var arr = [5, 5, 5]
var obj = {};
var index = false;
for (let i = 0; i < arr.length; i++) {
  var diff = 10 - arr[i];
  if (obj[diff] != undefined) {
    index = [obj[diff], i];
    break;
  }
  obj[arr[i]] = i;
}
console.log(index)