Debugging Node.js application All Versions

Examples

• 3

  Core node.js debugger and node inspector

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  3

  Using core debugger

  Node.js provides a build in non graphical debugging utility. To start the build in the debugger, start the application with this command:

  node debug filename.js
  

  Consider the following simple Node.js application contained in the debugDemo.js

  'use strict';
  
  function addTwoNumber(a, b){
  // function returns the sum of the two numbers
  debugger
    return a + b;
  }
  
  var result = addTwoNumber(5, 9);
  console.log(result);
  

  The keyword debugger will stop the debugger at that point in the code.

  Command reference

  1. Stepping

  cont, c - Continue execution
  next, n - Step next
  step, s - Step in
  out, o - Step out
  

  2. Breakpoints

  setBreakpoint(), sb() - Set breakpoint on current line
  setBreakpoint(line), sb(line) - Set breakpoint on specific line
  

  To Debug the above code run the following command

  node debug debugDemo.js

  Once the above commands runs you will see the following output. To exit from the debugger interface, type process.exit()

  

  Use watch(expression) command to add the variable or expression whose value you want to watch and restart to restart the app and debugging.

  Use repl to enter code interactively. The repl mode has the same context as the line you are debugging. This allows you to examine the contents of variables and test out lines of code. Press Ctrl+C to leave the debug repl.

  Using Built-in Node inspector

  As of version 6.3 you can run node's built in v8 inspector! The node-inspector plug-in is not needed anymore.

  Simply pass the inspector flag and you'll be provided with a URL to the inspector

  node --inspect server.js
  

  Using Node inspector

  Install the node inspector:

  npm install -g node-inspector
  

  Run your app with the node-debug command:

  node-debug filename.js
  

  After that, hit in Chrome:

  http://localhost:8080/debug?port=5858
  

  Sometimes port 8080 might not be available on your computer. You may get the following error:

  Cannot start the server at 0.0.0.0:8080. Error: listen EACCES.

  In this case, start the node inspector on a different port using the following command.

  $node-inspector --web-port=6500
  

  You will see something like this:

  

  edited Sep 21 '16 at 16:40