Javascript performance optimization

Question

I created the following js function

function csvDecode(csvRecordsList)
{
    var cel;
    var chk;
    var chkACB;
    var chkAF;
    var chkAMR;
    var chkAN;
    var csvField;
    var csvFieldLen;
    var csvFieldsList;
    var csvRow;
    var csvRowLen = csvRecordsList.length;
    var frag = document.createDocumentFragment();
    var injectFragInTbody = function () {tblbody.replaceChild(frag, tblbody.firstElementChild);};
    var isFirstRec;
    var len;
    var newEmbtyRow;
    var objCells;
    var parReEx = new RegExp(myCsvParag, 'ig');
    var tblbody;
    var tblCount = 0;
    var tgtTblBodyID;

    for (csvRow = 0; csvRow < csvRowLen; csvRow++)
    {
        if (csvRecordsList[csvRow].startsWith(myTBodySep))
        {
            if (frag.childElementCount > 0)
            {
                injectFragInTbody();
            }
            tgtTblBodyID = csvRecordsList[csvRow].split(myTBodySep)[1];
            newEmbtyRow = getNewEmptyRow(tgtTblBodyID);
            objCells = newEmbtyRow.cells;
            len = newEmbtyRow.querySelectorAll('input')[0].parentNode.cellIndex; // Finds the cell index where is placed the first input (Check-box or button)

            tblbody = getElById(tgtTblBodyID);
            chkAF = toBool(tblbody.dataset.acceptfiles);
            chkACB = toBool(tblbody.dataset.acceptcheckboxes) ;
            chkAN = toBool(tblbody.dataset.acceptmultiplerows) ;
            tblCount++;
            continue;
        }

        csvRecordsList[csvRow] = csvRecordsList[csvRow].replace(parReEx, myInnerHTMLParag); // Replaces all the paragraph symbols ¶ used into the db.csv file with the tag <br> needed into the HTML content of table cells, this way will be possible to use line breaks into table cells
        csvFieldsList = csvRecordsList[csvRow].split(myEndOfFld);

        csvFieldLen = csvFieldsList.length;
        for (csvField = 0; csvField < csvFieldLen; csvField++)
        {
            cel = chkAN ? csvField + 1 : csvField;
            if (chkAF && cel === 1) {objCells[cel].innerHTML =  makeFileLink(csvFieldsList[csvField]);} 
            else if (chkACB && cel === len) {objCells[cel].firstChild.checked = toBool(csvFieldsList[csvField]);}
            else {objCells[cel].innerHTML = csvFieldsList[csvField];}
        }
        frag.appendChild(newEmbtyRow.cloneNode(true));
    }
    injectFragInTbody();

    var recNum = getElById(tgtTblBodyID).childElementCount;
    customizeHtmlTitle();
    return csvRow - tblCount + ' (di cui '+ recNum + ' record di documenti)';
}

More than 90% of records could contain file names that have to be processed by the following makeFileLink function:

function makeFileLink(fname)
{
    return ['<a href="', dirDocSan, fname, '" target="', previewWinName, '" title="Apri il file allegato: ', fname, '" >', fname, '</a>'].join('');
}

It aims to decode a record list from a special type of *.db.csv file (= a comma-separated values where commas are replaced by another symbol I hard-coded into the var myEndOfFld). (This special type of *.db.csv is created by another function I wrote and it is just a "text" file).

The record list to decode and append to HTML tables is passed to the function with its lone parameter: (csvRecordsList).

Into the csv file is hosted data coming from more HTML tables.

Tables are different for number of rows and columns and for some other contained data type (which could be filenames, numbers, string, dates, checkbox values).

Some tables could be just 1 row, others accept more rows.

A row of data has the following basic structure:

data field content 1|data field content 2|data field content 3|etc...

Once decoded by my algorithm it will be rendered correctly into the HTML td element even if into a field there are more paragraphs. In fact the tag
will be added where is needed by the code:

csvRecordsList[csvRow].replace(par, myInnerHTMLParag)

that replaces all the char I choose to represent the paragraph symbol I have hard-coded into the variable myCsvParag.

Isn't possible to know at programming time the number of records to load in each table nor the number of records loaded from the CSV file, nor the number of fields of each record or what table field is going to contain data or will be empty: in the same record some fields could contain data others could be empty. Everything has to be discovered at runtime.

