# Performance Timing API
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> Stability: 1 - Experimental

The Performance Timing API provides an implementation of the
[W3C Performance Timeline][] specification. The purpose of the API
is to support collection of high resolution performance metrics.
This is the same Performance API as implemented in modern Web browsers.

```js
const { performance } = require('perf_hooks');
performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
  const measure = performance.getEntriesByName('A to B')[0];
  console.log(measure.duration);
  // Prints the number of milliseconds between Mark 'A' and Mark 'B'
});
```

## Class: Performance
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The `Performance` provides access to performance metric data. A single
instance of this class is provided via the `performance` property.

### performance.clearFunctions([name])
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* `name` {string}

If `name` is not provided, removes all `PerformanceFunction` objects from the
Performance Timeline. If `name` is provided, removes entries with `name`.

### performance.clearMarks([name])
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* `name` {string}

If `name` is not provided, removes all `PerformanceMark` objects from the
Performance Timeline. If `name` is provided, removes only the named mark.

### performance.clearMeasures([name])
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* `name` {string}

If `name` is not provided, removes all `PerformanceMeasure` objects from the
Performance Timeline. If `name` is provided, removes only objects whose
`performanceEntry.name` matches `name`.

### performance.getEntries()
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* Returns: {Array}

Returns a list of all `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime`.

### performance.getEntriesByName(name[, type])
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* `name` {string}
* `type` {string}
* Returns: {Array}

Returns a list of all `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime` whose `performanceEntry.name` is
equal to `name`, and optionally, whose `performanceEntry.entryType` is equal to
`type`.

### performance.getEntriesByType(type)
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* `type` {string}
* Returns: {Array}

Returns a list of all `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime` whose `performanceEntry.entryType`
is equal to `type`.

### performance.mark([name])
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* `name` {string}

Creates a new `PerformanceMark` entry in the Performance Timeline. A
`PerformanceMark` is a subclass of `PerformanceEntry` whose
`performanceEntry.entryType` is always `'mark'`, and whose
`performanceEntry.duration` is always `0`. Performance marks are used
to mark specific significant moments in the Performance Timeline.

### performance.measure(name, startMark, endMark)
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* `name` {string}
* `startMark` {string}
* `endMark` {string}

Creates a new `PerformanceMeasure` entry in the Performance Timeline. A
`PerformanceMeasure` is a subclass of `PerformanceEntry` whose
`performanceEntry.entryType` is always `'measure'`, and whose
`performanceEntry.duration` measures the number of milliseconds elapsed since
`startMark` and `endMark`.

The `startMark` argument may identify any *existing* `PerformanceMark` in the
the Performance Timeline, or *may* identify any of the timestamp properties
provided by the `PerformanceNodeTiming` class. If the named `startMark` does
not exist, then `startMark` is set to [`timeOrigin`][] by default.

The `endMark` argument must identify any *existing* `PerformanceMark` in the
the Performance Timeline or any of the timestamp properties provided by the
`PerformanceNodeTiming` class. If the named `endMark` does not exist, an
error will be thrown.

### performance.nodeTiming
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* {PerformanceNodeTiming}

An instance of the `PerformanceNodeTiming` class that provides performance
metrics for specific Node.js operational milestones.

### performance.now()
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* Returns: {number}

Returns the current high resolution millisecond timestamp.

### performance.timeOrigin
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* {number}

The [`timeOrigin`][] specifies the high resolution millisecond timestamp from
which all performance metric durations are measured.

### performance.timerify(fn)
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* `fn` {Function}

Wraps a function within a new function that measures the running time of the
wrapped function. A `PerformanceObserver` must be subscribed to the `'function'`
event type in order for the timing details to be accessed.

```js
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

function someFunction() {
  console.log('hello world');
}

const wrapped = performance.timerify(someFunction);

const obs = new PerformanceObserver((list) => {
  console.log(list.getEntries()[0].duration);
  obs.disconnect();
  performance.clearFunctions();
});
obs.observe({ entryTypes: ['function'] });

// A performance timeline entry will be created
wrapped();
```

## Class: PerformanceEntry
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### performanceEntry.duration
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* {number}

The total number of milliseconds elapsed for this entry. This value will not
be meaningful for all Performance Entry types.

### performanceEntry.name
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* {string}

The name of the performance entry.

### performanceEntry.startTime
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* {number}

The high resolution millisecond timestamp marking the starting time of the
Performance Entry.

### performanceEntry.entryType
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* {string}

The type of the performance entry. Current it may be one of: `'node'`, `'mark'`,
`'measure'`, `'gc'`, or `'function'`.

### performanceEntry.kind
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* {number}

When `performanceEntry.entryType` is equal to `'gc'`, the `performance.kind`
property identifies the type of garbage collection operation that occurred.
The value may be one of:

* `perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR`
* `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR`
* `perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL`
* `perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB`

## Class: PerformanceNodeTiming extends PerformanceEntry
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Provides timing details for Node.js itself.

### performanceNodeTiming.bootstrapComplete
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* {number}

The high resolution millisecond timestamp at which the Node.js process
completed bootstrap.

### performanceNodeTiming.clusterSetupEnd
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* {number}

The high resolution millisecond timestamp at which cluster processing ended.

### performanceNodeTiming.clusterSetupStart
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* {number}

The high resolution millisecond timestamp at which cluster processing started.

### performanceNodeTiming.loopExit
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* {number}

The high resolution millisecond timestamp at which the Node.js event loop
exited.

