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function and other functions that we can
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write for ourselves functions like hello
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but we also have the ability to include
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things like variables inside of our
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program as well so what might that look
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like
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well I'll go ahead and create a new file
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it will call counter HTML and counter is
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going to have some of the similar code
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to hello so I'll just go ahead and copy
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it for now but I'll clear out the script
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section change the title from hello to
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counter and now inside I'll get rid of
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the button or actually I'll keep the
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button but instead of saying click here
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the button is going to say count I'd
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like to create a program that just
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counts for me from zero to one two two
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three four so on and so forth and now in
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order to do that in order to have some
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way of counting repeatedly zero one two
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three four five I'm going to need a have
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some sort of variable inside of my
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program something that is keeping track
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of data like the number that I'm
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currently have counted to so in order to
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do that in JavaScript I can say
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something like let counter equal zero
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this is the way in JavaScript that I
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define a new variable I first say let
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counter meaning let there be a new
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variable called counter and I'm going to
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initially set the value of counter equal
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to the number zero and now when I click
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on the button instead of running the
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hello function I'm going to go ahead and
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run the count function which doesn't
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exist yet but I'll now write it I'll
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define a function called count and what
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the count function is going to do is it
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is going to first increment the value of
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counter and a number of ways I could do
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that one is my saying counter equals
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counter plus one to say go ahead and
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reset the value of counter to whatever
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it counter is plus one and there are a
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couple of shorthand notations for this I
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could equivalently say counter plus
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equals one to say add one to the counter
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or in the case of adding one javascript
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much like languages like c if you've
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seen them before and support notation
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like this counter plus plus which just
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means take the value of counter and
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increment it add 1 to it
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so I'll add 1 to the value of counter
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and then I'll just display an alert
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that has whatever the current value of
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counter happens to be and so I've
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incremented the value of counter and
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then displayed an alert that shows me
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what's contained inside of counter and
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so now if I go ahead and not go to hello
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dot HTML but to counter dot HTML instead
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I now see that I still see a button that
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says count and if I click on that button
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I get an alert that this time says one
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we've incrementing the value of counter
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from zero to one the alert now says 1
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and I can press ok if I press count
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again the count now goes to 2 I press ok
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press count again it goes to 3 and every
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time I click count it is going to
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increment the value of the variable
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counter inside of my JavaScript webpage
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and then it's going to display an alert
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that is going to contain the value of
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that variable
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so using alerts now we're able to
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manipulate that we can inside a
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functions manipulate the values of
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variables and then display alerts that
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show the contents of those variable ism
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but ultimately when users are
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interacting with the page it's going to
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be fairly annoying if the only way that
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we have to interact with the user is via
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displaying these alerts the equivalent
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of like printing something out except
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instead of printing it to a terminal
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we're printing it via an alert that
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appears on the screen what would be more
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interesting and more powerful and much
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more useful in the context of a webpage
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is if we could programmatically update
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the website change something about the
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webpage the user is actually seeing
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changing the content that happens to be
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on the page and turns out Java Script is
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going to enable us to do that as well
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