Willard Roseberry's List: Learning JavaScript

• In your first example you were using the variable images inside your BlockID function. Rather you should be using the IDs array you declare inside BlockID and return that array. When you do that, this line will work as expected: var images = BlockID(); because BlockID is now returning the proper array.

• • This is where I started understanding Functions for the first time.

• int i = 10;

  02

  03	while(i>0)

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  {

  05

  try

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  {

  07	System.out.println("Still have " + i + " loops left");

  08	Thread.sleep(1000);

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  i--;

  10

  }

  11	catch(InterruptedException e)

  12

  {

  13

  }

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  }

• function example(a,b,c,data,data2) {  number += a;  alert('You have chosen: ' + b);  (do lots more with c, data and data2) }

• If you forget to pass one argument the function complains. Suppose you do

    

  example(1,'house',16,'more stuff') 

    

  then 1 is passed to a, 'house' is passed to b, 16 is passed to c and 'more stuff' is passed to data. But now there is nothing for data2 since you did not pass anything to it in the function call. When the function arrives at the part it is supposed to do something with data2, it finds that data2 does not exist and gives error messages.

    

  So always be careful to pass enough arguments. Personally I always try to write functions that need as little arguments as possible. This keeps my code clean and makes updating easy.

• Using no arguments

    

  You can also write a function without any arguments:

    

  function example() { 	number += 1; 	alert('You have chosen: house'); } 

    

  Of course the function does exactly the same whenever you call it. If you try to give it arguments, it ignores them since they are not defined in the function header.

    

  Returning a value

    

  One more thing a function can do is return a value. Suppose we have the following function:
• • Really informative about functions and how to use them.
• function calculate(a,b,c) {  d = (a+b) * c;  return d; }

• This function calculates a number from the numbers you pass to it. When it's done it returns the result of the calculation. This is in fact the opposite of passing arguments: the function now passes the result back to its calling handle. After any return the function stops working: it doesn't execute any more lines, if there are any.

    

  In practice this works as follows:

    

  var x = calculate(4,5,9); var y = calculate((x/3),3,5); 

    

  Now you declare a variable x and tell JavaScript to execute calculate() with the arguments 4, 5 and 9 and to put the returned value (81) in x. Then you declare a variable y and execute calculate() again. The first argument is x/3, which means 81/3 = 27, so y becomes 150.

    

  Of course you can also return strings or even Boolean values (true or false). When using JavaScript in forms, you write a function that returns either true or false and thus tells the browser whether to submit a form or not.

    

  Returning to stop the function

    

  Sometimes it's useful to add a return to your script to stop the script. If you use object detection and you find out the browser can't handle a certain script, use a return to end the function before the browser reaches the dangerous lines it can't handle.

    

  Suppose you write a nice W3C DOM script. The first thing you'd have to do is seeing if the browser supports the W3C DOM:

    

  function initPage() { 	var supported = document.createElement && document.getElementsByTagName; 	if (!supported) return; 	// start of DOM script 	var x = document.getElementById('writeroot'); 	// etc. } 

    

  If supported is false (browser doesn't support the W3C DOM), the function stops (it would only lead to error messages in this browser). You do this by using return. Only if the browser supports the W3C DOM it makes it to the next line where the actual DOM script starts.

     

   

   

4 more annotations...

• Operator Usage Description Logical AND (&&) expr1 && expr2 Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false. Logical OR (||) expr1 || expr2 Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false. Logical NOT (!) !expr Returns false if its single operand can be converted to true; otherwise, returns true.