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JavaScript

jason chan
jason chan
31,008 Points

Arrays and functions? what's happening inside the braces of the parenthesis?

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

function print(message) {   // <---- what? where did message come from
  document.write(message);
}
function printList(list) {  // <---- what? where did list come from
    var listHTML = '<ol>';
    for (var i = 0; i < list.length; i += 1) {
        listHTML += '<li>' + list[i] + '</li>'
    }
    listHTML += '</ol>';
    print(listHTML);
}
printList(playList);

What's happening to message and list? I don't understand. The variables were not defined.

2 Answers

Chris Shaw
Chris Shaw
26,650 Points

Hi Jason,

This can be a confusing concept for most new comers to languages such as JavaScript but fear not as it's very easy to understand, whenever we declare a function (in most languages, Ruby is slightly different along with some others), we can pass it arguments which as part of the declaration for that particular function.

Arguments example
var foo = 'Hello';
var bar = 'World';

someFunction(foo, bar); // foo and bar are the function arguments

Makes sense? If not don't worry as the concept of function arguments will come naturally as you progress and advance your JavaScript knowledge, so far you've seen how arguments work but what about function parameters?

What are parameters?

Parameters are very similar to arguments, in fact, they're one-in-the same. The common difference between the two is parameters are whatever we want them be called whereas an argument is a physical object we're passing into a function declaration.

Parameters can be named anything, in the case of the example code you have above message is just a variable generated by the browser (it can be called anything such as valueOfSomeSort) and can be used throughout the entire code execution of the function.

Parameter names as I mentioned can be called anything, once defined the browser takes care of declaring it within the scope (you can learn more about scopes on the Mozilla Developer Network) of the function. This variable can't be used outside of the function but will always contain a new value anytime the function is called.

function doSomething(foo, bar) {
  // foo and bar don't require the "var" keyword because they are function parameters
}

Hope that helps.

jason chan
jason chan
31,008 Points

so its' similar to var anything; It's like undefined variable. I think i got it.