16   Errors

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Program containing the error. An implementation must report early errors in a Program prior to the first evaluation of that Program. Early errors in eval code are reported at the time eval is called but prior to evaluation of any construct within the eval code. All errors that are not early errors are runtime errors.

An implementation must treat any instance of the following kinds of errors as an early error:

• Any syntax error.
• Attempts to define an ObjectLiteral that has multiple get property assignments with the same name or multiple set property assignments with the same name.
• Attempts to define an ObjectLiteral that has both a data property assignment and a get or set property assignment with the same name.
• Errors in regular expression literals that are not implementation-defined syntax extensions.
• Attempts in strict mode code to define an ObjectLiteral that has multiple data property assignments with the same name.
• The occurrence of a WithStatement in strict mode code.
• The occurrence of an Identifier value appearing more than once within a FormalParameterList of an individual strict mode FunctionDeclaration or FunctionExpression.
• Improper uses of return, break, and continue.
• Attempts to call PutValue on any value for which an early determination can be made that the value is not a Reference (for example, executing the assignment statement 3=4).

(scroll to the next page)

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed. An implementation shall report all errors as specified, except for the following:

• An implementation may extend program syntax and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the program syntax or regular expression pattern or flag syntax.
• An implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).
• An implementation may define behaviour other than throwing RangeError for toFixed, toExponential, and toPrecision when the fractionDigits or precision argument is outside the specified range.