Constants and variables in Java

Generate Code 39 Extended in Java Constants and variables

There are two kinds of values you ll be using in JavaScript (or in any other kind of computer programming): constants (values that never change) and variables (which change depending on the circumstances) The first kind is like the dates of Christmas Eve or New Year s Eve these will always fall on the 24th and the 31st of December, no matter what else is going on The latter, however, are like Easter which might fall on a different day in a particular year depending on the circumstances The first thing you have to do with a variable is to define it; no variable can have any value unless, of course, that variable actually exists first As you d imagine, this is a fairly simple affair: To establish that a particular variable exists, you simply declare that this is so by adding a line like this to your JavaScript: var thisvariableexists If you want to assign a starting value to a variable at the same time that you declare it to exist, the process is just a tad different: var thisvariableexists=1 This line of code both declares the existence of a variable called thisvariableexists and assigns to it a value of 1 You actually don t have to use the var part at all Most Web browsers will understand what you mean if you just say thisvariableexists=1 Even so, it s good programming practice to specify exactly what you mean rather than just leaving things to chance (and maybe hoping that, years later, you ll remember exactly what you meant by some nonstandard usage)

You ll probably want to use lots of comments or notes of just exactly what you meant when you entered certain lines of code To do so in a JavaScript situation, all you have to do is to use the figures // at the beginning of a line of code, as in the following example: //This is a JavaScript comment Normally that s all you need to do just the double slash lines with nothing more However, if a comment requires more space than one line can provide, you d use opening and ending tags to specify it, as in the following example: /*This is an example of JavaScript code that exceeds several lines */ The opening and ending tags, of course, would be /* and */ Even if a value will never vary (in which case it s not technically a true variable) the date of New Year s Eve, for example you can still assign that value with a var declaration Variables can have one of three value types, and those types influence the kinds of calculations that you can perform based upon them The first type (that we ve already dealt with) is the numeric kind Numeric values, as their name implies, are nothing more than numbers: 31 for the end of December, 12 for that month, and so on Strings, however, are composed of textual values rather than numerical ones, and a string (of letters) can have any value that human languages may impute to it Thus the string value December may have many more possible interpretations than merely that it is 12th in a series of 12 Depending upon the particular application involved in interpreting that string, it may mean anything from the month in which Christmas and Hanukkah occur to the time when winter begins In addition to numerical or string values, you ll have to deal with Boolean values This sounds like a fearsome thing, but all it really means is whether something is or isn t logically true (By the way, everything in computing revolves around this concept; the simplest possible computer program is one that tells whether or not a light bulb is lit, and the most complex conceivable 3D graphics software is merely a descendant of this sort of statement) String values, unlike their numeric or Boolean equivalents, are always enclosed within quotation marks