Beginning JavaScript with DOM Scripting and Ajax, 2nd Edition

APPENDIX A ■ Debugging JavaScript
Trying to Access Undefined Variables
I talked about it in the second chapter of the book: you define variables either by declaring them with or without
an additional var keyword. (The latter is necessary to define the scope of the variable.) Keep in mind that strict
mode will prevent you from implicitly declaring variables (variables without the var keyword). Because of this, it is
recommended that you use the var keyword for every variable.
Stewie = "Son of Peter and Lois";
var Chris = "Older Son of Peter and Lois";
If you try to access a variable that hasn’t been defined yet, you’ll get an error. The alert() in the following script
throws an error because Meg is not defined yet:
Peter = "The Family Guy";
Lois = "The Family Guy's Wife";
Brian = "The Dog";
Stewie = "Son of Peter and Lois";
Chris = "Older Son of Peter and Lois";
alert( Meg );
Meg = "The Daughter of Peter and Lois";
This is easy when it is an obvious example like this one, but how about trying to guess where the bug in the
following example is?
exampleFamilies.html
function getFamilyData( outptID, isTree, familyName ) {
var father, mother, child;
switch( familyName ) {
case 'Griffin':
father = "Peter";
mother = "Lois";
child = "Chris";
break;
case 'Flintstone':
father = "Fred";
mother = "Wilma";
child = "Pebbles";
break;
}
var out = document.getElementById( outputID );
if( isTree ) {
var newUL = document.createElement( 'ul' );
newUL.appendChild( makeLI( father ) );
newUL.appendChild( makeLI( mother ) );
newUL.appendChild( makeLI( child ) );
out.appendChild( newUL );
} else {
var str = father + ' ' + mother + ' ' + child;
out.appendChild( document.createTextNode( str ) );
}
}
getFamilyData( 'tree', true, 'Griffin' );
344
www.it-ebooks.info