[{'a': 'something', 'b':12},
{'a': 'something', 'b':12},
{'a': 'somethingElse', 'b':12},
{'a': 'something', 'b':12},
{'a': 'somethingElse', 'b':12}]

let newArray = yourArray.filter((each)=>{
    return (each.a === something)
});
newArray[newArray.length-1];

let reversedArray = yourArray.reverse();
reversedArray.find((each)=>{return each.a === something})

var arr = [
   {'a': 'something', 'b':12},
   {'a': 'something', 'b':12},
   {'a': 'somethingElse', 'b':12},
   {'a': 'something', 'b':12},
   {'a': 'somethingElse', 'b':12}
];

var item_count = 0;
var traverse_count = 0;
var last_item_traverse_count = 0;
arr = arr.reverse();

arr.filter(function(element) {
   traverse_count += 1;
   if(item_count < 1 && element.a == 'something') {
       last_item_traverse_count = traverse_count;
       item_count += 1;
       return true;
   }

   return false;
});

var item_last_index = arr.length - last_item_traverse_count;

console.log(item_last_index);

[{'a': something, 'b':12},
{'a': something, 'b':12},
{'a': somethingElse, 'b':12},
{'a': something, 'b':12},
{'a': somethingElse, 'b':12}]

for(let i=(arr.length - 1); i>=0; i--){
    if(arr[i].a === "something"){
        index = i;
        break;
    }
}

Array.prototype.lastIndex = function(cond) {
  if (!this.length) return -1;
  if (!cond) return this.length-1;

  for (var i=this.length-1; i>=0; --i) {
    if (cond(this[i])) return i;
  }

  return -1;
}

Array.prototype.lastOrDefault = function(cond, defaultValue) {
  if (!this.length) return defaultValue;
  if (!cond) return this[this.length-1];

  for (var i=this.length-1; i>=0; --i) {
    if (cond(this[i])) return this[i];
  }

  return defaultValue;
}

myArr = [1,2,3,4,5];
var ind1 = myArr.lastIndex(function(e) { return e < 3; });
var num2 = myArr.lastOrDefault(function(e) { return e < 3; });
var num8 = myArr.lastOrDefault(function(e) { return e > 6; }, /* explicit default */ 8);

var arr = [ 'test1', 'test2', 'test3' ];

var last arrIndex = arr[arr.length - 1];