v8/test/mjsunit/arguments-enum.js

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function countArguments() {
  var count = 0;
  for (var prop in arguments)
    count++;
  return count;
}

function setArgumentCount() {
  arguments[10] = 5;
  arguments.x = 4;
  var count = 0;
  for (var prop in arguments)
    count++;
  return count;
}

assertEquals(0, countArguments());
assertEquals(1, countArguments(1));
assertEquals(2, countArguments(1, 2));
assertEquals(5, countArguments(1, 2, 3, 4, 5));

assertEquals(2, setArgumentCount());
assertEquals(3, setArgumentCount(1));
assertEquals(4, setArgumentCount(1, 2));
assertEquals(7, setArgumentCount(1, 2, 3, 4, 5));