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JavaScript

Language Fundamentals I

About JavaScript

• JavaScript is not Java, or even related to Java
  • The original name for JavaScript was “LiveScript”
  • The name was changed when Java became popular
  • Now that Microsoft no longer likes Java, its name for their JavaScript dialect is “Active Script”
• Statements in JavaScript resemble statements in Java,

because both languages borrowed heavily from the C

language

• JavaScript should be fairly easy for Java programmers to learn
• However, JavaScript is a complete, full-featured, complex language
• JavaScript is seldom used to write complete “programs”
• Instead, small bits of JavaScript are used to add functionality to HTML pages
• JavaScript is often used in conjunction with HTML “forms”
• JavaScript is reasonably platform-independent

Using JavaScript in a browser

• JavaScript code is included within - Notes: - The type attribute is to allow you to use other scripting languages (but JavaScript is the default) - This simple code does the same thing as just putting

  Hello World!

  in the same place in the HTML document - The semicolon at the end of the JavaScript statement is optional - You need semicolons if you put two or more statements on the same line - It’s probably a good idea to keep using semicolons

  JavaScript isn’t always available

  • Some old browsers do not recognize script tags
    • These browsers will ignore the script tags but will display the included JavaScript
    • To get old browsers to ignore the whole thing, use:
    • The <!-- introduces an HTML comment
    • To get JavaScript to ignore the HTML close comment, -->, the // starts a JavaScript comment, which extends to the end of the line
  • Some users turn off JavaScript
    • Use the to display a message in place of whatever the JavaScript would put there

  Where to put JavaScript

  • JavaScript can be put in the or in the of an

  HTML document

  • JavaScript functions should be defined in the
    • This ensures that the function is loaded before it is needed
  • JavaScript in the will be executed as the page loads
  • JavaScript functions can be put in a separate .js file
  • - - Put this in the - An external .js file lets you use the same JavaScript on multiple HTML pages - The external .js file cannot itself contain a