Expected variables not defined when using node inspect?

Question

I'm trying to familiarize myself with debugging using the node inspect command. I've written as simple script use_debugger.js which defines a function and calls it:

function count(nums, maxes) {
  debugger;
  return maxes;
}

count([1, 2, 3], [4, 5]);

When I drop into the debugger, I would expect nums to be defined and have the value [1, 2, 3]. However, if I run node inspect use_debugger.js and then type c and nums in the REPL, I get that it is not defined:

Kurts-MacBook-Pro:Scratch kurtpeek$ node --inspect-brk use_debugger.js
Debugger listening on ws://127.0.0.1:9229/7adfaada-c939-44a3-9150-6d1326b8c7d0
For help, see: https://nodejs.org/en/docs/inspector
^C
Kurts-MacBook-Pro:Scratch kurtpeek$ node inspect use_debugger.js
< Debugger listening on ws://127.0.0.1:9229/54fb6a12-82c8-4454-8307-6d120b0c26e8
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in use_debugger.js:1
> 1 (function (exports, require, module, __filename, __dirname) { function count(nums, maxes) {
  2   debugger;
  3   return maxes;
debug> c
break in use_debugger.js:2
  1 (function (exports, require, module, __filename, __dirname) { function count(nums, maxes) {
> 2   debugger;
  3   return maxes;
  4 }
debug> maxes
repl:1
maxes
^

ReferenceError: maxes is not defined
    at repl:1:1
    at Script.runInContext (vm.js:101:20)
    at Object.runInContext (vm.js:279:6)
    at REPLServer.controlEval (internal/deps/node-inspect/lib/internal/inspect_repl.js:521:25)
    at bound (domain.js:396:14)
    at REPLServer.runBound [as eval] (domain.js:409:12)
    at REPLServer.onLine (repl.js:621:10)
    at REPLServer.emit (events.js:182:13)
    at REPLServer.EventEmitter.emit (domain.js:442:20)
    at REPLServer.Interface._onLine (readline.js:290:10)
debug>

Should I not be able to 'access' the variables in the function's scope, nums and maxes?

Update

To further document estus' answer, the exec maxes command worked for me:

debug> exec maxes
[ 4, 5 ]

As stated in https://nodejs.org/api/debugger.html#debugger_information, the exec expr command executes an expression in the debugging script's context.

Estus Flask · Accepted Answer · 2018-09-23 13:50:24Z

9

maxes

evaluates maxes in REPL scope, not count function scope.

As explained in the documentation:

repl - Open debugger's repl for evaluation in debugging script's context

exec expr - Execute an expression in debugging script's context

To evaluate maxes in debugging scope, it should be either:

repl
maxes

Or:

exec maxes

edited Sep 23, 2018 at 13:50

answered Sep 23, 2018 at 1:09

Estus Flask

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2 Comments

Andrés Andrade Over a year ago

While debugging, eval doesn't evaluate the code in debugging script's context.

Estus Flask Over a year ago

@AndrésAndrade Thanks for noticing, that's a typo, it's exec.

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