The codebase is the path to the directory where the classes specified in the code property are located. The actual path to the required files is generated by a string concatenation of codebase+code to generate a fully specified URL.

Due to security limitations it is not permitted to access a codebase value that is outside the domain specified by the containing document.

There is conflicting information in the reference sources regarding the read/write ability of this property. Some suggest it is ReadOnly and others suggest you can assign a value to it. It may be that you can assign a value to it without the JavaScript interpreter complaining but that any value you assign is ignored.