Question

I have the following code:

var select_ids = [];
// syntax.filter.select_list is ["elem1", "elem2"]
for (var o = 0; o < syntax.filter.select_list.length; o += 1) {
    element = syntax.filter.select_list[o];
    if (priv.getPositionInArray(element,select_ids) === null ) {
      // this line hangs up the browser
      select_ids.unshift(element);
    }
}

I don’t understand why push() or unshift() into an empty array when looping two iterations hangs up the browser for 3-4sec.

If I leave away this line:

select_ids.unshift(element);

the script runs instantly, so

Question:
why could push/unshift to an empty array produce such a delay? FYI, this is inside another loop with 3 iterations. For every parent loop I’m resetting select_ids to an empty object to fill.

Thanks!

EDIT:
And I’m doing the if-clause to make sure I don’t add doubles to the array.

EDIT: (full code)

priv.findBestIndexForQuery = function (indices, syntax) {
    var i, j, k, l, m, n, p, o, element,
      search_ids = [], select_ids = [], use_index = [],
      index, query_param, search_param,
      // need to parse into object
      current_query = jIO.ComplexQueries.parse(syntax.query);

    // loop indices
    for (i = 0; i < priv.indices.length; i += 1) {
      index = {};
      index.reference = priv.indices[i];
      index.reference_size = index.reference.fields.length;

      // rebuild for iteration
      if (current_query.query_list === undefined) {
        search_ids.push(current_query.id);
      } else {
        for (j = 0; j < current_query.query_list.length; j += 1) {
            if (priv.getPositionInArray(current_query.query_list[j].id, 
                search_ids) === null ) {
                search_ids.push(current_query.query_list[j].id);
            }
        }
      }
      for (o = 0; o < syntax.filter.select_list.length; o += 1) {
         element = syntax.filter.select_list[o];
         if (priv.getPositionInArray(element,select_ids) === null ) {
              // line causing problems
              select_ids.unshift(element);
         }
      }
// there is a lot more, but I'm hanging on the line above
}

EDIT (getPostionInArray)

  priv.getPositionInArray = function (element, array) {
     var i;
     for (i = 0; i < array.length; i += 1) {
        if (array[i] === element) {
           return i;
        }
     }
     return null;
  };

Answer 1

As requested by the OP, the issue is, indeed unshift: there have been bugs with this function (its context is ad-hoc) in the past, and this bug report is still marked as NEW.

As for your getPositionInArray function, why don’t you use the standard indexOf method? Most browsers support it, and for those that don’t you can always add this to your script:

if (!Array.prototype.indexOf)
{
    Array.prototype.indexOf = function(search)
    {
        var i;
        for (i = 0; i < this.length; i++)
        {
            if (this[i] === search)
            {//I use type & value check...
                return i;
            }
        }
        return -1;//-1 is the standard behaviour
    };
}

Or, if you want this method to return null, instead of -1 in all browsers:

Array.prototype.indexOf = (function (nativeFunc)
{
    return function(search)
    {
        var retVal = nativeFunc.apply(this,[search]);//use native method
        return (retVal === -1 ? null : retVal);//return null if the native method returned -1
    };
}(Array.prototype.indexOf));//pass initial method to IIFE

Just a side-note, some prototype fun 🙂