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In this video, I want to go over two more pieces of syntax around interfaces, so to get started,

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I'm going to go down to the vehicle function inside of here.

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You'll notice that our implementation now only accesses one property on the vehicle argument.

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The only thing we are referencing is these summary property.

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So I got a question for you.

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We are saying that the argument must be a vehicle and must satisfy this interface is the terminology

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we would use.

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And we've said earlier that the vehicle must have a name a year broken and the summary function.

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However, our function down here is now only accessing the summary property.

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So here's the question.

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Is it important for us to say that a vehicle must have these properties available?

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I don't think it really is.

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So what happens if I delete those additional properties?

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I can delete them, they go away, and now you'll notice I do not see any errors inside my file.

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Why is that?

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Well, when I called vehicle right here, we are saying that in order to be passed into this function,

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you must satisfy the vehicle interface.

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So we pass an old civic and type script behind the scenes, does a quick check for us.

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It says, does this object right here satisfied the vehicle interface?

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So TypeScript is going to go back to the vehicle interface definition and it says in order to be considered

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to be a vehicle, you must have a function called summary that returns a string.

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It then looks at the old civic object and it says, OK, does this object have a summary property?

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That is a function that returns a string?

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Yes, it does.

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And that's the only question that gets asked.

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It doesn't matter if we have additional properties on old Civic, doesn't matter when TypeScript tries

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to decide if old Civic is a vehicle or not, it's not going to say, oh, you've got these extra properties,

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sorry, you can't be a vehicle.

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It's only going to check to see if it has this list of properties right here.

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If it has that list, awesome.

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It is qualified to be a vehicle.

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So the code we have here still works, but we've got a much more condensed interface definition.

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So here's the important thing here.

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Something I want to point out as like kind of step two of this little refactor, now that our interface

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only has a single function definition in it.

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Do you think the name vehicle is really appropriate here?

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Like is calling an object that has a summary property, that is a function that returns a string?

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Does it really seem appropriate to call that a vehicle?

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I would say maybe it doesn't.

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Maybe a more appropriate interface or a name for this interface from now be something like, I don't

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know, reportable or something like that.

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So now we're saying that in order to be considered the type reportable, you must have a function called

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summary that returns a string.

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And so now to fix up our file here, down here inside a print vehicles, there is no more vehicle types.

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I'm going to replace that with reportable.

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Now, just to be clear, the word reportable, I'm kind of meaning to say something that is reportable,

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has a summary function that returns a string.

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So this is like something that can be reported.

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All right, so notice how our definition for the interface is now a lot shorter and a little bit more

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generic than it was before.

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It's now back down inside the print vehicle interface or function down here, I want to just point out

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one more thing here.

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Notice how we've got a function called print vehicle.

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It takes an argument called vehicle, and it refers to that function or that argument inside the function.

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But once again, I got a question for you.

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Does anything about this function now really seem like it has anything to do with vehicles?

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I don't think it really does.

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The only thing that this function really does is taken something that can be reported on and then console

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log a summary about that thing.

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So maybe we should update some of the terminology inside this function.

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Maybe instead of an argument called vehicle, we call this something that's called like, I don't know.

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Reports about item, and maybe that makes sense.

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And then inside our rather than Collins vehicle, I want to call it maybe something a little bit more

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appropriate, like print summary or something like that.

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I'm going to update the reference down here as well to be simply print summary.

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OK, so all we've really done here is kind of pointed out how our interface no longer really has anything

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to do specifically with vehicles, and so I just decided to rename it into something a little bit more

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generic.

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And I did the same thing with the function down here.

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So let's take a quick pause right here.

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We're going to come back to the next video and we're going to discuss why this was a very significant

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factor by adding in just a little bit more code.

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So quick pause and we'll see you in just a minute.