How to make buttons that increment and decrement a value when clicked?

Question

I am new to programming. Every time I run this code, nothing happens. Can you please tell me why this is?

<body>
  <input type=button value="increment" onclick="button1()" />
  <input type=button value="decrement" onclick="button2()" />
  <script type="text/javascript">
    var x = 0
    document.write(x)

    function button1() {
      document.write(x++)
    }
    function button2(){
      document.write(x--)   
    }
  </script>
</body>

Does this answer your question? [Incrementing value continuously on mouse hold](https://stackoverflow.com/questions/28127507/incrementing-value-continuously-on-mouse-hold) — Pietro, Feb 02 '22 at 15:23

Angelos Chalaris · Answer 1 · 2017-08-04T08:25:02.287

The problem is that you put ++ and -- after the variable, meaning that it will increment/decrement the variable after printing it. Try putting it before the variable, like below.

Also, as mentioned, you have some trouble with document.write(). Consider the following documentation from the W3Schools page:

The write() method writes HTML expressions or JavaScript code to a document.

The write() method is mostly used for testing. If it is used after an HTML document is fully loaded, it will delete all existing HTML.

Thus, document.write() will remove all your existing content as soon as you click on a button. If you want to write to the document, use an element's .innerHTML like this:

var x = 0;

document.getElementById('output-area').innerHTML = x;

function button1() {
  document.getElementById('output-area').innerHTML = ++x;
}

function button2() {
  document.getElementById('output-area').innerHTML = --x;
}

<input type=button value="increment" onclick="button1()" />
<input type=button value="decrement" onclick="button2()" />
<span id="output-area"></span>

No, the problem is not the post increment. It's the `document.write`. Could you please mention that explicitly? — PeterMader, Aug 04 '17 at 08:21
how to make it with same button, like (Facebook Like) button? — MADMAX, Feb 07 '20 at 04:54

score 4 · Answer 2 · edited Aug 04 '17 at 08:02

4

Why don't you change your code a bit? Instead of document.write(x++) and document.write(x--) use document.write(++x) and document.write(--x).

edited Aug 04 '17 at 08:02

Angelos Chalaris

6,611
8
49
75

answered Aug 04 '17 at 08:00

p.mathew13

920
8
16

But that won't make it work. Just don't use `document.write`. – PeterMader Aug 04 '17 at 08:19

score 2 · Answer 3 · answered Aug 04 '17 at 08:08

document write will delete full html: The write() method is mostly used for testing: If it is used after an HTML document is fully loaded, it will delete all existing HTML. As in w3schools

try this instead

<body>
<input type=button value="increment" onclick="button1()" />
<input type=button value="decrement" onclick="button2()" />
<div id="value"></div>
<script type="text/javascript">
    var x=0

    var element = document.getElementById("value");
    element.innerHTML = x;

    function button1(){
        element.innerHTML = ++x;
    }
    function button2(){
        element.innerHTML = --x;
    }
</script>

I changed the x-- and x++ to ++x and --x so the changes are immediatly. With this change your code would have worked aswell. showing 1 or -1.

PeterMader · Accepted Answer · 2017-08-04T08:26:54.470

The document.write is the problem. It only works before the browser is done loading the page completely. After that, document.write doesn't work. It just deletes all of the existing page contents.

Your first document.write is executed before you the page has loaded completely. This is why you should see the 0 next to the two buttons.

Then however, the page has loaded. Clicking on a button causes the event handler to be executed, so document.write will be called, which doesn't work at that point, because the page already has loaded completely.

document.write shouldn't be used anymore. There are many modern ways of updating the DOM. In this case, it would create a <span> element and update it's content using textContent.

Moreover, use addEventListener instead of inline event listeners:

var x = 0;
var span = document.querySelector('span'); // find the <span> element in the DOM
var increment = document.getElementById('increment'); // find the element with the ID 'increment'
var decrement = document.getElementById('decrement'); // find the element with the ID 'decrement'

increment.addEventListener('click', function () {
  // this function is executed whenever the user clicks the increment button
  span.textContent = x++;
});

decrement.addEventListener('click', function () {
  // this function is executed whenever the user clicks the decrement button
  span.textContent = x--;
});

<button id="increment">increment</button>
<button id="decrement">decrement</button>
<span>0</span>

As others have mentioned, the first x++ won't have a visible effect, because the value of x is incremented after the content of the <span> is updated. But that wasn't not your original problem.

score 1 · Answer 5 · answered Jul 11 '20 at 12:52

HTML code for UI

<div class="card">
      <div class="card-body">
       <h5 class="card-title">How many guests can stay?</h5>
       <div class="row">
       <ul class="guestCounter">
          <li data-btn-type="increment"><span class="romoveBtn"><i class="fa fa-plus"></i></span> </li>
          <li class="counterText"><input type="text" name="guestCount" id="btnGuestCount" value="1" disabled /> </li>
        <li data-btn-type="decrement"><span class="romoveBtn"><i class="fa fa-minus"></i></span></li>
       </ul>
  </div>

Java Script:

