Inspector#
Source Code: lib/inspector.js
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#
Added in: v19.0.0
Class: inspector.Session#
The inspector.Session is used for dispatching messages to the V8 inspector
back-end and receiving message responses and notifications.
new inspector.Session()#
Added in: v8.0.0
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'#
Added in: v8.0.0
- Type: <Object> The notification message object
Emitted when any notification from the V8 Inspector is received.
session.on('inspectorNotification', (message) => console.log(message.method));
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>#
Added in: v8.0.0
- Type: <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);
});
Caveat Breakpoints with same-thread session is not recommended, see
support of breakpoints.
session.connect()#
Added in: v8.0.0
Connects a session to the inspector back-end.
session.connectToMainThread()#
Added in: v12.11.0
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()#
Added in: v8.0.0
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])#
Added in: v19.0.0
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);
}
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');
const { profile } = await session.post('Profiler.stop');
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#
The inspector.Session is used for dispatching messages to the V8 inspector
back-end and receiving message responses and notifications.
new inspector.Session()#
Added in: v8.0.0
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'#
Added in: v8.0.0
- Type: <Object> The notification message object
Emitted when any notification from the V8 Inspector is received.
session.on('inspectorNotification', (message) => console.log(message.method));
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>;#
Added in: v8.0.0
- Type: <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);
});
Caveat Breakpoints with same-thread session is not recommended, see
support of breakpoints.
session.connect()#
Added in: v8.0.0
Connects a session to the inspector back-end.
session.connectToMainThread()#
Added in: v12.11.0
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()#
Added in: v8.0.0
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])#
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));
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', () => {
session.post('Profiler.stop', (err, { profile }) => {
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#
- Type: <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()#
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()#
Added in: v12.7.0
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.
inspector.Network.requestWillBeSent({
requestId: 'request-id-1',
timestamp: Date.now() / 1000,
wallTime: Date.now(),
request: {
url: 'https://nodejs.org/en',
method: 'GET',
},
});
inspector.Network.dataReceived([params])#
Added in: v22.17.0
This feature is only available with the --experimental-network-inspection flag enabled.
Broadcasts the Network.dataReceived event to connected frontends, or buffers the data if
Network.streamResourceContent command was not invoked for the given request yet.
Also enables Network.getResponseBody command to retrieve the response data.
inspector.Network.dataSent([params])#
Added in: v22.18.0
This feature is only available with the --experimental-network-inspection flag enabled.
Enables Network.getRequestPostData command to retrieve the request data.
inspector.Network.requestWillBeSent([params])#
Added in: v22.6.0
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])#
Added in: v22.6.0
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])#
Added in: v22.6.0
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])#
Added in: v22.7.0
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.
inspector.NetworkResources.put#
Added in: v22.19.0
This feature is only available with the --experimental-inspector-network-resource flag enabled.
The inspector.NetworkResources.put method is used to provide a response for a loadNetworkResource
request issued via the Chrome DevTools Protocol (CDP).
This is typically triggered when a source map is specified by URL, and a DevTools frontend—such as
Chrome—requests the resource to retrieve the source map.
This method allows developers to predefine the resource content to be served in response to such CDP requests.
const inspector = require('node:inspector');
async function setNetworkResources() {
const mapUrl = 'http://localhost:3000/dist/app.js.map';
const tsUrl = 'http://localhost:3000/src/app.ts';
const distAppJsMap = await fetch(mapUrl).then((res) => res.text());
const srcAppTs = await fetch(tsUrl).then((res) => res.text());
inspector.NetworkResources.put(mapUrl, distAppJsMap);
inspector.NetworkResources.put(tsUrl, srcAppTs);
};
setNetworkResources().then(() => {
require('./dist/app');
});
For more details, see the official CDP documentation: Network.loadNetworkResource
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.