46 down vote accepted
	

Complete the following steps in Eclipse to get plugins for JavaScript files:

    Open Eclipse -> Go to "Help" -> "Install New Software"
    Select the repository for your version of Eclipse. I have Juno so I selected http://download.eclipse.org/releases/juno
    Expand "Programming Languages" -> Check the box next to "JavaScript Development Tools"
    Click "Next" -> "Next" -> Accept the Terms of the License Agreement -> "Finish"
    Wait for the software to install, then restart Eclipse (by clicking "Yes" button at pop up window)
    Once Eclipse has restarted, open "Window" -> "Preferences" -> Expand "General" and "Editors" -> Click "File Associations" -> Add ".js" to the "File types:" list, if it is not already there
    In the same "File Associations" dialog, click "Add" in the "Associated editors:" section
    Select "Internal editors" radio at the top
    Select "JavaScript Viewer". Click "OK" -> "OK"

To add JavaScript Perspective: (Optional)
10. Go to "Window" -> "Open Perspective" -> "Other..."
11. Select "JavaScript". Click "OK"

To open .html or .js file with highlighted JavaScript syntax:
12. (Optional) Select JavaScript Perspective
13. Browse and Select .html or .js file in Script Explorer in [JavaScript Perspective] (Or Package Explorer [Java Perspective] Or PyDev Package Explorer [PyDev Perspective] Don't matter.)
14. Right-click on .html or .js file -> "Open With" -> "Other..."
15. Select "Internal editors"
16. Select "Java Script Editor". Click "OK" (see JavaScript syntax is now highlighted )