Table of Contents

This proposal has progressed to the Draft ECMAScript 6 Specification, which is available for review here: specification_drafts. Any new issues relating to them should be filed as bugs at The content on this page is for historic record only and may no longer reflect the current state of the feature described within.


  • AKA “optional parameters” and “optional arguments”, but without arity checking, all trailing arguments are optional in a real sense
  • 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 ::=
  | FormalParameterList , Identifier
  • to this pair of productions:
  FormalParameterList ::=
  | FormalParameterList , FormalParameter

  FormalParameter ::=
    Identifier Initialiser?
  | Pattern Initialiser?
  • 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

The length property of a function should be updated to not include optional parameters.

Erik Arvidsson 2010/11/05 22:37

Right now this works as if arguments.length is checked. However, sometimes when chaining functions with optional parameters this will lead to problems since the optional parameter will be passed as undefined. Would it make sense to also allow undefined in the tail of the parameter list?

This example is pretty contrived and it can easily be fixed but I think it points out the issue clearly:

function add(x = 0, y = 0) {
  return x + y;
function inc(opt_x) {
  return add(1, opt_x);
assert(1, inc());

Erik Arvidsson 2011/05/03 16:52

We could treat trailing undefined as missing. CoffeeScript (after apply and call in their first parameter handling, and after ==) equate null and undefined in this way. You can’t therefore pass an explicit null or undefined, and I expect that will be a sticking point.

The scenario you describe above could be handled by passing a “magic undefined” to inc when it is called via inc(), and propagating that through to add to trigger defaulting.

This is doable but a hassle for implementors (not terrible from where I sit, but other implementors should weigh in; JavaScriptCore already lets objects and/or strings masquerade as undefined, IIRC).

Brendan Eich 2011/05/03 18:34

harmony/parameter_default_values.txt · Last modified: 2013/07/11 23:52 by rwaldron
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