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javascript - document.querySelector always returns null

I've looked at the other answers but still don't understand.

For some reason this line of code always returns null.

var els = document.querySelector("[id='MTG_INSTR$2']");

I checked the value of document in the console so I'm pretty sure that's correct.

The id is buried deep inside a table though, could that be an issue?

EDIT 2: If it helps, here's the full code.

content.js

chrome.runtime.onMessage.addListener(
      function(request, sender, sendResponse) {
        if( request.message === "start" ) {
         start();
             }
      }
    );

    function start(){
        var els = document.querySelector("[id='MTG_INSTR$2']");
        console.log(els);
        alert("started");
    }

popup.html

<!DOCTYPE html>
<html>
<head></head>
<script src="popup.js"></script>
<body>
<input id="button1" type=button value=clickme>
</body></html>

popup.js

 function popup() {
    chrome.tabs.query({currentWindow: true, active: true}, function (tabs){
    var activeTab = tabs[0];
    chrome.tabs.sendMessage(activeTab.id, {"message": "start"});
   });
}

document.addEventListener("DOMContentLoaded", function() {
  document.getElementById("button1").addEventListener("click", popup);
});

manifest.json

{
"content_security_policy": "script-src 'self' 'unsafe-eval'; object-src 'self'",
  "manifest_version": 2,
  "name": "extensiontest",
  "version": "0.2",
  "content_scripts": [
  {
    "matches": [
      "<all_urls>"
    ],
    "js": ["content.js"]
  }
],
"browser_action": {
  "default_icon": "icon.png",
    "default_popup":"popup.html"
},
//"background": {
// "scripts": ["background.js"]
//},
"permissions": [
    "tabs"
  ]
}

EDIT: I've included a screen shot of what the document structure looks like.

enter image description here

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1 Answer

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by (71.8m points)

I doubt your JS gets loaded before the markup and css. In that case, you could try using <script src="" defer></script>

The contents are loaded first and your java script should work fine after that.


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