---
title: "V8 Inspector from Node"
description: "Using the inspector module to interact with the V8 inspector from Node.js. Practical notes and code examples on V8 Inspector."
url: "https://h3manth.com/scribe/v8-inspector-from-node/"
canonical_url: "https://h3manth.com/scribe/v8-inspector-from-node/"
markdown_url: "https://h3manth.com/scribe/blog/v8-inspector-from-node.md"
publishDate: "2021-04-13T00:00:00.000Z"
author: "Hemanth HM"
category: "node"
tags: ["node","javascript","debugging","v8"]
word_count: 238
reading_time_minutes: 2
x_markdown_tokens: 615
content_signal: "ai-train=yes, search=yes, ai-input=yes"
---

# V8 Inspector from Node

> Using the inspector module to interact with the V8 inspector from Node.js. Practical notes and code examples on V8 Inspector.

The [inspector](https://nodejs.org/api/inspector.html) module provides an API that makes it easy to interact with the V8 inspector.

## Opening an Inspector

One can easily open an inspector like:

```javascript
const inspector = require("inspector");

inspector.open(); // <- inspector.open([port[, host[, wait]]])
/*
Debugger listening on ws://127.0.0.1:9229/ab01a630-90ad-4c87-91c9-a665632c1d8b
For help see https://nodejs.org/en/docs/inspector
*/

inspector.url();
// 'ws://127.0.0.1:9229/ab01a630-90ad-4c87-91c9-a665632c1d8b'

inspector.console.log("This should be visible on the debugger console");
```

## Waiting for Debugger

Most of the time we would want to wait until the debugger is connected to our process. That's where `inspector.waitForDebugger()` will be useful:

```javascript
// foo.js
(() => {
  const inspector = require("inspector");
  inspector.open();
  inspector.waitForDebugger();
})();
```

```bash
> node foo.js
# This would make the process wait till the debugger is connected.
# Debugger listening on ws://127.0.0.1:9229/b4b0e48d-2e4c-4cde-8177-6924c662aef4
# For help, see: https://nodejs.org/en/docs/inspector
# Debugger attached.
# Waiting for the debugger to disconnect...
```

## Session API

Dispatching and receiving message responses and notifications is possible with the `session` API:

```javascript
const inspector = require("inspector");

inspector.open();
inspector.waitForDebugger();

const session = new inspector.Session();

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

session.post("Runtime.evaluate", { expression: "5 + 4" }, (error, { result }) =>
  console.log(result)
);
// 9
```

`session.on(<inspector-protocol-method>)` where the methods are from [devtools-protocol](https://chromedevtools.github.io/devtools-protocol/v8/).

P.S: This is an experimental API with Stability of 1, meaning it is not subject to Semantic Versioning rules. Non-backward compatible changes or removal may occur in any future release. Use of the feature is not recommended in production environments.