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00:00:00,510 --> 00:00:03,866
Now we need to make sure that
the getRandomNumber function

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00:00:03,866 --> 00:00:06,405
receives two numeric values as arguments.

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00:00:06,405 --> 00:00:08,390
And no other type of values.

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00:00:08,390 --> 00:00:11,081
If the function is called
with a string for example,

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00:00:11,081 --> 00:00:14,031
then the program should display
a custom error message.

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00:00:14,031 --> 00:00:18,310
Inside the function add an if statement
just above the return statement.

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00:00:19,890 --> 00:00:22,270
But what should we test here?

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00:00:22,270 --> 00:00:25,400
Well, we need to make sure that the two
arguments passed to the function

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are numbers.

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00:00:26,510 --> 00:00:28,150
There's a few ways we might do this.

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I’ll teach you a new method for
testing number values.

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00:00:31,330 --> 00:00:36,138
JavaScript provides a special function
called isNaN, or is not number.

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That takes one argument and
returns a Boolean value.

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It returns true if the value is
not a number, and false if it is.

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For example, if you pass the string 'six'
to the function, you get the value true.

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00:00:49,920 --> 00:00:53,460
However if you pass a number, like six,
to the function, you get false.

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00:00:54,520 --> 00:00:58,354
So in the if statement,
I'll first test that the lower argument

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passed to the function is a number,
like this.

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00:01:04,214 --> 00:01:07,339
In other words,
if the lower value is not a number,

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then the code block with
this if statement will run.

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00:01:11,190 --> 00:01:13,330
But we want to test both arguments.

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If either is not a number,
then we'll display an error.

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00:01:17,430 --> 00:01:22,838
To test both you can use the logical or
operator, by typing two pipe characters.

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00:01:22,838 --> 00:01:25,740
Then add a test for
the upper argument like this.

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00:01:30,140 --> 00:01:35,130
So now if either argument is not a number,
we can display a custom error.

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00:01:36,610 --> 00:01:41,394
We could maybe use console.log to
display a message in the console.

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Like 'Both arguments must be numbers.'.

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00:01:50,160 --> 00:01:53,814
In this case, it's probably
best to throw a custom error.

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00:01:53,814 --> 00:01:57,770
So the programmer can immediately see that
they've used the function incorrectly.

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00:01:57,770 --> 00:02:01,670
JavaScript lets you create your own errors
which print directly to the JavaScript

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00:02:01,670 --> 00:02:04,630
console, just like
regular syntax errors do.

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00:02:04,630 --> 00:02:05,460
It looks like this.

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00:02:06,590 --> 00:02:10,350
The throw keyword,
followed by JavaScript's Error object.

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00:02:12,300 --> 00:02:14,176
This is a built in keyword and

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00:02:14,176 --> 00:02:17,780
object JavaScript provides
to create a custom error.

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00:02:17,780 --> 00:02:20,900
Or, as developers say, throw an exception.

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00:02:20,900 --> 00:02:24,100
Since it's inside the if statement,
the error is thrown

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00:02:24,100 --> 00:02:27,870
only when the function receives anything
but a number for either argument.

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00:02:29,890 --> 00:02:33,570
All right, now I'll test the latest
changes by calling the function correctly

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00:02:33,570 --> 00:02:36,060
with numbers a couple of times.

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00:02:37,080 --> 00:02:40,794
Then call it with a string
value like 'three hundred'.

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00:02:46,397 --> 00:02:47,960
I'll save the file.

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00:02:47,960 --> 00:02:52,977
And over in the console,
I get two random numbers, 4 and 17.

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00:02:52,977 --> 00:02:56,819
And below them is the Uncaught Error,
displaying my custom error message,

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00:02:56,819 --> 00:02:59,070
Both arguments must be numbers.

46
00:02:59,070 --> 00:03:02,810
Notice how it looks just like a regular
error thrown by the JavaScript engine.

47
00:03:02,810 --> 00:03:06,950
It even displays the line number
producing the error, great.

48
00:03:06,950 --> 00:03:09,800
So now you've learned how
to program defensively.

49
00:03:09,800 --> 00:03:14,650
Or looking out for problems that might
arise in your program and taking action.

50
00:03:14,650 --> 00:03:16,440
As with many things in programming,

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00:03:16,440 --> 00:03:19,960
getting comfortable with the details
of functions takes time and practice.

52
00:03:19,960 --> 00:03:23,608
But once you get to hang of working with
functions, you'll be well on your way to

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00:03:23,608 --> 00:03:26,690
mastering one the most essential
features of JavaScript.

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00:03:26,690 --> 00:03:31,820
In fact, functions are often referred to
as first-class citizens in JavaScript.

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00:03:31,820 --> 00:03:35,044
Which means that you have the ability
to do almost anything with functions.

56
00:03:35,044 --> 00:03:39,160
Like store them in variables,
pass them as arguments to other functions.

57
00:03:39,160 --> 00:03:41,890
You can even set a function
to return another function.

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00:03:41,890 --> 00:03:45,170
This lets you take advantage of powerful
design patterns you'll learn about in

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later courses.

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00:03:46,480 --> 00:03:47,887
So you should feel excited and

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00:03:47,887 --> 00:03:51,607
proud of reaching another important
milestone in your programming journey.

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00:03:51,607 --> 00:03:53,004
And remember, we're here to help.

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00:03:53,004 --> 00:03:56,226
So if you have questions about anything
covered in this course, you can always get

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00:03:56,226 --> 00:03:59,590
in touch with the friendly Treehouse
staff or other students in the community.

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00:03:59,590 --> 00:04:01,037
Thanks everyone, and happy coding.