Commits on Aug 6, 2024

1. doc: broken PerformanceObserver code sample …

   The code sample at the top of the "Performance measurements API"
   section of the docs does not run.
   
   The code in question:
   
   ```js
   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:
   
   ```js
   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:
   
   ```sh
   $ 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:
   
   ```js
   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');
   })()
   ```
   
   ```sh
   $ node performance-test.js
   17.761083
   5002.468417
   ```

   

   domharrington committed Aug 6, 2024
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