pvdz.ee - 11.4.4 Prefix increment operator

UnaryExpression : ++ UnaryExpression

Code: (Meta Ecma)

function evaluate(++ UnaryExpression) {
  var expr = evaluate(UnaryExpression);
  if (type(expr) == 'reference' && IsStrictExpression(expr) && Type(GetBase(expr)) == 'environmentRecord' && (GetReferencedName(expr) == 'eval' || GetReferencedName(expr) == 'arguments')) throw SyntaxError;
  var oldValue = ToNumber(GetValue(expr));
  var newValue = oldValue + 1; // using rules in 11.6.3
  PutValue(expr, newValue);
  return newValue;
}

Note: unlike the postfix expression, this immediately returns the new value of expr (but also replaces any previous value, regardless).

Code: (Ecma)

var x = {valueOf: function(){ return 5; }};
alert(x); // object
alert(++x); // 6
alert(x); // 6