// set event for guest counter
$(".guestCounter li").on("click", function (element) {
    var operationType = $(this).attr("data-btn-type");
    //console.log(operationType);
    var oldValue = $(this).parent().find("input").val();
    //console.log(oldValue);

    let newVal;
    if (operationType == "increment") {
         newVal = parseFloat(oldValue) + 1;
    } else {
        // Don't allow decrementing below zero
        if (oldValue > 1) {
             newVal = parseFloat(oldValue) - 1;
        } else {
            newVal = 1;
        }
    }
    $(this).parent().find("input").val(newVal);
});

score 0 · Answer 6 · answered Dec 26 '21 at 15:46

0

var x = 1;
document.getElementById('output-area').innerHTML = x;
function button1() {
  document.getElementById('output-area').innerHTML = ++x;

}
function button2() {
  if(x <= 0 ){
    alert(' minimum value 0 // By Khaydarov Marufjon marvell_it academy uzb ')
    return false ;
  }
  document.getElementById('output-area').innerHTML = --x;
}

input{
  width: 70px;
   background-color: transparent;
   padding: 20px;
   text-align: center;
 }
 button{
   padding: 20px;
 }

<input type='button' value="plus" onclick="button1()" />
<span id="output-area"></span>
<input type='button' value="minus" onclick="button2()" />

answered Dec 26 '21 at 15:46

Khaydarov Marufjon

11
4

2

Please don't post only code as an answer, but also provide an explanation what your code does and how it solves the problem of the question. Answers with an explanation are usually more helpful and of better quality, and are more likely to attract upvotes. – Tyler2P Dec 26 '21 at 16:07
Not relevant for the demo, but in general you should be using an `output` element with its `.value` rather than `span` and innerHTML for values produced from Javascript. There is also innerText to use instead of innerHTML whenever your content is not actually HTML (learning this habit avoids security holes). – Tronic Dec 26 '21 at 19:29

Pietro · Answer 7 · 2022-02-10T09:46:15.423

This question is very common. I have developed a solution with Bootstrap and pure JavaScript in another very similar thread here. There is autoincrement input on keeping button pressed down. Use ontouchstart and ontouchend instead than onmousedown and onmouseup method for mobile. To make it work for both mobile and desktop browser without headache use onpointerdown, onpointerup, onpointerleave

https://stackoverflow.com/a/70957862/13795525

function imposeMinMax(el) {
    if (el.value != '') {
        if (parseInt(el.value) < parseInt(el.min)) {
            el.value = el.min;
        }
        if (parseInt(el.value) > parseInt(el.max)) {
            el.value = el.max;
        }
    }
}


var index = 0;
var interval;
var timeout;
var stopFlag=false;

function clearAll(){
   clearTimeout(timeout);
   clearInterval(interval);
}


function modIn(el) {
   var inId = el.id;
   if (inId.charAt(0) == 'p') {
      var targetId = inId.charAt(2);      
      var maxValue = Number(document.getElementById(targetId).max);
      var actValue = Number(document.getElementById(targetId).value);
      index = actValue;
      if(actValue < maxValue){
         stopFlag=false;
         document.getElementById(targetId).value++;
      }else{
      stopFlag=true;
      }
      timeout = setTimeout(function(){
      interval = setInterval(function(){        
         if(index+1 >= maxValue){
            index=0;
            stopFlag=true;
         }  
         if(stopFlag==false){
            document.getElementById(targetId).value++;
         } 
         index++;
      }, 100);
      }, 500);      
   imposeMinMax(document.getElementById(targetId));
   }
   if (inId.charAt(0) == 'm') {
      var targetId = inId.charAt(2);
      var minValue = Number(document.getElementById(targetId).min);
      var actValue = Number(document.getElementById(targetId).value);
      index = actValue;
      if(actValue > minValue){
         stopFlag=false;
         document.getElementById(targetId).value--;
      }else{
         stopFlag=true;
      }
      timeout = setTimeout(function(){
         interval = setInterval(function(){        
            if(index-1 <= minValue){
               index=0;
               stopFlag=true;
            }  
            if(stopFlag==false){
               document.getElementById(targetId).value--;
            } 
            index--;
         }, 100);
         }, 500);      
      imposeMinMax(document.getElementById(targetId));
   }
}

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Button example</title>
  <meta charset="utf-8">
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">
</head>
<body>

  <button type='button' class='btn btn-danger btn-sm ' id='mbA' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>-</button>
  <input type='number' id='A'  onchange='imposeMinMax(this)' value='200' max='350' min='150' step='1' style='width: 50px;'>                 
  <button type='button' class='btn btn-danger btn-sm ' id='pbA' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>+</button>&nbsp;

  <button type='button' class='btn btn-danger btn-sm signBut' id='mbB' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>-</button>
  <input type='number'  id='B'  onchange='imposeMinMax(this)' value='250' max='450' min='150' step='1' style='width: 50px;'>                 
  <button type='button' class='btn btn-danger btn-sm ' id='pbB' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>+</button>&ensp;

  <button type='button' class='btn btn-danger btn-sm signBut' id='mbC' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>-</button>
  <input type='number'  id='C'  onchange='imposeMinMax(this)' value='3' max='10' min='1' step='1' style='width: 50px;'>                 
  <button type='button' class='btn btn-danger btn-sm ' id='pbC' onmousedown='modIn(this)' onmouseup='clearAll()' onmouseleave='clearAll()'>+</button>

</body>



</html>

How to make buttons that increment and decrement a value when clicked?

7 Answers7