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doc: broken `PerformanceObserver` code sample

Rodrigo Muino Tomonari requested to merge github/fork/domharrington/patch-1 into main

The code sample at the top of the "Performance measurements API" section of the docs does not run.

The code in question:

const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});

If you replace doSomeLongRunningProcess with an IIFE with a sleep() at the top of it, you get this:

const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');

(async function doSomeLongRunningProcess() {
  await new Promise(r => setTimeout(r, 5000));
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
})()

When you run this, you get the following output:

$ node performance-test.js
17.873416

node:internal/per_context/domexception:53
    ErrorCaptureStackTrace(this);
    ^
DOMException [SyntaxError]: The "A" performance mark has not been set
    at new DOMException (node:internal/per_context/domexception:53:5)
    at __node_internal_ (node:internal/util:695:10)
    at getMark (node:internal/perf/usertiming:65:11)
    at calculateStartDuration (node:internal/perf/usertiming:202:13)
    at measure (node:internal/perf/usertiming:220:7)
    at Performance.measure (node:internal/perf/performance:135:12)
    at /private/tmp/performance-test.js:14:15

Node.js v20.11.1

I believe it's due to the call to performance.clearMarks(); in the PerformanceObserver callback. If you remove that, it works as expected:

const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');

(async function doSomeLongRunningProcess() {
  await new Promise(r => setTimeout(r, 5000));
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
})()
$ node performance-test.js
17.761083
5002.468417

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