How to clear text field automatically when enter minus value

`

<input type="number" id="qty">
<input type="button" onclick="add()" value="Add">

<script>
  function add(){
    qty = document.getElementById('qty').value
    if(qty>0){
        var tot = qty*25
        do开发者_StackOverflow社区cument.write(tot)
    
    }else{
        qty = ""
        alert("no")
    }
  }
</script>

'qty=""' is the correct way as mentioned on google. but not working for me.

Try

document.getElementById("qty").value = "";

and cast qty to number in your if

if(Number(qty) > 0)

Values of inputs are always string , try this one in your condition :

if(Number(qty)>0){
...
}

Use this below code to clear the input

qty.value = "";

The reason why qty = '' isn't working is that you've assigned the string value of the input to that variable. Setting it to an empty string won't make the value of the input change - you need to explicitly change the element's value.

So, a few things first:

1. It's often useful to cache your elements first.
2. Remove the inline JS and replace it with addEventListener and attach it to the button element (a more modern approach).
3. Use a <button> element instead of an <input type="button">
4. Assign the total to an element's text content/innerText instead of document.write for various reasons.

Then when the handler is called coerce the string value of the input to a number, and use that in the condition. If the number is less than zero assign an empty string to the input value.

// Cache the elements
const qty = document.querySelector('#qty');
const result = document.querySelector('#result');
const button = document.querySelector('button');

// Add a listener to the button
button.addEventListener('click', add);

function add() {

  // Coerce the input value to a number
  const val = Number(qty.value);

  if (val > 0) {

    // Instead of `document.write` assign
    // the total to the text content of an element
    result.textContent = val * 25;

  } else {

    // Set the value of the qty element to
    // an empty string
    qty.value = '';
    console.log('no');
  }
}

#result { margin-top: 0.5em; }

<input type="number" id="qty">
<button type="button">Add</button>
<div id="result"></div>

if you want to check in time when you enter a value (as I understood from the question), you must have a listener on <input type=" number" id="qty"> and event 'input'. qty.addEventListener('input', (e)=>{ // .... make your validation })