• AKA “optional parameters” and “optional arguments”, but without arity checking, all trailing arguments are optional in a real sense
• Waldemar’s original JS2 optional parameters
• ES4 default parameters, gleaned from the normative grammar and the reference implementation
• var foo = arg || defaultFoo idiom for a function taking an optional parameter arg motivates one or both of
  • ||=, or perhaps ??= (a more precise operator inspired by C#)
  • default parameter values (these may abate the need for ||=)

• Allow trailing formal parameters to receive default values if no corresponding actual parameters are passed
• The default values should be computed in the scope of the callee, thus enabling use of
  • any parameters to the left of the one being given its default value, and of
  • any lexical and free (global) variables in enclosing functions and the top level

• The ES5 FormalParameterList production changes from:

  FormalParameterList ::=
    Identifier
  | FormalParameterList , Identifier

• to this pair of productions:

  FormalParameterList ::=
    FormalParameter
  | FormalParameterList , FormalParameter

  FormalParameter ::=
    Identifier
  | Identifier = AssignmentExpression

• Added restriction not expressed by this grammar: only trailing formal parameters may have default values specified by = AssignmentExpression

• When binding actual to formal parameters (see ES5 10.6 steps under 3(d)), for each formal parameter P from left to right:
  • If no actual parameter was passed for P, and
  • = AssignmentExpression was given to supply P with a default value, then:
    • Evaluate that AssignmentExpression in the scope of the fresh activation (localEnv in ES5 10.4.3) for the current function
    • Assign the result of evaluating the AssignmentExpression to P

• It’s easy enough to elaborate the grammar to require that only trailing formal parameters may be given default values. Worth doing?
• Clearly ES5’s 10.6 steps under 3(d) could be adapted, but I’m both (a) lazy and (b) hopeful that we’ll switch from pseudocode to a testable spec

— Brendan Eich 2009/05/05 06:06