Debugging

This page explains how to debug OCaml programs compiled to JavaScript.

Compiler flags

OCaml flags

Use -g when compiling and linking OCaml bytecode. Js_of_ocaml will attempt to preserve variable names.

Js_of_ocaml flags

Flag	Description
`--pretty`	Format JavaScript readably, preserve OCaml variable names
`--no-inline`	Prevent function inlining
`--debug-info`	Add source location comments
`--source-map`	Generate source map

Dune configuration

For development builds, add to your dune file:

(executable
 (name main)
 (modes js)
 (js_of_ocaml (flags --pretty --source-map)))

Source maps

Source maps map generated JavaScript back to OCaml sources. With source maps enabled, you can set breakpoints, step through code, and inspect variables directly in your OCaml files.

To enable source maps:

js_of_ocaml --source-map program.byte

This generates program.js and program.js.map.

Breakpoints

In browser developer tools

Open browser developer tools (F12), navigate to the Sources panel, and set breakpoints in your OCaml files (when using source maps) or JavaScript files.

See the Chrome DevTools documentation for details.

Programmatic breakpoints

Insert breakpoints in your OCaml code using Js.debugger:

let my_function x =
  Js_of_ocaml.Js.debugger ();  (* Breaks here when devtools are open *)
  x + 1

Note: Browsers only pause at debugger statements when developer tools are open.

Debugging in Node.js

Using --inspect

Run your program with Node.js inspector:

node --inspect _build/default/main.bc.js

This prints a URL like:

Debugger listening on ws://127.0.0.1:9229/...

Chrome DevTools for Node.js

Run node --inspect or node --inspect-brk (breaks on first line)
Open Chrome and navigate to chrome://inspect
Click "Open dedicated DevTools for Node"
Your program appears in the Sources panel

Stack traces

Attaching JavaScript backtraces

To get JavaScript stack traces attached to OCaml exceptions:

OCAMLRUNPARAM=b=1 node program.js

Or compile with:

js_of_ocaml --enable with-js-error program.byte

See Error handling for details.

Chrome DevTools tips

Useful settings in Chrome DevTools (Settings > Experiments):

Display variable values inline while debugging - Shows values next to variables
Resolve variable names - Maps minified names back to original names