If your compairing javascript you might find it not displaying.
https://bugs.eclipse.org/bugs/show_bug.cgi?id=509820
Here is a workround...
- Window > Preferences > Compare/Patch > General Tab
- Deselect checkbox next to "Open structure compare automatically"
If one or both of the files you wish to compare isn't in an Eclipse project:
Open the Quick Access search box
Type compare and select Compare With Other Resource
Select the files to compare ? OK
You can also create a keyboard shortcut for Compare With Other Resource by going to Window ? Preferences ? General ? Keys
Just select all of the files you want to compare, then open the context menu (Right-Click on the file) and choose Compare With, Then select each other..
Other than using the Navigator/Proj Explorer and choosing files and doing 'Compare With'->'Each other'... I prefer opening both files in Eclipse and using 'Compare With'->'Opened Editor'->(pick the opened tab)... You can get this feature via the AnyEdit eclipse plugin located here (you can use Install Software via Eclipse->Help->Install New Software screen): http://andrei.gmxhome.de/eclipse/
Source: Stackoverflow.com