Wait for inject.js to execute completely before continuing in popup.js

I have a problem with my chrome extension, the extension reads an object from the page and displays its contents in popup.js.

To read the object, popup executes injects.js which in turn injects script.js.

So what happens is, when inject.js completes execution the popup.js callback is called and renderData(mydata)
is executed. I don't want this to happen until script.js is executed completely

Popup.js

chrome.tabs.executeScript(null, { file: "inject.js" }, function() {
  renderData(mydata);
});

inject.js

var s = document.createElement("script");
s.src = chrome.runtime.getURL("script.js");
(document.head || document.documentElement).appendChild(s);

script.js

 var data = {
    type: "FROM_PAGE",
    text: JSON.stringify(WIO.sessionInfo.context)
  };
  window.postMessage(data, "*");

The problem is that executeScript returns the last synchronously evaluated expression as the callback's parameter but a script with src runs asynchronously. BTW you forgot to specify the parameter.

Use textContent with literal code to run the script synchronously and send a DOM message, which is also synchronous, then leave the result in the last expression:

popup.js

chrome.tabs.executeScript(null, { file: "inject.js" }, ([mydata]) => {
  renderData(mydata);
});

inject.js

var data;
var eventId = 'event' + Math.random();
document.addEventListener(eventId, e => {
  data = JSON.parse(e.detail);
}, {once: true});

var script = document.createElement('script');
script.textContent = `(${eventId => {
  var data = JSON.stringify(WIO.sessionInfo.context);
  document.dispatchEvent(new CustomEvent(eventId, {detail: data}));
}})('${eventId}')`;
document.documentElement.appendChild(script);
script.remove();

data;

Alternatively you can use extension messaging to send the data to the popup (example).