;

= Q2 ;

= Q3 ;

= Q4 ;