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Inspector

Stability: 2 - Stable

The node:inspector module provides an API for interacting with the V8 inspector.

It can be accessed using:

import * as inspector from 'node:inspector/promises';
const inspector = require('node:inspector/promises');

or

import * as inspector from 'node:inspector';
const inspector = require('node:inspector');

Promises API

Stability: 1 - Experimental

Class: inspector.Session

  • Extends: {EventEmitter}

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

new inspector.Session()

Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend.

When using Session, the object outputted by the console API will not be released, unless we performed manually Runtime.DiscardConsoleEntries command.

Event: 'inspectorNotification'

  • {Object} The notification message object

Emitted when any notification from the V8 Inspector is received.

session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed

Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

It is also possible to subscribe only to notifications with specific method:

Event: <inspector-protocol-method>;

  • {Object} The notification message object

Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value.

The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

session.on('Debugger.paused', ({ params }) => {
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]

Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

session.connect()

Connects a session to the inspector back-end.

session.connectToMainThread()

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

session.disconnect()

Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

session.post(method[, params])

  • method {string}
  • params {Object}
  • Returns: {Promise}

Posts a message to the inspector back-end.

import { Session } from 'node:inspector/promises';
try {
  const session = new Session();
  session.connect();
  const result = await session.post('Runtime.evaluate', { expression: '2 + 2' });
  console.log(result);
} catch (error) {
  console.error(error);
}
// Output: { result: { type: 'number', value: 4, description: '4' } }

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

Example usage

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

CPU profiler

Here's an example showing how to use the CPU Profiler:

import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();
session.connect();

await session.post('Profiler.enable');
await session.post('Profiler.start');
// Invoke business logic under measurement here...

// some time later...
const { profile } = await session.post('Profiler.stop');

// Write profile to disk, upload, etc.
fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
Heap profiler

Here's an example showing how to use the Heap Profiler:

import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();

const fd = fs.openSync('profile.heapsnapshot', 'w');

session.connect();

session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
  fs.writeSync(fd, m.params.chunk);
});

const result = await session.post('HeapProfiler.takeHeapSnapshot', null);
console.log('HeapProfiler.takeHeapSnapshot done:', result);
session.disconnect();
fs.closeSync(fd);

Callback API

Class: inspector.Session

  • Extends: {EventEmitter}

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

new inspector.Session()

Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend.

When using Session, the object outputted by the console API will not be released, unless we performed manually Runtime.DiscardConsoleEntries command.

Event: 'inspectorNotification'

  • {Object} The notification message object

Emitted when any notification from the V8 Inspector is received.

session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed

Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

It is also possible to subscribe only to notifications with specific method:

Event: <inspector-protocol-method>;

  • {Object} The notification message object

Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value.

The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

session.on('Debugger.paused', ({ params }) => {
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]

Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

session.connect()

Connects a session to the inspector back-end.

session.connectToMainThread()

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

session.disconnect()

Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

session.post(method[, params][, callback])

  • method {string}
  • params {Object}
  • callback {Function}

Posts a message to the inspector back-end. callback will be notified when a response is received. callback is a function that accepts two optional arguments: error and message-specific result.

session.post('Runtime.evaluate', { expression: '2 + 2' },
             (error, { result }) => console.log(result));
// Output: { type: 'number', value: 4, description: '4' }

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

You can not set reportProgress to true when sending a HeapProfiler.takeHeapSnapshot or HeapProfiler.stopTrackingHeapObjects command to V8.

Example usage

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

CPU profiler

Here's an example showing how to use the CPU Profiler:

const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();
session.connect();

session.post('Profiler.enable', () => {
  session.post('Profiler.start', () => {
    // Invoke business logic under measurement here...

