Vertical scrollbar causing column width different between header and cell in spreadsheet

Vertical scrollbar causing column width different between header and cell in spreadsheet. This is a common issue when the HTML page that display on a computer. Whereas, it won't have issue if it is on Android or iOS because the scrollbar is floating on top of the content.

To explain this issue, let says we have a spreadsheet (or "table" view) built with DIV which looks like below:

<style>
.header {
  display:flex;
  flex-direction:row;
}

.item {
  display:flex;
  flex-direction:row;
}

.c1 {
  width:50%;
}

.item-list {
  overflow: hidden;
  overflow-y: scroll;
  height: 80px;
}
</style>

<div class="my-table">

<div class="header">
  <div class="c1">Date</div>
  <div class="c1">Ref #</div> 
</div> 
<div class="item-list">
  <div class="item">
    <div class="c1">1 Sep 2019</div>
    <div class="c1">0001</div> 
  </div>
  <div class="item">
    <div class="c1">1 Sep 2019</div>
    <div class="c1">0002</div> 
  </div>
  <div class="item">
    <div class="c1">1 Sep 2019</div>
    <div class="c1">0003</div> 
  </div>
  <div class="item">
    <div class="c1">1 Sep 2019</div>
    <div class="c1">0004</div>
  </div>
  <div class="item">
    <div class="c1">1 Sep 2019</div>
    <div class="c1">0005</div>
  </div>
</div>
</div>


With the CSS for item-list, a vertical scrollbar will appear once the number of items increased.

The problem with this CSS is that the date and reference number column in the header will have a different width if you compared it with the column in the items.

To solve this, you need to use this style:

(1) you have to use "sticky:

.header{
  position: sticky;
  top: 0;
  left: 0;
  background-color: white;
}

(2) Move the overflow attribute for "item-list" to "my-table":

.my-table {
  position:relative;
  overflow: hidden;
  overflow-y: scroll;
  height: 80px;
}

And that solves the column width issue when there is a vertical scrollbar.

Server-sent event

Server sent event supports the server push from server to browser only.

In Javascript

var sse = null;

function runNow() {
    if (!sse) {
        sse = new EventSource('/sse_stream');

        sse.addEventListener('message',
            function (e) {
                document.getElementById('lbl1').innerText = e.data;
            });

    }
}

function stop() {
    if (sse) {
        sse.close();
        sse = null;
    }
}


Reference:

https://www.html5rocks.com/en/tutorials/eventsource/basics/
https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events

Web worker

Summary of web worker

• Every web worker is a thread on it's own!! Instantiate too many worker instances might degrade your computer performance.
• The worker will not be able to access the HTML page.
• All data exchange/communication must be done through postMessage().
• With web worker, the communication between the HTML/JS and the worker will be more complex.
• Good thing is that process in the web worker won't affect the HTML page update/redraw.

The JavaScript to that create a web worker 

This script will be added to the HTML page like a normal JS file.

var w;
var lbl1;

function onStart() {
    if (w) {
        return;
    }

    w = new Worker('demo_worker.js');

    lbl1 = $('#lbl1');

    w.onmessage = function (e) {
        if (e.data.t == 'number') {
            lbl1.text('running value=' + e.data.i);
        }
        else {
            console.log(e.data)
        }
    }
}


function onStop() {
    if (w != null) {
        w.terminate();
        w = null;
    }
}

function onSayHelo() {
    if (w) {
        w.postMessage('helo from tab...' + (new Date()));
    }
}

The web worker Javascript file "demo_worker.js"

• This file will run in it's own thread. 
• All data exchange must be done through postMessage().
• The worker is able to execute AJAX call.
• The worker will not be able to access the HTML page.

var i = 0;

function timedCount() {
    i = i + 1;

    postMessage({
        t: 'number',
        i: '<b>'+ i + '</b>',
        ts: new Date()
    });

    setTimeout("timedCount()", 1000);   
}

function testAjax() {
    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function (e) {
        if (xhr.readyState == 4 && xhr.status == 200) {
            // upon loaded succesfully, pass it to the HTML page.
            postMessage({
                t: 'data',
                data: xhr.response
            });
        }
    }

    xhr.open('get', 'test-data.txt');
    xhr.send();   
}

// message from the HTML page.
self.onmessage = function (e) {
    console.log('worker...', e.data)
}

timedCount();
setTimeout(testAjax, 3000)

Print the call stack in console

Here's the code in printing the call stack in console:

var stack = new Error().stack;   
console.log(stack);

CSS fill pattern

To create the fill pattern image, check out the following website:

     http://www.patternify.com/

/*dots*/

.inactive_cell2 {
    background-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAQAAAAECAYAAACp8Z5+AAAAEklEQVQImWNgYGD4z0AswK4SAFXuAf8EPy+xAAAAAElFTkSuQmCC");
    background-repeat: repeat;
}

To create strips without using any image data,

    https://css-tricks.com/stripes-css/

/*stripes*/
.inactive_cell {   
    background: repeating-linear-gradient( 48deg, #fefefe, #e0e0e1 5px, #eaeaea 5px, #eaeaea 10px );
}

Simple pie chart & doughnut chart with SVG + CSS

You can draw a simple pie chart & doughnut chart with SVG + CSS. This is quite simple and straight forward. But the limitation is that it is able to show 2 sections only.

To change the value of the section, look for stroke-dasharray and change the first value.

.pie2 {

        fill: yellow;

        stroke: green;

        /* max=50 (radius x 2) */

        stroke-width: 50;

        /* 158 = (radius x 2) x 3.142 = 25 x 2 x 3.142 */

        stroke-dasharray: 40 158;

    }

    .chart2 {

        background-color: yellow;

        border-radius: 50%;

        -webkit-transform: rotate(-90deg);

                transform: rotate(-90deg);

    }

    <svg width="100" height="100" class="chart2">    

        <circle r="25" cx="50" cy="50" class="pie2"/>  

    </svg>

To draw a doughnut chart,  

    .doghnut {

        fill: yellow;

        stroke: green;

        stroke-width: 20;

        stroke-dasharray: 180 251;

    }

    .doghnut_chart {

        background-color: yellow;

        border-radius: 50%;

        -webkit-transform: rotate(-90deg);

                transform: rotate(-90deg);

    }

    <svg width="100" height="100" class="doghnut_chart">    

        <circle r="40" cx="50" cy="50" class="doghnut"/>  

    </svg>

Javascript that prevents double click on submit button

This is what happening:

• The user loves "double clicking" on any button even though single click will trigger the action.
• The Internet connection slows down and the user thought that he did not click on the correct button. So, he decided to click it again even though the waiting time is lesser than 1 second (?!).

To solve this, you need the following JavaScript to ensure that the submit is working with single click only. Note: you may adjust the value of "500" (ms) to whatever value that you want.

var _prev_save_click_ts = null;

function beforeSubmit() {

    if (_prev_save_click_ts != null) {
        var ts = new Date();

        if (ts - _prev_save_click_ts < 500) {
            // user is double clicking.
            return false;
        }
    }

    _prev_save_click_ts = new Date();
    return true;
}