Node.js Debugger

Véase Debugger

The built-in Node debugger supports placing debugger statements in your source code.

// myscript.js
x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

When you run your app in debug mode via node debug [your app], it will break when a debugger statement is hit.

[~/javascript/node.js/debugging/debugger(master)]$ node debug myscript.js
< Debugger listening on port 5858
connecting to 127.0.0.1:5858 ... ok
break in myscript.js:2
  1 // myscript.js
> 2 x = 5;
  3 setTimeout(() => {
  4   debugger;
debug>

From here, we can use the following debug commands to step through our code :

• continue – cont, c
• step – next, n
• step in – step, s
• step out – out, o

debug> help
Commands: run (r), cont (c), next (n), step (s), out (o), backtrace (bt), setBreakpoint (sb), clearBreakpoint (cb),
watch, unwatch, watchers, repl, exec, restart, kill, list, scripts, breakOnException, breakpoints, version
debug>

debug> cont
< hello
break in myscript.js:4
  2 x = 5;
  3 setTimeout(() => {
> 4   debugger;
  5   console.log('world');
  6 }, 1000);

debug> next
break in myscript.js:5
  3 setTimeout(() => {
  4   debugger;
> 5   console.log('world');
  6 }, 1000);
  7 console.log('hello');
debug>

At any point, we can open a REPL by typing repl to inspect or reassign variable values.

debug> repl
Press Ctrl + C to leave debug repl
> x
5
Ctrl-C

debug> sb(7)
  1 // myscript.js
  2 x = 5;
  3 setTimeout(() => {
  4   debugger;
> 5   console.log('world');
  6 }, 1000);
* 7 console.log('hello');
  8 
  9 
 10 });

debug> c
< world
debug>

Watchers

It is possible to watch expression and variable values while debugging.

On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type

watch('my_expression')

The command watchers will print the active watchers.

To remove a watcher, type

unwatch('my_expression')

Breakpoints

• setBreakpoint(), sb() - Set breakpoint on current line
• setBreakpoint(line), sb(line) - Set breakpoint on specific line
• setBreakpoint('fn()'), sb(...) - Set breakpoint on a first statement in functions body
• setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
• clearBreakpoint('script.js', 1), cb(...) - Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that isn't loaded yet:

debug> setBreakpoint('mod.js', 23)