Calling a Function

In order to make use of a function, you need to call it. You call a function simply by using its name followed by any parameters in parentheses. "To invoke" a function is another way of saying "to call".

Let's call the function sum(), passing two parameters and assigning the value that the function returns to the variable result:

>>> var result = sum(1, 2);
>>> result;

3

Parameters

When defining a function, you can specify what parameters the function expects to receive when it is called. A function may not require any parameters, but if it does and you forget to pass them, JavaScript will assign the value undefined to the ones you skipped. In the next example, the function call returns NaN because it tries to sum 1 and undefined:

>>> sum(1)

NaN

JavaScript is not picky at all when it comes to parameters. If you pass more parameters than the function expects, the extra parameters will be silently ignored:

>>> sum(1, 2, 3, 4, 5)

3

What's more, you can create functions that are flexible about the number of parameters they accept. This is possible thanks to the arguments array that is created automatically inside each function. Here's a function that simply returns whatever parameters are passed to it:

>>> function args() { return arguments; }
>>> args();

[]

>>> args( 1, 2, 3, 4, true, 'ninja');

[1, 2, 3, 4, true, "ninja"]

By using the arguments array you can improve the sum() function to accept any number of parameters and add them all up.

function sumOnSteroids() { 
  var i, res = 0; 
  var number_of_params = arguments.length;
  for (i = 0; i < number_of_params; i++) {
    res += arguments[i];
  }
  return res; 
} 

If you test this function by calling it with a different number of parameters (or even no parameters at all), you can verify that it works as expected:

>>> sumOnSteroids(1, 1, 1);

3

>>> sumOnSteroids(1, 2, 3, 4);

10

>>> sumOnSteroids(1, 2, 3, 4, 4, 3, 2, 1);

20

>>> sumOnSteroids(5);

5

>>> sumOnSteroids();

0

The expression arguments.length returns the number of parameters passed when the function was called. Don't worry if the syntax is unfamiliar, we'll examine it in detail in the next chapter. We'll also see that arguments is technically not an array, but an array-like object.