The short-circuiting logical-or connective || is often used as a default operator. If an expression would produce a falsy value, then appending || and another expression will fill in a default value instead.

possible_value || default_value

This works well for things like text fields, where the value from an empty field is easily replaced with a default value.

This is also used with parameters and object members, but there is a problem. With parameters and object members, the only falsy value of interest usually is undefined. Unfortunately, the number 0 is also a falsy value, which can lead to an unintended replacement with the default value.

We propose that a default operator ?? be added. It is a short-circuiting operator. If the first operand evaluates to a value other than undefined, then the result is the first operand and the second operand is not evaluated. But if the result of evaluating the first operand is undefined, then the second operand is evaluated and used as the result.

possible_value ?? default_value

is a short form of

possible_value !== undefined ? possible_value : default_value

but with possible_value evaluated exactly once.

For convenience, we propose adding an ??= assignment operator:

variable ??= default_value;  // variable = variable ?? default_value

As with the existing assignment operators, the Reference internal type instance evaluated from the left-hand side variable is evaluated once, and used via GetValue in the expansion on the right of =.

— Douglas Crockford 2011/02/13 00:48 — Brendan Eich 2012/06/17 18:52

The production ConditionalExpression : LogicalORExpression ?? AssignmentExpression is evaluated as follows:

1. Let lref be the result of evaluating LogicalORExpression.
2. Let lval be GetValue(lref).
3. If lval is not undefined, then return lval.
4. Let rref be the result of evaluating AssignmentExpression.
5. Return GetValue(rref).

The production AssignmentExpression : LeftHandSideExpression ??= AssignmentExpression is evaluated as follows:

1. Let lref be the result of evaluating LeftHandSideExpression.
2. Throw a SyntaxError exception if the following conditions are all true:
   • Type(lref) is Reference is true
   • IsStrictReference(lref) is true
   • Type(GetBase(lref)) is Environment Record
   • GetReferencedName(lref) is either “eval” or “arguments”
3. Let lval be GetValue(lref).
4. If lval is not undefined, then return lval.
5. Let rref be the result of evaluating AssignmentExpression.
6. Let rval be GetValue(rref).
7. Call PutValue(lref, rval).
8. Return rval.

— Brendan Eich 2012/06/17 18:52

Ryan Florence provided some good examples of pain points that exist today, that could be mitigated by the default operator:

https://mail.mozilla.org/pipermail/es-discuss/2012-June/023356.html

var events = {
  _callbacks: {},
 
  on: function (topic, callback) {
    (this._callbacks[topic] ??= []).push(callback);
    ...
  },
  ...
};
 

function ajaxWithError (settings) {
  settings.method ??= 'POST';
 
  settings.elementPosition ??= this._getElementPositions();
 
  settings.somethingWithInitialization ??= (function(){
    var thing = new Thing();
    thing.foo = settings.foo;
    return thing;
  })();
  ...
};

— Rick Waldron 2012/06/12 17:08

• Does ||= deserve another look, even though it would be a new (Ruby-like) idiom?
  • Pro: paves a cow-path.
  • Pro: avoids adding a new operator.
  • Con: leads cows down the wrong path (|| means falsy value gets overridden by default value).
  • Con: does not mean op= (assignment operator) semantics, rather avoids assigning L = L in truthy-L case.
• Should null be equated to undefined? Need more source code surveys.
  • CoffeeScript equates null and undefined in its ? and when triggering parameter default values.