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- Now the next basic
type I want you to learn

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is going to be the array type.

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So in order to understand
how the array type works,

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let's just take a variable
and assign it to an array

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and see what TypeScript
infers the type as.

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So I can say const a equals,

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let's do an array of one, two, three.

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So this is just three different numbers.

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I hover over this, you
can see that my type

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has been set to number with
this square brackets at the end.

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And that's just a noting
that this is a number array.

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So an array of numbers.

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So if I wanted to manually type this,

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I could say that this is going to be

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a number array just like that.

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So now if I were to
move the default value,

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and I said that A is equal to
for example, one, two, three

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you can see that works fine.

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Also I need to make sure I give this

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some type of default value,

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otherwise it's gonna gimme an error.

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'Cause undefined is not a number array.

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Also, I should probably
use let here instead

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just to make sure all that's working.

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The really nice thing about
manually defining these,

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is if I come in here with const,

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and I give this a value of
like let's say one, two, three

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like we had before,

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and then I can do inside of
here, is I can say dot push.

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And I say I wanna push a new
value to the end of this.

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TypeScript is smart enough to know,

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"Okay, this is an array of numbers,

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so I can only add numbers
to be added onto this."

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So I can only put numbers in here.

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If I were to try to put a string,

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you can see I'm going to get an error

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because I'm trying to add a
string to an array of numbers.

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So this again, is where
the power of TypeScript

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really explodes because
by typing just one thing,

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and I don't even need to include the type

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'cause it's already inferred,

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you can see that I'm getting
this awesome auto complete

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and this error protection in my code.

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And this code is just plain JavaScript.

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I haven't even written any TypeScript

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and I'm getting all of
these awesome errors

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just by all of this inference.

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So if you want to type an array,

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just make sure that you're coming in here

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and you're doing whatever the type is.

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For example, a number

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followed by these square bracket syntax

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to denote that this is
specifically an array of numbers

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and not just a single number.

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Now another thing to note is
I have defined this const a

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as one, two, three.

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So you may think if it's following
the literal type practice

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that this a would be typed as an array

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of values one, two, and three.

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I could manually type
that out if I wanted to,

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by just saying one, two, three, like that.

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And now I'm saying that a

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must be the values of one, two and three,

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which means I can't push
anything at all to it.

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If I try to push another number,

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for example, I push the
value of four into here,

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you can see that I'm getting an error

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because four is not a value
of three, one, or two.

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So the reason that I don't get this actual

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like explicit literal type when
I define my array like this

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is because arrays just like objects

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they have a pass by reference
as opposed to pass by value,

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which essentially means that
I can call this push method

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which actually modifies my array

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without actually redefining it.

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Just like I would have to do
with saying something like,

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a equals one, two, three, four.

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I can do the exact same thing as this code

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just by calling a.push.

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And that doesn't actually
redefine the const variable.

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So the fact that this uses
references instead of values

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and the fact you can modify a

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without actually redefining it,

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TypeScript says that this
knows that it's a number array

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and not explicitly the values
of one, two, and three.

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Which is really great because

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if it assumed the values
were one, two, and three,

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it would almost always be
wrong, since you're always

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pushing and removing elements from arrays.

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And the great thing is this works

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with all the different types
you could possibly think of.

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For example, if we had
an array of strings,

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you can see I get a string
array here, here and so on.

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So all your different primitive types,

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and you can combine this together.

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For example, if I were to
put dates inside of here,

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or if I just say, this is a new date,

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you can now see I have an array of dates.

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So specifically I could say
that this is a date array

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just like that, and
that's going to work fine.

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Now in this case,

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a date is essentially an object
type inside a TypeScript,

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and we're gonna be covering
that in a little bit.

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So it's not super important to know,

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but just know that you can use things

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that aren't primitives all over the place.

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So everywhere you could
normally use a primitive

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you can use other things
like dates and so on.