seen

seen.js

function seen( prop_val, arr, prop_name=null ) {
  if( prop_name == null ) {
    for ( var i = 0; i < arr.length; i++ ) {
      if ( prop_val == arr[i] ) {
        return i;
      }
    }
  }
  else {
    for( var i = 0; i < arr.length; i++) {
      if( prop_val == arr[i][prop_name] ) {
        return i;
      }
    }
  }
  return -1;
}