Code Quality Rank: L2
Programming language: JavaScript
License: MIT License
Latest version: v4.10.2

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🔥 single-command flamegraph profiling 🔥

Discover the bottlenecks and hot paths in your code, with flamegraphs.

Visualize Stack Traces

0x can profile and generate an interactive flamegraph for a Node process with a single command, on any platform which Node runs on (macOs, Linux, Windows, Android...).


  • Node v8.5.0 and above
  • Default usage supports any Operating System that Node runs on!
  • Chrome
    • Other browsers may open flamegraphs in a degraded, but functional form


Older versions of Node are supported via previous 0x versions:

0x Node macOS/SmartOS Linux Windows
v4 v8.5.0+ ☑️ ☑️ ☑️
v3 v6 – v8.4.0 ☑️ ☑️
v2 v4 ☑️ ☑️


An example interactive flamegraph can be viewed at http://davidmarkclements.github.io/0x-demo/


npm install -g 0x


Use 0x to run a script:

0x my-app.js

Immediately open the flamegraph in the browser:

0x -o my-app.js

Automatically execute profiling command against the first port opened by profiled process:

0x -P 'autocannon localhost:$PORT' my-app.js

Use a custom node executable:

0x -- /path/to/node my-app.js

Pass custom arguments to node:

0x -- node --zero-fill-buffers my-app.js


When ready to generate a flamegraph, send a SIGINT.

The simplest way to do this is pressing CTRL+C.

When 0x catches the SIGINT, it process the stacks and generates a profile folder (<pid>.0x), containing flamegraph.html.

The UI

The flamegraph.html file contains the 0x UI, which is explained in [docs/ui.md](docs/ui.md).

Production Servers

A lightweight, production server friendly, approach to generating a flamegraph is described in [docs/production-servers.md](docs/production-servers.md).

The Profile Folder

By default, a Profile Folder will be created and named after the PID, e.g. 3866.0x (we can set this name manually using the --output-dir flag).

The Profile Folder is explained in more detail in [docs/profile-folder.md](docs/profile-folder.md)


Clone this repo, run npm i -g and from the repo root run

0x examples/rest-api

In another tab run

npm run stress-rest-example

To put some load on the rest server, once that's done use ctrl + c to kill the server.

Command Line API

--help | -h

Print usage info.

--open | -o

Open the flamegraph in the browser using open or xdg-open (see https://www.npmjs.com/package/open for details).

--on-port | -P

Run a given command and then generate the flamegraph. The command as specified has access to a $PORT variable. The $PORT variable is set according to the first port that profiled process opens.

For instance, here's an example of using autocannon to load-test the process:

0x -P 'autocannon localhost:$PORT' app.js

When the load-test completes, the profiled processed will be sent a SIGINT and the flamegraph will be automatically generated.

Remember to use single quotes to avoid bash interpolation, or else escape variable (e.g. 0x -P "autocannon localhost:$PORT" app.js won't work wheras 0x -P "autocannon localhost:\$PORT" app.js will).

Note: On Windows interpolation usually occurs with %PORT%, however in this case the dollar-prefix $PORT is the correct syntax (because the interpolation is not shell based).

Default: ''


The name of the HTML file, without the .html extension Can be set to - to write HTML to STDOUT (note due to the nature of CLI argument parsing, this must be set using =, e.g. --name=-).

If either this flag or --output-html-file is set to - then the HTML will go to STDOUT.

Default: flamegraph


Set the title to display in the flamegraph UI.

Default: the command that 0x ran to start the process

--output-dir | -D

Specify artifact output directory. This can be specified in template form with possible variables being {pid}, {timestamp}, {name} (based on the --name flag) and {outputDir}(variables must be specified without whitespace, e.g. { pid } is not supported).

Default: {pid}.0x

--output-html | -F

Specify destination of the generated flamegraph HTML file. This can be specified in template form with possible variables being {pid}, {timestamp}, {name} (based on the --name flag) and {outputDir} (variables must be specified without whitespace, e.g. { pid } is not supported). It can also be set to - to send the HTML output to STDOUT (note due to the nature of CLI argument parsing, this must be set using =, e.g. --output-html=-).

If either this flag or --name is set to - then the HTML will go to STDOUT.

Default: {outputDir}/{name}.html


Use an OS kernel tracing tool (perf on Linux or dtrace on macOS and SmartOS). This will capture native stack frames (C++ modules and Libuv I/O), but may result in missing stacks on Node 8.

See [docs/kernel-tracing.md](docs/kernel-tracing.md) for more information.

Default: false

--quiet | -q

Limit output, the only output will be fatal errors or the path to the flamegraph.html upon successful generation.

Default: false

--silent | -s

Suppress all output, except fatal errors.

Default: false


Don't generate the flamegraph, only create the Profile Folder, with relevant outputs.

Default: false


Supply a path to a profile folder to build or rebuild visualization from original stacks.

Default: undefined


Supply a path to a CPU profile. See examples/cpu-profile for an example.

CPU Profile output does not have as much information but it can be exported from Chrome Devtools in the browser. There's also an automated headless tool for doing so: automated-chrome-profiling. For creating Node.js Cpu Profiles in Node see v8-profiler or v8-profiler-next.

Default: undefined


Show output from DTrace or perf(1) tools.

Default: false


Save the intermediate tree representation of captured trace output to a JSON file.

Default: false

Programmatic API

0x can also be required as a Node module and scripted:

const zeroEks = require('0x')
const path = require('path')

async function capture () {
  const opts = {
    argv: [path.join(__dirname, 'my-app.js'), '--my-flag', '"value for my flag"'],
    workingDir: __dirname
  try {
    const file = await zeroEks(opts)
    console.log(`flamegraph in ${file}`)
  } catch (e) {


The Programmatic API is detailed in [docs/api.md](docs/api.md).


Memory Issues

Very complex applications with lots of stacks may hit memory issues.

The --stack-size flag can be used to set the memory to the maximum 8GB in order to work around this when profiling:

node --stack-size=8024 $(which 0x) my-app.js

There may still be a problem opening the flamegraph in Chrome. The same work around can be used by opening Chrome from the command line (platform dependent) and nesting the --stack-size flag within the --js-flags flag: --js-flags="--stack-size 8024".


DEBUG=0x* 0x my-app.js



Sponsored by nearForm

This tool is inspired from various info and code sources and would have taken much longer without the following people and their Open Source/Info Sharing efforts:



*Note that all licence references and agreements mentioned in the 0x README section above are relevant to that project's source code only.