    // some time later...
    session.post('Profiler.stop', (err, { profile }) => {
      // Write profile to disk, upload, etc.
      if (!err) {
        fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
      }
    });
  });
});
Heap profiler

Here's an example showing how to use the Heap Profiler:

const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();

const fd = fs.openSync('profile.heapsnapshot', 'w');

session.connect();

session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
  fs.writeSync(fd, m.params.chunk);
});

session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => {
  console.log('HeapProfiler.takeHeapSnapshot done:', err, r);
  session.disconnect();
  fs.closeSync(fd);
});

Common Objects

inspector.close()

Attempts to close all remaining connections, blocking the event loop until all are closed. Once all connections are closed, deactivates the inspector.

inspector.console

  • {Object} An object to send messages to the remote inspector console.
require('node:inspector').console.log('a message');

The inspector console does not have API parity with Node.js console.

inspector.open([port[, host[, wait]]])

  • port {number} Port to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • host {string} Host to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • wait {boolean} Block until a client has connected. Optional. Default: false.
  • Returns: {Disposable} A Disposable that calls inspector.close().

Activate inspector on host and port. Equivalent to node --inspect=[[host:]port], but can be done programmatically after node has started.

If wait is true, will block until a client has connected to the inspect port and flow control has been passed to the debugger client.

See the security warning regarding the host parameter usage.

inspector.url()

  • Returns: {string|undefined}

Return the URL of the active inspector, or undefined if there is none.

$ node --inspect -p 'inspector.url()'
Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
For help, see: https://nodejs.org/en/docs/inspector
ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34

$ node --inspect=localhost:3000 -p 'inspector.url()'
Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
For help, see: https://nodejs.org/en/docs/inspector
ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a

$ node -p 'inspector.url()'
undefined

inspector.waitForDebugger()

Blocks until a client (existing or connected later) has sent Runtime.runIfWaitingForDebugger command.

An exception will be thrown if there is no active inspector.

Integration with DevTools

The node:inspector module provides an API for integrating with devtools that support Chrome DevTools Protocol. DevTools frontends connected to a running Node.js instance can capture protocol events emitted from the instance and display them accordingly to facilitate debugging. The following methods broadcast a protocol event to all connected frontends. The params passed to the methods can be optional, depending on the protocol.

// The `Network.requestWillBeSent` event will be fired.
inspector.Network.requestWillBeSent({
  requestId: 'request-id-1',
  timestamp: Date.now() / 1000,
  wallTime: Date.now(),
  request: {
    url: 'https://nodejs.org/en',
    method: 'GET',
  }
});

inspector.Network.requestWillBeSent([params])

Stability: 1 - Experimental

  • params {Object}

This feature is only available with the --experimental-network-inspection flag enabled.

Broadcasts the Network.requestWillBeSent event to connected frontends. This event indicates that the application is about to send an HTTP request.

inspector.Network.responseReceived([params])

Stability: 1 - Experimental

  • params {Object}

This feature is only available with the --experimental-network-inspection flag enabled.

Broadcasts the Network.responseReceived event to connected frontends. This event indicates that HTTP response is available.

inspector.Network.loadingFinished([params])

Stability: 1 - Experimental

  • params {Object}

This feature is only available with the --experimental-network-inspection flag enabled.

Broadcasts the Network.loadingFinished event to connected frontends. This event indicates that HTTP request has finished loading.

inspector.Network.loadingFailed([params])

Stability: 1 - Experimental

  • params {Object}

This feature is only available with the --experimental-network-inspection flag enabled.

Broadcasts the Network.loadingFailed event to connected frontends. This event indicates that HTTP request has failed to load.

Support of breakpoints

The Chrome DevTools Protocol Debugger domain allows an inspector.Session to attach to a program and set breakpoints to step through the codes.

However, setting breakpoints with a same-thread inspector.Session, which is connected by session.connect(), should be avoided as the program being attached and paused is exactly the debugger itself. Instead, try connect to the main thread by session.connectToMainThread() and set breakpoints in a worker thread, or connect with a Debugger program over WebSocket connection.