### performanceNodeTiming.loopStart
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* {number}

The high resolution millisecond timestamp at which the Node.js event loop
started.

### performanceNodeTiming.moduleLoadEnd
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* {number}

The high resolution millisecond timestamp at which main module load ended.

### performanceNodeTiming.moduleLoadStart
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* {number}

The high resolution millisecond timestamp at which main module load started.

### performanceNodeTiming.nodeStart
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* {number}

The high resolution millisecond timestamp at which the Node.js process was
initialized.

### performanceNodeTiming.preloadModuleLoadEnd
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* {number}

The high resolution millisecond timestamp at which preload module load ended.

### performanceNodeTiming.preloadModuleLoadStart
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* {number}

The high resolution millisecond timestamp at which preload module load started.

### performanceNodeTiming.thirdPartyMainEnd
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* {number}

The high resolution millisecond timestamp at which third_party_main processing
ended.

### performanceNodeTiming.thirdPartyMainStart
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* {number}

The high resolution millisecond timestamp at which third_party_main processing
started.

### performanceNodeTiming.v8Start
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* {number}

The high resolution millisecond timestamp at which the V8 platform was
initialized.


## Class: PerformanceObserver(callback)
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* `callback` {Function} A `PerformanceObserverCallback` callback function.

`PerformanceObserver` objects provide notifications when new
`PerformanceEntry` instances have been added to the Performance Timeline.

```js
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });

performance.mark('test');
```

Because `PerformanceObserver` instances introduce their own additional
performance overhead, instances should not be left subscribed to notifications
indefinitely. Users should disconnect observers as soon as they are no
longer needed.

### Callback: PerformanceObserverCallback(list, observer)
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* `list` {PerformanceObserverEntryList}
* `observer` {PerformanceObserver}

The `PerformanceObserverCallback` is invoked when a `PerformanceObserver` is
notified about new `PerformanceEntry` instances. The callback receives a
`PerformanceObserverEntryList` instance and a reference to the
`PerformanceObserver`.

### Class: PerformanceObserverEntryList
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The `PerformanceObserverEntryList` class is used to provide access to the
`PerformanceEntry` instances passed to a `PerformanceObserver`.

#### performanceObserverEntryList.getEntries()
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* Returns: {Array}

Returns a list of `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime`.

#### performanceObserverEntryList.getEntriesByName(name[, type])
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* `name` {string}
* `type` {string}
* Returns: {Array}

Returns a list of `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime` whose `performanceEntry.name` is
equal to `name`, and optionally, whose `performanceEntry.entryType` is equal to
`type`.

#### performanceObserverEntryList.getEntriesByType(type)
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* `type` {string}
* Returns: {Array}

Returns a list of `PerformanceEntry` objects in chronological order
with respect to `performanceEntry.startTime` whose `performanceEntry.entryType`
is equal to `type`.

### performanceObserver.disconnect()
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Disconnects the `PerformanceObserver` instance from all notifications.

### performanceObserver.observe(options)
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* `options` {Object}
  * `entryTypes` {Array} An array of strings identifying the types of
    `PerformanceEntry` instances the observer is interested in. If not
    provided an error will be thrown.
  * `buffered` {boolean} If true, the notification callback will be
    called using `setImmediate()` and multiple `PerformanceEntry` instance
    notifications will be buffered internally. If `false`, notifications will
    be immediate and synchronous. Defaults to `false`.

Subscribes the `PerformanceObserver` instance to notifications of new
`PerformanceEntry` instances identified by `options.entryTypes`.

When `options.buffered` is `false`, the `callback` will be invoked once for
every `PerformanceEntry` instance:

```js
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // called three times synchronously. list contains one item
});
obs.observe({ entryTypes: ['mark'] });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);
```

```js
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // called once. list contains three items
});
obs.observe({ entryTypes: ['mark'], buffered: true });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);
```

## Examples

### Measuring the duration of async operations

The following example uses the [Async Hooks][] and Performance APIs to measure
the actual duration of a Timeout operation (including the amount of time it
to execute the callback).

```js
'use strict';
const async_hooks = require('async_hooks');
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const set = new Set();
const hook = async_hooks.createHook({
  init(id, type) {
    if (type === 'Timeout') {
      performance.mark(`Timeout-${id}-Init`);
      set.add(id);
    }
  },
  destroy(id) {
    if (set.has(id)) {
      set.delete(id);
      performance.mark(`Timeout-${id}-Destroy`);
      performance.measure(`Timeout-${id}`,
                          `Timeout-${id}-Init`,
                          `Timeout-${id}-Destroy`);
    }
  }
});
hook.enable();

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries()[0]);
  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ entryTypes: ['measure'], buffered: true });

setTimeout(() => {}, 1000);
```

### Measuring how long it takes to load dependencies

The following example measures the duration of `require()` operations to load
dependencies:

<!-- eslint-disable no-global-assign -->
```js
'use strict';
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');
const mod = require('module');

// Monkey patch the require function
mod.Module.prototype.require =
  performance.timerify(mod.Module.prototype.require);
require = performance.timerify(require);

// Activate the observer
const obs = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  entries.forEach((entry) => {
    console.log(`require('${entry[0]}')`, entry.duration);
  });
  obs.disconnect();
  // Free memory
  performance.clearFunctions();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');
```

[`timeOrigin`]: https://w3c.github.io/hr-time/#dom-performance-timeorigin
[Async Hooks]: async_hooks.html
[W3C Performance Timeline]: https://w3c.github.io/performance-timeline/
