Get the js script from a rest api hit and execute it on browser

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I need to get user details behind the scene when he is accessing our browser based application. So a plug and play JavaScript component need to be created which can be easily embedded in the header of the web page. The logic of capturing the data points should sit on server side. So an API can be invoked from this component and then "User-Agent" header can be leveraged in that API to get the data points.

However, not all data points can be Identified from User-Agent such as language, screen resolution, color depth, etc. Capturing these attributes need an execution of JavaScript on client side.

Two approaches:

  1. With Redirect(If allowed and component is an iFrame so that parent page is not reloaded)

When the component loads initially, redirect it to another page which will contain the JavaScript that will capture these details and it will be auto submitted and sent to the API which will use the data points and then redirect to the component with a flag to not perform the capture activity again.

  1. Without redirect

So, The approach am thinking is to have a component in which, during onLoad event, an API will be invoked and the JavaScript code (To capture the data points from client) will be returned as response. Then this returned JavaScript will be executed on client and result will be saved in a hidden field. There will be an Allow button(part of requirement to take user consent) in this component, which when clicked, the data from the hidden field will be submitted to the API.

Please review the approach and let me know if it sounds good and viable.

rest html ajax javascript
1 Answers

I bet that the second approach would be better. And even better, you don't actually need to write a dedicated API to generate the JS file s required for tracking.

Now, write a simple JavaScript file which will collect the required data. I'll name this as track.js.

function collectAndSubmit(){
  var data = {};
  
  // Then collect the required data
  data.screenWidth = window.innerWidth;
  data.screenHeight = window.innerHeight;
  data.preferredLanguages = navigator.languages; // In the form of an array

  // Submit the data through XHR
  var xhr = new XMLHttpRequest();
  xhr.open("POST", "https://example.com/track.php");
  xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8"); // Note that we're sending a JSON request
  xhr.send(JSON.stringify(data));
}

The last 6 lines of code will send all of the collected data in JSON format to a server. However, I don't know which server are you using (e.g. PHP, Node.js) since they have different ways to parse the JSON data. For example, PHP you can use $data = json_decode(file_get_contents('php://input')); to obtain the requested data or var data = req.body; if you're using Node.js with Express and the body-parser module.

The best part here is that track.js doesn't have to be generated by an api, e.g. by performing a GET request to /gettrackingscript. Instead, you can serve this as a static file, putting it on your CDN and so on.

And now, what is the best way to download this track.js file? Well, it depends on how would you use it. You can simply include a <script src="track.js"> tag when the page loads and call the collectAndSubmit() method when you would like to send the data. Or by getting the script through another XMLHttpRequest and execute them with eval().

Since you're planning to redirect the user after all of the data has been sent, you don't have to refresh the page unless if you're sticking to the MVC (mode-view-controller) pattern when building the site. After the xhr.send(), you can set your backend server to send additional data back to the browser by sending a HTTP response, which you can parse it here by using:

xhr.onreadystatechange = function() {
  if (this.readyState === XMLHttpRequest.DONE && this.status === 200) {
    // Assuming that you're using JSON
    var additionalData = JSON.parse(xhr.responseText);
  }
}
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