= Debugging

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

==@@id="compiler-flags"@@ 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)))
}}}

==@@id="source-maps"@@ 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}}}.

==@@id="breakpoints"@@ 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 https://developer.chrome.com/docs/devtools/javascript for details.

=== Programmatic breakpoints

Insert breakpoints in your OCaml code using <<a_api subproject="js_of_ocaml" text="Js.debugger"|val Js_of_ocaml.Js.debugger>>:

<<code language="ocaml"|
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.

==@@id="nodejs"@@ 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

==@@id="stack-traces"@@ 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 <<a_manual chapter="errors" fragment="stack-traces"|Error handling>> for details.

==@@id="devtools-tips"@@ 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

== See also

* <<a_manual chapter="errors"|Error handling>> - Exception handling between OCaml and JavaScript
* <<a_manual chapter="options"|Command line options>> - Full list of compiler flags
