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JavaScript JavaScript Loops, Arrays and Objects Tracking Multiple Items with Arrays Two-Dimensional Arrays

Simon Tanner
Simon Tanner
5,776 Points
Posted by Simon Tanner
Simon Tanner
5,776 Points

Understanding parameters

Hi, please could you help me understand how the list parameter in function printList( list ) knows how to access (sorry can't think of the right term?) the elements in the array var playlist. I'm a bit lost and thought it would have needed to be function printList (playList) for it to work? Thanks for any help!

var playList = [
  'I Did It My Way',
  'Respect',
  'Imagine',
  'Born to Run',
  'Louie Louie',
  'Maybellene'
];

function print(message) {
  document.write(message);
}

function printList( list ) {
  var listHTML = '<ol>';
  for ( var i = 0; i < list.length; i += 1) {
    listHTML += '<li>' + list[i] + '</li>';
  }
  listHTML += '</ol>';
  print(listHTML);
}

printList(playList);

4 Answers

Jonathan Grieve
MOD
Jonathan Grieve
Treehouse Moderator 91,253 Points
Jonathan Grieve
Jonathan Grieve
Treehouse Moderator 91,253 Points

Hi Simon,

In the function, list is simply a variable that's been passed into the function as a parameter. It's simply a placeholder for a value for which an action will be taken.

In the example, list is a placeholder for the array. Rather than reference the array directly, it's actually passed into the function call as an argument.

so list is in actuality the playList array. :-)

Vinnie Dauer
Vinnie Dauer
12,824 Points
Vinnie Dauer
Vinnie Dauer
12,824 Points

The function doesn't automatically know to access playList. It accesses playList when you call the function with playList as the "list" parameter. A function is written like this so that you can call any array parameter when the function is called. I hope that clears it up.

Simon Tanner
Simon Tanner
5,776 Points
Simon Tanner
Simon Tanner
5,776 Points

Thanks a lot Jonathan and Vinnie - both really helpful!

Vinnie Dauer
Vinnie Dauer
12,824 Points

No problemo!

Fabio Silva
Fabio Silva
5,097 Points
Fabio Silva
Fabio Silva
5,097 Points

I still don't get it! What if there were more arrays? How the function would know which array to access?

Jonathan Grieve
Jonathan Grieve
Treehouse Moderator 91,253 Points
Jonathan Grieve
Jonathan Grieve
Treehouse Moderator 91,253 Points

If you wanted to include more arrays you'd declare the array outside the function and then pass it in as another parameter, 

var playList = [
  'I Did It My Way',
  'Respect',
  'Imagine',
  'Born to Run',
  'Louie Louie',
  'Maybellene'
];

var playListTwo = [
  'new',
  'songs',
  'would',
  'go',
  'here'
];
function print(message) {
  document.write(message);
}

function printList( list, list2 ) {
  var listHTML = '<ol>';
  for ( var i = 0; i < list.length; i += 1) {
    listHTML += '<li>' + list[i] + '</li>';
  }
  for(var i=0; i <list2.length; i +=1) {    
    listHTML += '<li>' + list2[i] + '</li>';
  }
  listHTML += '</ol>';
  print(listHTML);
}

printList(playList, playList2);

There comes a point where you'd have to find anther way to do this if you have too many parameters but that's another story. :)