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Debugging

Tracing Errors In CLI Output

Programs can have errors. These can/will throw an Error in the code. Errors that are thrown will display in the console, and include a "stack" of lines that describe "where" the error occurred: this is referred to as a stack-trace.
Node allows for passing of a specific V8's command-line arg --stack-trace-limit

  • defaults to last 10 lines of stacktrace on error
  • to see as much as possible, set the limit HIGH!
    • more stack-trace for dev && maybe qa only...
    • causes some cpu overhead, not worth it on prod
node --stack-trace-limit=99999 app.js

Debugging With Inspect

Adding Breakpoints

  • pauses the running code at a specific 'line' in the process

in dev tools

  • find the vertical "column" of numbers, which represents the line numbers of the code being run
  • click a number - this adds a blue 'flag' on-top-of the number, representing a break-point
  • Now, when the code re-visits this line (via a loop or some other re-visiting mechanisms), the code will 'pause', && stop running at this line

adding debugger to the code

Create an automatic "breakpoint" on a specific line in a program's source code.
Pause the execution of the program on that breakpoint:

  • create a new line and write debugger;, where the code should pause
  • add a few details to the node cli when running the program: NODE_INSPECT_RESUME_ON_START=1 node inspect file.js
    • NODE_INSPECT_RESUME_ON_START tells node and the debugger to run and STOP on the debugger; line in the program
    • inspect instructs node to use the debugger, the "inspect" tool

A different way to use the inspect argument is to use it withouth the NODE_INSPECT_RESUME_ON_START=1 env var.
Without that env var set, node will pause on the first "runnable" piece/function of code.

Debugging with cli

A program can be paused immediately when starting the program by utilizing the cli.
This is different than the above debugger. One primary way it is different is that the debugger method requires adding a line to the source code - here, though, no new line in required.

  • --inspect
    • triggers the v8 inspector
    • takes an optional port: --inspect=9876
  • --inspect-brk
    • inspect mode
    • breaks on first line of the code

CLI Output After Flags

After entering the inspect-brk flag, (something like) this should appear in the terminal

Debugger listening on ws://127.0.0.1:9229/b23039cd-cd8b-48f6-ba79-83ab9ecd5660
For help, see: https://nodejs.org/en/docs/inspector

Accessing the Node Debugger

The Chrome browser comes with a builtiin inspect detail that node works with in order to allow us devs to "inspect" the node code during the --inspected process. In chrome, set the url to chrome://inspect

Some Chrome Developer tools allow us to...

  • Pause on Exception
    • 'break' the code when an exception is thrown
    • open the 'sources' tab
    • click the hexagon-with-a-pause sign
  • Adding a breakpoint in devtools during inspections
    • open the 'sources' tab
    • click one of the vertical 'columns' that are numbering the code lines
      • should see a blue shape appear on the line, indicating a 'break-point' has been added to the running of the code
    • click th 'play'-style button, to continue the execution of the code until the breakpoint is reached

Triggering A Breakpoint In The Code

Adding a debugger keyword to the code, directly, can allow for the chrome inspector devtools to stop the node process at the keyword.

References

Tags:
server
node
javascript
debugging