The JavaScript Diaries: Part 6

By Lee Underwood

  function computer(monitor,hd,ram) {
    this.monitor=monitor;
    this.hd=hd;
    this.ram=ram;
  }

Let's look at what we did here:

  1. We used the function reserved word to create a function called computer.
  2. Next, we assigned parameters to the function: monitor, hd, and ram.
  3. We then assigned those parameters to properties by the same name.
    1. The properties are assigned using the this reserved word. The statement says "use the instance of the parameter contained in this object," e.g.:
      • Create a property called monitor, using the parameter monitor from this object
      • Create a property called hd, using the parameter hd from this object
      • Create a property called ram, using the parameter ram from this object

Now we need to create a new instance of the object. We'll create two of them. (You can create as many instances of the object as you need, limited by memory of course.)

  var sysNeed = new computer("17\"","80GB","512MB");
  var sysWant = new computer("21\"","120GB","1GB");

Let's break this down and see what we've done:

  1. First, we declared a variable called sysNeed
  2. Next, we did two things, which happen at the same time:
    1. We created a new instance of the computer object using the new reserved word.
    2. We initialized it using a function call to the computer constructor function, assigning the string 17 to the monitor parameter, the string 80GB to the hd parameter, and the string 512MB to the ram parameter. (Note the "\" before the double quote after the number 17. This tells the JavaScript interpreter to literally interpret the following character, in this case the double quote.)
  3. We then repeated the process, creating the variable sysWant.

Now, we need to write some code to display those properties:

  document.write("I want a "+sysWant.monitor+" screen<br>");
  document.write("but I'll settle for a "+sysNeed.monitor+".<br><br>");
  document.write("The "+sysWant.hd+" hard drive<br>");
  document.write(" should work with "+sysNeed.ram+" of RAM.<br><br>");
  document.write("I really would like to have "+sysWant.ram+" of RAM.");

(I have shortened the lines above for display purposes but you could code them into three lines, or less, if you wish.)

Let's take a look at how this is written (we'll only break down the first two lines as the others are basically the same):

  1. We created a document.write statement using the following:
    1. We created a string, "I want a". (Note the blank space at the end of the string.)
    2. That was then concatenated to the value of the monitor property taken from the new instance of the computer object, which we named sysWant, e.g. sysWant.monitor.
    3. That was then concatenated to the string " screen but I'll settle for a". (Note the blank space at the beginning and at the end of the string.)
    4. That was then concatenated to the value of the monitor property taken from the new instance of the computer object, which we named sysNeed, e.g. sysNeed.monitor.
    5. Then we concatenated the string ".<br><br>" to finish off the statement.

The printout from the script should look like this:

I want a 21" screen
but I'll settle for a 17".

The 120GB hard drive
should work with 512MB of RAM.

I really would like to have 1GB of RAM.

An Object Initializer (a.k.a. "Object Literal")

Another method that could be used when you have a limited amount of information is an object initializer (a.k.a. object literal). There is no need to create a new instance of the object. The format is:

  objectName={property:value}

In our preceding script, in place of the constructor function, you would write it as follows:

  sysNeed = {monitor:"17\"",hd:"80GB",ram:"512MB"}
  sysWant = {monitor:"21\"",hd:"120GB",ram:"1GB"}

You wouldn't need to set up the variables. Just replace the script in the head with the one above and then reuse the same document.write statements.

Let's take another look at the document.write() statement that we've been using. Does it look familiar now, in light of our study of objects? JavaScript has several built-in objects and document is one of them. It's also a property of the window object. Technically, we should write it as: window.document.write() but since there can only be one document in a window at a time, references to objects inside the document can eliminate the window portion of the code, as it is assumed. So we see that the document is a property of the window object. It's also an object itself. The write command is a method of the document property. (Remember, methods are like verbs.) We'll study the built-in objects in a later installment.

Now, let's take a look at methods.




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