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Debugging Node.js

V8 Inspector

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol.

V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

$ node --inspect index.js
Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
For help, see: https://nodejs.org/en/docs/inspector

To break on the first line of the application code, pass the --inspect-brk flag instead of --inspect.

$ node --inspect-brk app.js

In order to begin debugging the process, the next step is to set a Chrome browser tab's address bar to chrome://inspect.

Debugger command

The debugger statement can be used to explicitly pause on the line that the statement appears when debugging.

function f (n = 99) {
  if (n === 0) throw Error()
  debugger
  f(n - 1)
}
f()

Profile

The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector (--inspect flag).

console.profile

The console.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector.

console.profile('MyLabel');
// Some code
console.profileEnd('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.

console.profileEnd

Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. If this method is called without a label, the most recently started profile is stopped.

console.timeStamp

This method does not display anything unless used in the inspector. The console.timeStamp() method adds an event with the label 'label' to the Timeline panel of the inspector.