How to fix 'Cannot read property 'value' of null' with an input in Javascript

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I want to show the value of an in my console. But when I try to console.log the value, it says that value of 'null' can't be read

I have tried making this in my HTML file with but it doesn't work too

Here is my code :

var grayscale = document.getElementById('grayscale');
console.log(grayscale.value)

grayscale.onchange = function(){
    console.log(grayscale.value)
}
<input type='range' id='grayscale' min='0' max='100'>

Use window.onload and call a function, which performs all the tasks you want. It will wait till window loads and then your function will be executed. Right now it is getting executed before the html is getting rendered, means it is accessing the value of the slider before even it is loaded. So you get the error.

window.onload=function(){var grayscale = document.getElementById('grayscale');
console.log(grayscale.value)

grayscale.onchange = function(){
    console.log(grayscale.value)
}
}
<input type='range' id='grayscale' min='0' max='100'>

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Your JavaScript is running before the DOM is ready. Try wrapping it in an event listener like:

document.addEventListener("DOMContentLoaded", function(event) { 
  var grayscale = document.getElementById('grayscale');
  console.log(grayscale.value)

  grayscale.onchange = function(){
    console.log(grayscale.value)
  }
}

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Put the Javascript after the element. If grayscale is null, it is not part of the DOM yet. Put the Javascript file or snippet in the body section at the very end after your element.

<html>
  ....
  <body>
     ....
     <script type="text/javascript" src="theJs.js"></script>
  </body>
</html>

Or you can use embedded javascript:

<input type='range' id='grayscale' min='0' max='100'>
<script>
var grayscale = document.getElementById('grayscale');
console.log(grayscale.value)

grayscale.onchange = function(){
    console.log(grayscale.value)
}
</script>

</body>

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Try this instead:

const grayscale = document.getElementById('grayscale');

grayscale.addEventListener('change', ({target}) => console.log(target.value));

ps: forgot to mention, when launch script, set attribute defer, like below:

<script defer src="./index.js"></script>

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Comments
  • When does your JavaScript run, exactly? Before or after the main HTML document is constructed?
  • This is your answer, Or place at the bottom of your documents, before the closing body tag.