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A story about JavaScript error detection in browser

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Hello! Thank you for reading! My name is Nakamoto and I develop the front end of KINTO FACTORY ('FACTORY' in this article), a service that allows you to upgrade your current car.

In this article, I would like to introduce a method of how to detect errors that occur in clients such as browsers using AWS CloudWatch RUM.

Getting Started

The reason why we introduced it was due to an enquiry we received by our Customer Center (CC), where a user tried to order products from the FACTORY website, only to encounter an error where the screen did not transition. This prompted an investigation request. I immediately parsed the API log and checked if there were any errors, but I could not find anything that would lead to an error.

Next, I checked what kind of model and browser was being used to access the front end. When examining the access logs from Cloud Front, I looked into the access of the relevant user and checked the User-Agent where I could see:

Android 10; Chrome/80.0.3987.149

It was accessed from a relatively old Android device.

With that in mind, while analyzing the source of the page where the problem occurred, a front end development team member advised that replaceAll in JavaScript might be the culprit... This function requires compatibility with Chrome version 85 or higher... (Since FACTORY recommends using the latest version of each browser, we hadn't tested cases with old versions such as this case in QA.)

*Other members of the team also told me that you can easily search for functions here to see which browsers and versions are supported!

Until now, monitoring in FACTORY has detected errors in the BFF layer and notified PagerDuty and Slack, but it has not been possible to detect errors in the client-side, so it was the first time we noticed them through communication from customers. If we continued as-is, we would not be able to notice such errors on the client side unless we received customer feedback, so we decided to take countermeasures.

Detection Method

Originally, FACTORY's frontend had been loading client.js from AWS's CloudWatch RUM (Real-time User Monitoring). However, this function was not being used for anything in particular (user journeys, etc. are analyzed separately with Google Analytics), so it was a bit of a waste.

As I investigated, I learned that RUM allows JavaScript to send events to CloudWatch on a client such as a browser. So using this mechanism, I decided to create a system to send and detect custom events when some kind of error occurs.

Notification Method

The general flow of notifications are as follows:

  1. When an error is detected in the browser, CloudWatch RUM sends a custom event with the error description in the message
window.crm("recordEvent", {
  type: "error_handle_event",
  data: {
    /* Information required for analysis. The contents of the exception error */
  1. Cloud Watch Alerm detects the above events and sends the error details via SNS when the event occurs
  2. The above SNS notifies SQS, Lambda picks up the message and notifies the error to OpenSearch (this mechanism uses the existing API error detection and notification mechanism)

After Implementation

After implementing this mechanism in the production environment and operating it for several months, I can luckily say that critical issues, such as the JavaScript error that resulted in its introduction, have not occurred.

However, I have been able to detect cases where errors occur due to unintended access from search engine crawlers and bots, and I have become aware of accesses that I did not pay particular attention to until I introduced it, so it became a reminder of the importance of monitoring and being vigilant.


In order to enable the best online purchase experiences on websites such as FACTORY, it's very important to prevent as many errors as possible (such as problems when buying items, viewing pages, etc.). However, there is unfortunately a limit as to how much we can guarantee that it works on all customers' devices and browsers. That is why, if an error occurs, it is necessary to show easy to understand messages for the customers (with what they should do next), and a mechanism in place for us, the developers on the operation side, so that we can quickly identify the occurrence and details of the problem.

I would like to continue using different tools and mechanisms to ensure stable website operation.


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【PdM(KINTO FACTORY)】プロジェクト推進G/東京

KINTO FACTORYについて自動車のソフトウェア、ハードウェア両面でのアップグレードを行う新サービスです。トヨタ・レクサスの車をお持ちのお客様にOTAやハードウェアアップデートを通してリフォーム、アップグレード、パーソナライズなどを提供し購入後にも進化続ける自動車を提供するモビリティ業界における先端のサービスの開発となります。

【KINTO FACTORYバックエンドエンジニア】プロジェクト推進G/大阪

KINTO FACTORYについて自動車のソフトウェア、ハードウェア両面でのアップグレードを行う新サービスです。トヨタ・レクサスの車をお持ちのお客様にOTAやハードウェアアップデートを通してリフォーム、アップグレード、パーソナライズなどを提供し購入後にも進化続ける自動車を提供するモビリティ業界における先端のサービスの開発となります。