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var _0x = require("0x")

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0x v4.7.2

🔥 single-command flamegraph profiling 🔥



🔥 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:

v3v6 – v8.4.0☑️☑️


An example interactive flamegraph can be viewed at


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/

Production Servers

A lightweight, production server friendly, approach to generating a flamegraph is described in docs/

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/


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 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/ 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


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/


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



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