Opal does not support some language/runtime features of ruby. These are documented here when possible, as well as the reasons why they are not supported.
For performance and ease of runtime features, all strings in Opal are immutable, i.e. #<<, #gsub!, etc. do not exist. Also, symbols are just strings. There is no class, runtime or feature difference between Symbols and Strings. Their syntaxes can be used interchangeably.
Encodings only have a very small implementation inside Opal.
JavaScript does not have a native Thread implementation, so they are not present inside Opal. There is a placeholder Thread class just to provide some small level of compatibility with libraries that expect it. It does not have any function.
Opal does not currently support frozen objects, but has placeholder methods to prevent other libraries breaking when expecting these methods. Opal could support frozen objects in the future once a similar implementation becomes available across JavaScript runtimes.
method_added and method_removed hooksThese are not currently supported by Opal, but this is considered a bug and will be implemented soon.
All methods in Opal are defined as public to avoid additional runtime overhead. Module#private and Module#protected exist as just placeholder methods and are no-op methods.