All language subtitles for 12. Custom Arguments

Our conversion pipe definitely works but I think you'll agree with me that it's not very full featured

right now.

We called this thing convert pipe but it can only do a conversion from miles over 2 kilometers.

Let's figure out how we can add in some custom argument to this thing to customize exactly exactly what

units we convert those miles into do so we can go back over to our app component template and on our

convert pipe we can add on some syntax very similar to some of the syntax we saw earlier where we were

using some other built in pipes and we were customizing how those pipes were behaving right after convert.

Right here I'll put down a colon and then a string and inside there I'm going to add in about in any

four meters.

This means that I want to take that Miles value right there and I want to convert it into meters instead

of kilometers.

Let's save this and then go back over to our pipe and figure out how we can somehow receive that argument.

The makeover inside my pipe you'll notice that there's the second argument right here of dot dot dot

args.

If you've never seen the syntax before it essentially means take whatever other arguments were provided

to the transform function and stuffed them all into a single array called args if we console log args

outright now and then save this and flip back over I can then type inside the miles and put right here

and I can see what args is though it is a array with one element inside of it just that in that we had

added in as we start to add in more arguments to our pipe they're all gonna show up inside of that args

array.

For example if I decided that in addition to that meter's input right here if for some reason I also

wanted to have some like other value inside if you're like some number for who knows what reason I save

that and flip back over.

Now when I type something in I'm going to see first the M and then whatever other value I threw inside

there.

If we need multiple different arguments to pass in we just separate them all with the colon.

In our case we really just need a designation of the output units.

So again in this case meters we only really expect to receive one argument and usually best practice

is to say that rather than passing in like this.

Dot dot dot args are receiving that argument.

We might instead specifically say hey you got to give us some kind of conversion element or some kind

of conversion unit here I might call that or get units and say it needs to be a string.

Now I can print out simply or get units by itself and if I say this enter number no longer do I have

the array.

I just have the m..

I had entered okay.

Let's figure out how we can adjust the logic inside of here to convert the input miles into some different

target units based upon that target units arguments to do so I think maybe a good way to handle this

would be a switch statement.

We can delete that console log I'll put a switch statement set off here and we're gonna switch over

the value of target units and we'll write out a couple of cases here.

If Target units is equal to about limiters we'll start with that first and let's return value times

one point 6 0 9 3 4 we can also add in a case to handle say meters.

And that would be the same thing.

We would multiply that by 1000 and it may be one more case that's for demonstration.

We'll do a conversion over two centimeters I'd take the same thing and multiply it once again by a thousand.

I might also want to put in a default case as well.

In case someone tries to use my pipe and throws in some kind of target unit that I don't know how to

handle I could put in a default right here and it's really up to us how to best handle this.

We could just return our value and say well if you give us some kind of conversion metric that we don't

understand let's just throw the value back at whoever is using this pipe but maybe a better way would

be to throw an error.

So a signal to other engineers that hey you're trying to convert into a unit that is not supported we

could instead throw a new error and say ah get unit not supported except all right.

Let's say this look back over now when I type inside of here if I say 1 mile I get 16 0 9 meters which

I don't know sounds right to me.

And if I go back to my template I can now change that to say kilometers.

So now convert miles in two kilometers yep that looks good.

And I can change convert over to the M instead give me miles in centimeters though one mile is many

many many centimeters.

Okay.

So that's how we add in custom arguments to a pipe.

We've got to do is use the same syntax we already saw around the built in pipes and we received that

argument as additional arguments inside this transform function over here.

So it looks good.

Now in the next video there's a couple of other kind of gotchas around pipes I want you to be aware

of.

We're gonna take a look at a couple of corner cases in just a moment.