This book usually mentions if a feature is part of ECMAScript 5 (as required by older browsers) or a newer version. For more detailed information (incl. pre-ES5 versions), there are several good compatibility tables available online:
Please consult the following sources:
First example: If the operands of an operator don’t have the appropriate types, they are converted as necessary.
Second example: If an arithmetic computation fails, you get an error value, not an exception.