pvdz.ee - 11.12 Conditional operator ?:

ConditionalExpression :
LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression

ConditionalExpressionNoIn :
LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

Code: (Meta Ecma)

function evaluate(LogicalORExpression ? AssignmentExpression(1) : AssignmentExpression(2)) {
  var lref = evaluate(LogicalORExpression);
  if (ToBoolean(GetValue(lref))) {
    var trueRef = evaluate(AssignmentExpression(1)); // the first one...
    return GetValue(trueRef);
  }
  var falseRef = evaluate(AssignmentExpression(2)); // the second one...
  return GetValue(falseRef);
}

The NoIn versions behave identical.

The specification notes that this implementation of the ternary operator is different from C and Java because it doesn't restrict the second AssignmentExpression to be a ConditionalExpression. This allows an assignment in both "arms" of the operator. It also eliminates a case of a comma expression as the center expression (??).