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When you use React.js, every JavaScript file in your application is invisible to every other JavaScript file by default. ProfilePage.js and NavBar.js can't see each other.

This is a problem!

On line 9, you just added an instance of the NavBar component class. But since you're in ProfilePage.js, JavaScript has no idea what NavBar means.

If you want to use a variable that's declared in a different file, such as NavBar, then you have to import the file that you want. To import a file, you can use require and pass in a filepath:

var NavBar = require('./NavBar.js');

We've used require before, but not like this.

If you pass require a string that begins with either a dot or a slash, then require will treat that string as a filepath. require will follow that filepath, and import the file that it finds.

If your filepath doesn't have a file extension, then ".js" is assumed. So the above example could be shortened:

var NavBar = require('./NavBar');

One final, important note:
None of this behavior is specific to React! Module systems of independent, importable files are a very popular way to organize code. React's specific module system comes from Node.js. More on all of that later.

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