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JavaScript lets you assign default
values to your function parameters.

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That way if for any reason you do not
pass a certain argument to a function,

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the function uses or
falls back to the default value.

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Open the file default-params.js,

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then update the script tag in index.html
to point to default-params.js.

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Let's first take a look at what happens
when you don't pass an argument

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to a function that requires one.

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I'll define a function named sayGreeting
that takes the parameter name.

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I'm writing this as
a function declaration,

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but you can also write this as an arrow
function expression if you'd like.

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The function should return a greeting
using the value of the name parameter.

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I'll set it to return a string using
a template literal with the message,

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Good morning,
followed by the value of name.

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The function now expects a name
value passed to it as an argument.

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Over in the JavaScript console, let's see
what happens if you call the sayGreeting

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function without passing
it a name argument.

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The console prints Good morning,
undefined!

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Undefined is one of
JavaScript's built-in values.

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In this case,

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it means that there's no value assigned
to the name parameter, it's undefined.

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In some cases, missing a function
argument can break your entire program.

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To prevent any of that from happening,
you can assign a default value to your

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parameters right inside
the function definition.

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Let's assign the name parameter a default
value using the equals sign, or

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the assignment operator, and
set it to the string student.

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I'll save the file, and
back in the console, I'll once again call

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the sayGreeting function without
passing it a name argument.

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This time,
the console prints Good morning, student!

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If you pass the function an argument,
like the string Maria,

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now the value Maria gets assigned
to the name parameter, and

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the function returns the greeting
Good morning, Maria!, good.

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The default parameter
acts as a fallback and

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safeguards your program from breaking or
returning undefined.

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Now let's look at how default values work
when you have multiple parameters in

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your function.

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I want to pass a custom
greeting to the function

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instead of always displaying Good morning!

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So I'll add a greeting parameter
to the function definition,

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then in the return statement,

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replace Good morning with the value that
gets assigned to the greeting parameter.

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I'll save my file, and
over in the console,

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I'll call the sayGreeting function,
passing it a greeting argument only,

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like Hi there,
the function returns Hi there, student!

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All right, now I'll assign a default
value to the greeting parameter,

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I'll set it to Good morning.

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I'll test that this works by calling the
sayGreeting function without passing it

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arguments.

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And it returns Good morning, student!,
the default greeting and name.

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But what if you want to pass
the function a name argument only?

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In other words, skip the greeting
argument to use the default greeting and

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pass it just the name you want to display?

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Well, if you pass
the function a name only, for

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example Guil,
it's going to return Guil, student!

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Since the function's first
parameter is greeting,

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it's assigning the string
Guil to that parameter.

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Now, what if I just omit the argument,
like this?

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This breaks the function.

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JavaScript throws a syntax error,
it says Unexpected token, comma.

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Even if you've assigned a default
value to that first parameter,

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the function expects
you to pass it a value.

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In this case, you need to pass
the function undefined as the value for

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the greeting argument.

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This instructs the JavaScript engine
to use the default value assigned to

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the first parameter.

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It acts as a placeholder for
the argument you want to skip.

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Calling the function now returns
Good morning, Guil!, perfect.

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And keep in mind that you only
need to pass undefined for all but

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the last argument.

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If the last argument is missing,

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the function will use
the default value you set.

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All right, now that you've learned
how to work with default parameters,

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why don't you revisit the getArea
function you wrote earlier and

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assign the unit parameter a default value.

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You can copy this function snippet from
the teacher's notes with this video.

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Let's say that based on where
your site visitors live,

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you'll want your function to display
square feet or square meters.

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If most of your visitors are from the US,

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you'll likely set
the default to square feet.

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On the other hand, if most of
your visitors are from Canada or

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Europe, for example,
you'd set it to square meters.