Javascript Array.indexOf() shows strange behaviour when applied to array of objects

You are comparing 2 different objects that happen to have the same values inside, but they are still different objects from an equality perspective.

Your original code:

<script>
    var arr = [{a:1,b:2,c:3},{a:3,b:5,c:6}];
    var a = arr[0];
    alert(arr.indexOf(a));
    var b = {a:1,b:2,c:3};
    alert(arr.indexOf(b));
</script>

An updated version that shows how to get a match on the comparison.

<script>
    var b = {a:1,b:2,c:3};
    var arr = [b,{a:3,b:5,c:6}];
    var a = arr[0];
    alert(arr.indexOf(a));
    alert(arr.indexOf(b));
</script>

If you want to compare 2 different objects that happen to have the same values, you have to do so manually.

From MDN:

indexOf compares searchElement to elements of the Array using strict equality (the same method used by the ===, or triple-equals, operator).

Since no two objects are ever strictly equal, you can not use indexOf to search an array of objects, you must loop (or extend indexOf to loop if the elements of the array are objects)

Use this instead

add this code:

<script>
   Array.prototype.newIndexOf = function( item ) {
      var count = this.length;
      var j = 0;
      while (this[j++] !== item && j < count) {}
      return (j === count) ? -1 : j;
   }
</script>

indexOf is slow and no strict equal check.

http://jsperf.com/indexof-vs-equaling/2