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You have completed JavaScript Loops!
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Use a `for` loop to build a string holding HTML, then display it on a web page.
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Loops come in handy on many occasions.
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Now you'll use a for loop to
display content on the page.
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You'll create a loop that adds 10 div
elements to a webpage with numbers
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inside them.
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In other words,
use a for loop to repeat the same action,
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adding a div to the page 10 times.
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The divs are styled with CSS to display
as circles with white background colors.
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To code along with me,
open the file html-loop.js and
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make sure to update the script tag in
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index.html to point to js/html-loop.js.
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In this file,
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there's already a variable named main that
points to the main element in index.html.
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I'll start by declaring a new variable
that's going to hold a string containing
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the HTML I want to display on the page.
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I'll name it html and
set its initial value to an empty string.
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I'm using the variable name html because
it reflects the contents of the variable,
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a string that holds HTML div tags.
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And I'm using the let keyword because
the loop is going to update and
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add a string of HTML to the current
value of the variable each time it runs.
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And the HTML will be simple.
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Each div will look similar to this.
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An opening and closing div tag.
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And to make it more interesting,
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I'll display the number of the counter
each time we go through the loop.
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So each div will display
a different number.
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Now I'll write the for loop structure.
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First, between the parentheses, I'll
create or initialize the counter variable.
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I'll name it i, and set its value to zero.
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Remember, this variable declaration
is evaluated only once at the very
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beginning before the condition is even
tested and before the loop begins.
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Next I'll add the condition,
i is less than 10.
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As long as the value stored in i is
less than 10, the loop will run.
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Again, the condition gets evaluated
before each loop iteration.
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Now I need to make sure that
the loop ends at some point.
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I can do that by changing the value
of i each time through the loop.
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I'll use the increment operator to
add one to the current value of
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i each time the loop runs.
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In other words,
after the code in this code block runs.
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Now, I'll add the code
to run within the loop.
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Each time through the loop,
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I want to add a set of div tags to
the string assigned to the HTML variable.
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You can do that with the addition
assignment operator.
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I'll type html +=,
I'll assign it a template literal so
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that I can interpolate or insert the value
of the i variable into the string output.
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In other words,
the number of times the loop runs.
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Between the div tags I'll write ${} i.
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As soon as the value of i is no longer
less than 10, the loop ends and
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the program can display the entire
contents of the HTML variable on the page.
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Finally, I've selected the main element
and assigned it to the variable main.
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So I'll set the HTML to display inside
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main with main.innerHTML = html.
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I'll save the file and
preview the results in the browser.
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Notice how the browser displays 10
divs with the numbers 0 through 9.
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To display the numbers 1 through
10 initialize the counter or
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i variable to one and change the condition
to i is less than or equal to 10.
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Now the loop starts counting from one.
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Once the value in i is no longer
less than or equal to 10,
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the loop ends and the numbers 1
through 10 display on the page.
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Now if you don't want to
change your initialization and
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condition, another option is
to interpolate i + 1 like so.
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The results are the same, but I'll set
it back to how I previously had it.
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To make the program display more divs,
like 100,
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change the condition to i is less than or
equal to 100.
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And what if you want the loop to
count by a higher number, like five?
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Well, initialize the counter variable
to five then increment it by five at
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the end of each loop iteration like this.
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The result is 20 divs starting with
number five and counts to 100 by fives.
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Good, I encourage you to
experiment with this loop and
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try a few more values on your own.
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Another way you might write the for
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loop is by placing the main.innerHTML
statement inside the loop.
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Notice how this produces
the same results in the browser.
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Now with this approach, the browser has to
do extra work by replacing the contents
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inside the main element each time the loop
runs with more and more div tags.
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This happens so
fast you'll only see the final value.
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In fact, if you console.log the value
of the HTML variable inside the loop,
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you can explicitly see how
the loop builds the string from
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one div tag to two, to three,
all the way to 20 div tags.
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You might also add an if statement
inside the loop that for example checks
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if i strictly equals 100, then sets
the innerHTML of main with a final string.
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For this short and simple program I
prefer to build the entire string in
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a temporary HTML variable first, and
then let innerHTML update the page.
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By keeping this statement
outside of the loop,
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I can clearly see that when the loop ends,
the rest of the program continues
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by updating the content
inside the main element.
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It's also more performance, because
I'm not asking the browser to replace
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the main element's HTML over and
over again.
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There are yet other ways to insert HTML
into the page, many of which you'll learn
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in a later course on JavaScript and
the DOM or document object model.
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All right, so now you've learned how to
write and work with three different types
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of JavaScript loops, the while,
do while, and for loop.
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Hopefully you're starting to see how
useful they are and how quick and
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easy they make executing the same code 10,
20, or even 100 times.
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