This content originally appeared on web.dev and was authored by Eric Bidelman
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.
ReportingObserver has only shipped in Chrome 69. It is being considered by
other browsers.
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.
Key Term:
ReportingObserver is part of a larger spec, the Reporting
API, which provides a common way to send
these different reports to a back end. The Reporting API is a generic framework
to specify a set of server endpoints to report issues to.
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:
Hero image by Sieuwert Otterloo on Unsplash.
This content originally appeared on web.dev and was authored by Eric Bidelman
Eric Bidelman | Sciencx (2019-08-21T00:00:00+00:00) Know your code health with the `ReportingObserver` API. Retrieved from https://www.scien.cx/2019/08/21/know-your-code-health-with-the-reportingobserver-api/
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