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JavaScript JavaScript and the DOM (Retiring) Getting a Handle on the DOM Selecting Elements with the Same Class Name

Posted by emmanuel egunjobi
emmanuel egunjobi
4,475 Points

Color not changing getting error Uncaught TypeError: Cannot read property 'style' of undefined at app.js:5

I am not sure as to why it keeps showing the error I don't see any typos and I have already looked at other questions from community. It wont change to red.

const myList = document.getElementsByTagName('li');
const errorNotPurple = document.getElementsByClassName('error-not-purple');

for ( let i = 0; 0 < myList.length; i++ ) {
  myList[i].style.color = 'purple';
}

for ( let i = 0; 0 < errorNotPurple.length; i++ ) {
  errorNotPurple[i].style.color = 'red';
}

This is the html page 

<!DOCTYPE html>
<html>
  <head>
    <title>JavaScript and the DOM</title>
    <link rel="stylesheet" href="css/style.css">
  </head>
  <body>
    <h1 id="myHeading">JavaScript and the DOM</h1>
    <p>Making a web page interactive</p>
    <p>Things that are purple:</p>
    <ul>
      <li>grapes</li>
      <li class="error-not-purple">oranges</li>
      <li>lavender</li>
       <li class="error-not-purple">fire trucks</li>
       <li class="error-not-purple">snow</li>
      <li>plums</li>
    </ul>
    <script src="app.js"></script>
  </body>
</html>

1 Answer

Jassim Alhatem
Jassim Alhatem
20,883 Points
Jassim Alhatem
Jassim Alhatem
20,883 Points

Hi emmanuel egunjobi, I hope you found the answer. I'm a bit late but I'm going to answer it anyway. In your condition, the 0 Is always going to be less than the length of that element. So the loop is looping more than the length of the element. When the loop goes for more than the length of the element, for example errorNotPurple[3].style. It returns an error (a TypeError) because it's basically this undefined.style. What you should do is this:

const myList = document.getElementsByTagName('li');
const errorNotPurple = document.getElementsByClassName('error-not-purple');


for ( let i = 0; i < myList.length; i++ ) {
  myList[i].style.color = 'purple';
}



for ( let i = 0; i < errorNotPurple.length; i++ ) {
    console.log(errorNotPurple[i]);
    errorNotPurple[i].style.color = 'yellow';
}