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Understanding variables

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Most programs use variables. This lesson explains what variables are.

Common JavaScript Errors

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Here are some common errors people make in JavaScript.

The number of elements in a JavaScript array: length is misleading

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The JavaScript property length returns the number of elements in an array. Right? Er, well, ...

eMe: A quiz

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Create a quiz about yourself.


You’ll learn how to use focus() to improve a page’s usability.

Changing text

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Learn some more JavaScript/jQuery:

  • Learn how to set the text of an element, with text("stuff").
  • Learn how to get the text of an element, with text().
  • Learn how to add a CSS class to an element, with addClass().
  • Learn how to remove a CSS class from an element, with removeClass().

SitePoint reference

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Good reference for HTML, CSS, and JavaScript.

Programs in browsers

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In this lesson, you will learn that:

  • Some programs run inside browsers.
  • Those programs are downloaded along with pages.
  • Every major browser can run JavaScript programs.
  • Programs are triggered by events.
  • JavaScript is used for interface effects, validating form data, input widgets, and sending data to a server.

Finding JavaScript bugs

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  • Syntax bugs happen when you type something the browser doesn’t understand, like alrt().
  • Use Firefox’s error console to track down syntax errors.
  • Logic errors are when you tell the browser to do the wrong thing.
  • Use alert() statements to track program execution. If alert messages are supposed to show up, but don’t, you know what code is not executing.
  • Use alert() statements to look at the values of variables.

Better client-side error display

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  • How to report errors in a professional way.
  • How page-wide, global error messages help the user.
  • How JavaScript functions make error reporting easier to program.


How to...