Inspector | Node.js v8.9.2 Documentation

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# Inspector#

Stability: 1 - Experimental

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

It can be accessed using:

    const inspector = require('inspector');

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

- port <number> Port to listen on for inspector connections. Optional, defaults to what was specified on the CLI.
- host <string> Host to listen on for inspector connections. Optional, defaults to what was specified on the CLI.
- wait <boolean> Block until a client has connected. Optional, defaults to false.

Activate inspector on host and port. Equivalent to `node --inspect=[[host:]port]`, but can be done programatically 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.

### inspector.close()#

Deactivate the inspector. Blocks until there are no active connections.

### inspector.url()#

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

## Class: inspector.Session#

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

### Constructor: 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.

`inspector.Session` is an `EventEmitter` with the following events:

### Event: 'inspectorNotification'#

Added in: v8.0.0

- <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

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

### Event: <inspector-protocol-method>#

Added in: v8.0.0

- <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' ]

### session.connect()#

Added in: v8.0.0

Connects a session to the inspector back-end. An exception will be thrown if there is already a connected session established either through the API or by a front-end connected to the Inspector WebSocket port.

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

Added in: v8.0.0

- 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 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.

### 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.