Into the special csv file each table is separated from the next by a row witch contains just a string with the following pattern: myTBodySep = tablebodyid where myTBodySep = "targettbodydatatable" that is just a hard coded string of my choice. tablebodyid is just a placeholder that contains a string representing the id of the target table tbody element to insert new record in, for example: tBodyDataCars, tBodyDataAnimals... etc.

So when the first for loop finds into the csvRecordsList a string staring with the string into the variable myTBodySep it gets the tablebodyid from the same row: this will be the new tbodyid that has to be targeted for injecting next records in it

Each table is archived into the CSV file

The first for loop scan the csv record list from the file and the second for loop prepare what is needed to compile the targeted table with data.

The above code works well but it is a little bit slow: in fact to load into the HTML tables about 300 records from the CSV file it takes a bit more of 2.5 seconds on a computer with 2 GB ram and Pentium core 2 4300 dual-core at 1800 MHz but if I comment the row that update the DOM the function needs less than 0.1 sec. So IMHO the bottle neck is the fragment and DOM manipulating part of the code.

My aim and hope is to optimize the speed of the above code without losing functionalities.

Notice that I'm targeting just modern browsers and I don't care about others and non standards-compliant browsers... I feel sorry for them...

Any suggestions? Thanks in advance.

Edit 16-02.2018

I don't know if it is useful but lastly I've noticed that if data is loaded from browser sessionstorage the load and rendering time is more or less halved. But strangely it is the exact same function that loads data from both file and sessionstorage. I don't understand why of this different behavior considering that the data is exactly the same and in both cases is passed to a variable handled by the function itself before starting checking performance timing.

Edit 18.02.2018

1. Number of rows is variable depending on the target table: from 1 to 1000 (could be even more in particular cases)
2. Number of columns depending on the target table: from 10 to 18-20

Answer 1

In fact, building the table using DOM manipulations are way slower than simple innerHTML update of the table element.

And if you tried to rewrite your code to prepare a html string and put it into the table's innerHTML you would see a significant performance boost.

Browsers are optimized to parse the text/html which they receive from the server as it's their main purpose. DOM manipulations via JS are secondary, so they are not so optimized.

I've made a simple benchmark for you.

Lets make a table 300x300 and fill 90000 cells with 'A'. There are two functions.

The first one is a simplified variant of your code which uses DOM methods:

var table = document.querySelector('table tbody');
var cells_in_row = 300, rows_total = 300;

var start = performance.now();
fill_table_1();
console.log('using DOM methods: ' + (performance.now() - start).toFixed(2) + 'ms');

table.innerHTML = '<tbody></tbody>';


function fill_table_1() {
  var frag = document.createDocumentFragment();

  var injectFragInTbody = function() {
    table.replaceChild(frag, table.firstElementChild)
  }

  var getNewEmptyRow = function() {
    var row = table.firstElementChild;
    if (!row) {
      row = table.insertRow(0);
      for (var c = 0; c < cells_in_row; c++) row.insertCell(c);
    }
    return row.cloneNode(true);
  }

  for (var r = 0; r < rows_total; r++) {
    var new_row = getNewEmptyRow();
    var cells = new_row.cells;
    for (var c = 0; c < cells_in_row; c++) cells[c].innerHTML = 'A';
    frag.appendChild(new_row.cloneNode(true));
  }
  injectFragInTbody();
  return false;
}

<table><tbody></tbody></table>

The second one prepares html string and put it into the table's innerHTML:

var table = document.querySelector('table tbody');
var cells_in_row = 300, rows_total = 300;

var start = performance.now();
fill_table_2();
console.log('setting innerHTML: ' + (performance.now() - start).toFixed(2) + 'ms');

table.innerHTML = '<tbody></tbody>';

function fill_table_2() {// setting innerHTML
  var html = '';
  for (var r = 0; r < rows_total; r++) {
    html += '<tr>';
    for (var c = 0; c < cells_in_row; c++) html += '<td>A</td>';
    html += '</tr>';
  }
  table.innerHTML = html;
  return false;
}

<table><tbody></tbody></table>

I believe you'll come to some conclusions.