Horrible Howto - Attach A Stack To Debug

I remember a time while learning how to use JavaScript when I wanted to attach the function name and file name to my debug calls, before fully grasping the utility of error stacks.

The Circumstance

Imagine you’re working in a cloud function like AWS Lambda. Deploying your function takes some time, and your ability to inspect it via a debugger is severely limited. To solve this, I used the debug module.

Starting Out

In a file named blah.js:

const v = require('debug')('verbose');

v('test');

By setting DEBUG=verbose, the output is verbose test +0ms.

Building the Stack

JavaScript lacks a global stack, so the cheapest way to get stack history is to actually drop the function into an error status.

const buildStack = () => {
  const depth = 3;
  if (!this.stackIs) {
    Object.defineProperty(this, 'stackIs', {
      get: function() {
        const orig = Error.prepareStackTrace;
        Error.prepareStackTrace = function(_, stack) {
          return stack;
        };
        const err = new Error();
        Error.captureStackTrace(err, arguments.callee);
        const stack = err.stack;
        Error.prepareStackTrace = orig;
        return stack;
      },
    });
  }

  if (!this.lineIs) {
    Object.defineProperty(this, 'lineIs', {
      get: function() {
        return this.stackIs[depth].getLineNumber();
      },
    });
  }

  if (!this.functionIs) {
    Object.defineProperty(this, 'functionIs', {
      get: function() {
        return this.stackIs[depth].getFunctionName();
      },
    });
  }

  return {
    function: this.functionIs,
    line: this.lineIs,
  };
};

This gives you an object containing the location of the debug call, which makes for really awesome debugging.

The full snippet is available in this gist, and I originally found the idea on Stack Overflow.