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For...Of
3:18A new way to loop over items in an iterable like String, Array, Set, and Map
When developing an application we
inevitably need to iterate over a dataset.
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For example, an Ajax request sends back
a list of names or an array of objects.
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And our program needs to go
over each item in the array.
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The array for each method provided in
ES5 made looping over an array a breeze.
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ES2015 gives us a similar but
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slightly different technique the,
the for of statements.
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You can follow along by launching
the workspace for this video.
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So from the beginning of JavaScripts
existence, we've relied heavily
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on a basic for loop, where we
determine the length of an array and
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iterate by increment a variable by one for
each pass.
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For example let's take a look at the file
for-loop.js containing a basic for loop.
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If I run the file, the console
logs the numbers one through six.
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This works well enough but
it's cumbersome.
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Now ES5 gave us for each to process each
item of an array in a callback function.
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So if you wanted to control
the loop to end it early,
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we still had to use the for loop.
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Now let's take a look at the file
for-each.js which contains
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a collection of teachers.
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So say I wanted to log only
Nick's rating to the console.
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Well using for-each it would
look something like this.
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And when I run this code,
I see Nick's rating in the console.
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But what I don't see is how many
times this function was called so
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I'll add another log message
outside of my if statement.
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We'll say console.log teacher.name.
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And you've probably already guessed that
the names of online teachers will log
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to the console.
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So let's see.
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And sure enough all nine
teacher names are logged.
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Luckily, ES2015 has a way to
iterate over an array of values and
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break out of the loop when needed.
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So I'm going to rewrite the for
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each loop using the new for of syntax by
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typing for let teacher of teachers.
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And now I can short circuit the loop or
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break out of the loop after it logs
Nick by using the break keyword.
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So let's give it a try.
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Over and the console, I'll run the code.
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And now only seven teachers are logged
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followed by Nicks rating which
is exactly what I expected.
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Keep in mind that for
of can not be used with regular objects.
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You'd want to use the for
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in statement instead when
looping over object properties.
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So check the teachers notes
to learn more about for in.
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However, for of can be used with
any interval object like strings,
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arrays, maps, and sets.
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Coming up in the next video
we'll take a look at set,
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a new type of iterable that
lets you store unique values.
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