Function reference vs. function call

Here's a JavaScript function definition:

function foo() {
  alert('foo!');
}

You can refer to that function like this:

foo

And here is how to call (execute) the function:

foo()

Even experienced developers new to JS tend to get these confused. For example, jQuery's getJSON method allows you to specify a function to be executed once the operation is complete. Be careful not to plug in the function call when what you really need is a function reference:

$.getJSON( "action.php", {data:'some data here'}, foo() );

This will execute the foo function instantly. Not what you want. So leave off the parens:

$.getJSON( "action.php", {data:'some data here'}, foo );

This merely passes a reference to the function. It won't execute until it's called by jQuery.