Javascript: Is it safe to delete properties on an object being iterated with 'for' -

I am doing something like this:

  var myObj = {a: 1, b : 2, C: 3, D: 4}; (Var key in myObj) {if (some conditions) {delete (myObj [key]); }}   
 This is what I have tried to do well in the samples, but I'm not sure that this can be unexpected behavior in certain scenarios / browsers.  
 Is it OK to repeat the object?   
 
 
 indicates that  for..in  forms the "next asset" Let's say the name of the next property of P. Obitz, whose [[enumerable]] attribute is correct if there is no such property, return (normal, V, empty) .   
 Since the concept "next asset" is well defined in the presence of mutation (although the recurrence order is not there)  
 is a corner case where  is removed  Uni-masks as a prototype property      var obj = Object Create ({x: 1}); Obj.y = 2; Obj.x = 3; For (obj in var k) {if (k == 'y') {remove obj.x; } Alert (K); }   
 In this case, where you can repeat  y  and delete  x , you can still type in  X  from the prototype, but if you repeat the  x  before, then  y  then you should not see the second  x .