chrome.scripting.executeScript injected function can't call an outside function

I am writing a Chrome Extension and I have this page:

<html>
  <body>
    <button id="changeColor"></button>
    <script src="popup.js"></script>
  </body>
</html>

With this JS (popup.js):

let changeColor = document.getElementById("changeColor");

chrome.storage.sync.get("color", ({ color }) => {
  changeColor.style.backgroundColor = color;
});

changeColor.addEventListener("click", async () => {
    let [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
  
    chrome.scripting.executeScript({
      target: { tabId: tab.id },
      function: setPageBackgroundColor,
    });

});
  
function setPageBackgroundColor() {
  chrome.storage.sync.get("color", ({ color }) => {
    document.body.style.backgroundColor = color;
  });
  // Here, it says: Uncaught ReferenceError: getElementByXpath is not defined
  console.log(getElementByXpath("xpath").textContent);
}
  
function getElementByXpath(path) {
  return document.evaluate(path, document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
}

Why?

Solution

Solution: put all necessary code and functions inside the function you inject.

In your case getElementByXpath definition should be moved inside setPageBackgroundColor.

The reason is that executeScript injects the function as TEXT:

it takes the function's code as plain text IIFE i.e. (function foo() { ... })(),
it transfers the text to the web page,
it selects the ISOLATED world environment where all content scripts of your extension run by default or the unsafe MAIN world where all page scripts run if you specified it explicitly,
it executes that text as JavaScript code.

The injected function can also use the existing global variables/functions created in the same world by files and functions injected via executeScript/registerContentScript or manifest.json's content_scripts that already ran in this page.