Javascript performance strangeness: it seems doing a conditional test and assignment is faster than an assignment alone...

assignment-perf-test.js

var myObj = {};
var populateObj = function(obj) { for(var i = 0; i < 10000; i++) { if(i%5==0)obj[i] = i; }  };
//populateObj(myObj);
console.time('test-and-assign');
for(var i = 0; i < 10000; i++) {
   if(!myObj[i]){
      myObj[i] = i;
   }
}
console.timeEnd('test-and-assign');

var myObj = {};
//populateObj(myObj);
console.time('coalesce');
for(var i = 0; i < 10000; i++) {
   myObj[i] = myObj[i] || i;
}
console.timeEnd('coalesce');

var myObj = {};
//populateObj(myObj);
console.time('control');
for(var i = 0; i < 10000; i++) {
   myObj[i] = i;
}
console.timeEnd('control');