Know your code health with the ReportingObserver API

ReportingObserver lets you know when your site uses a deprecated API or runs into a browser intervention. The basic functionality originally landed in Chrome 69. As of Chrome 84, it can be used in workers.

const observer = new ReportingObserver((reports, observer) => {
  for (const report of reports) {
    console.log(report.type, report.url, report.body);
  }
}, {buffered: true});

observer.observe();

Use the callback to send reports to a backend or analytics provider for analysis.

Why is that useful? Until this API, deprecation and intervention warnings were only available in DevTools as Console messages. Interventions, in particular, are only triggered by various real-world constraints like device and network conditions. Thus, you may never even see these messages when developing/testing a site locally. ReportingObserver provides a solution to this problem. When users experience potential issues in the wild, web developers can be notified about them.

# Background

A while back, I wrote a blog post (Observing your web app) because I found it fascinating how many APIs there are for monitoring the "stuff" that happens in a web app. For example, there are APIs that can observe information about the DOM: ResizeObserver, IntersectionObserver, MutationObserver. PerformanceObserver captures performance measurements. And methods like window.onerror and window.onunhandledrejection even let us know when something goes wrong.

However, there are other types of warnings which are not captured by the existing APIs. When your site uses a deprecated API or runs up against a browser intervention, DevTools is the first to tell you about them:

One would naturally think window.onerror captures these warnings. It does not. That's because window.onerror does not fire for warnings generated directly by the user agent itself. It fires for runtime errors (JavaScript exceptions and syntax errors) caused by code execution.

ReportingObserver picks up the slack. It provides a programmatic way to be notified about browser-issued warnings such as deprecations and interventions. You can use it as a reporting tool and lose less sleep wondering if users are hitting unexpected issues on your live site.

# The API

ReportingObserver is not unlike the other Observer APIs such as IntersectionObserver and ResizeObserver. You give it a callback; it gives you information. The information that the callback receives is a list of issues that the page caused:

const observer = new ReportingObserver((reports, observer) => {
  for (const report of reports) {
    // → report.type === 'deprecation'
    // → report.url === 'https://reporting-observer-api-demo.glitch.me'
    // → report.body.id === 'XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload'
    // → report.body.message === 'Synchronous XMLHttpRequest is deprecated...'
    // → report.body.lineNumber === 11
    // → report.body.columnNumber === 22
    // → report.body.sourceFile === 'https://reporting-observer-api-demo.glitch.me'
    // → report.body.anticipatedRemoval === <JS_DATE_STR> or null
  }
});

observer.observe();

# Filtered reports

Reports can be pre-filtered to only observe certain report types. Right now, there are two report types: 'deprecation' and 'intervention'.

const observer = new ReportingObserver((reports, observer) => {
  …
}, {types: ['deprecation']});

# Buffered reports

Use the buffered: true option when you want to see the reports that were generated before the observer instance was created:

const observer = new ReportingObserver((reports, observer) => {
  …
}, {types: ['intervention'], buffered: true});

This option is great for situations like lazy-loading a library that uses a ReportingObserver. The observer gets added late, but you don't miss out on anything that happened earlier in the page load.

# Stop observing

Stop observing using the disconnect() method:

observer.disconnect();

# Examples

# Report browser interventions to an analytics provider

const observer = new ReportingObserver((reports, observer) => {
  for (const report of reports) {
    sendReportToAnalytics(JSON.stringify(report.body));
  }
}, {types: ['intervention'], buffered: true});

observer.observe();

# Be notified when APIs are going to be removed

const observer = new ReportingObserver((reports, observer) => {
  for (const report of reports) {
    if (report.type === 'deprecation') {
      sendToBackend(`Using a deprecated API in ${report.body.sourceFile} which will be
                     removed on ${report.body.anticipatedRemoval}. Info: ${report.body.message}`);
    }
  }
});

observer.observe();

# Conclusion

ReportingObserver gives you an additional way for discovering and monitoring potential issues in your web app. It's even a useful tool for understanding the health of your code base (or lack thereof). Send reports to a backend, know about the real-world issues, update code, profit!

# Future work

In the future, my hope is that ReportingObserver becomes the de facto API for catching all types of issues in JavaScript. Imagine one API to catch everything that goes wrong in your app:

• Browser interventions
• Deprecations
• Feature Policy violations. See Chromium Issue #867471.
• JS exceptions and errors (currently serviced by window.onerror).
• Unhandled JS promise rejections (currently serviced by window.onunhandledrejection)

Additional resources:

• W3c spec
• chromestatus.com entry

Hero image by Sieuwert Otterloo on Unsplash.