Make a replacement log function that doesn't require the parentheses.

LogUtil.js

// Get stdout off process as outPipe
const { stdout: outPipe } = process

// Import log and trace off of console
const { log, trace } = console

// Make a logRaw function
const logRaw = (...args) => outPipe.write(...args)

// Easy log fn for node
function echo() {
  const maybeArray = Array.prototype.slice.call(arguments)[0]
  const logString = (typeof(maybeArray) === "string") ? maybeArray : maybeArray[0]
  const templateVarToValue = (match, string, offset) => eval(Array.from(match).splice(1).toString())
  log(logString.replace(/(\$[^\W]+)/g, templateVarToValue))
}

// Give back an array of the stacktrace items
const traceIt = () => {
  var orig = Error.prepareStackTrace
  Error.prepareStackTrace = function(_, stack) {
    return stack
  }

  var err = new Error
  Error.captureStackTrace(err, arguments.callee)

  var stack = err.stack
  Error.prepareStackTrace = orig

  return stack
}

// Takes a string of variable names comma seperated
// In: `foo, bar` => Out: `foo: fooVal, bar: barVal`
function varDump(/*str, stackTrace=false*/) {
  // Parse the arguments
  const maybeArray = Array.prototype.slice.call(arguments)[0]

  // Parse first argument
  const varString = (typeof(maybeArray) === "string") ? maybeArray : maybeArray[0]

  // Take all arguments, variable names
  var varArray = varString.split(',')

  // Take the last argument
  const stackTrace = (varArray.pop().trim().toLowerCase() === 'true')

  // If we need to create a stack trace
  stackTrace && traceIt().forEach((site) => {
    log('  \033[36m%s\033[90m in %s:%d\033[0m',
      site.getFunctionName() || 'anonymous',
      site.getFileName(), site.getLineNumber()
    )
  })

  // For each to variable names log out its current value
  varArray
    .map((__) => __.trim())
    .forEach((__) => logRaw(`${__}: ${eval(__)} `))

  // Add a newline at the end
  logRaw("\n")
}

const a = "foo"
const b = "bar"
const c = "baz"

//echo`The value of: a=$a, b=$b, and c=$c`
//varDump`a, b, c, false`

module.exports = { varDump, echo }

templateStringAsArgExample.js

// First argument will be an `array` when called as: log`something`
// First argument will be a `string` when called as: log(`something`)
function log(/* arguments */) {
  var maybeArray = Array.prototype.slice.call(arguments)[0]
  var resultString = (typeof(maybeArray) === "string") ? maybeArray : maybeArray[0]
  console.log(resultString)
}

// Can call function without `()`
log`something`

// Can call function with `()`
log(`something`)

LogUtil.js has methods that can help with debugging, but if used while inside of a function block (and not in the global scope) then you will not be able to get the values of the variables.

TODO: Refactor approach to use something similar to this after refactoring to include some observed limitations listed below..

echo  = function() { /* noop */ }
debug = {}

Object.defineProperty(debug, 'fn', {
  get: function() {
    const toLog = (full, matchedTemplateStr, offset) => {
      const wrapWithConsoleLog  = (line)      => ("console.log('" + line + "')")
      const wrapAsVariableInStr = (variable)  => ("'+" + variable + "+'")
      const normalizeDollarVar  = (dollarVar) => (Array.from(dollarVar).splice(1).toString())
      const replaceDollarVar = (full, matchedDollarVar, offset) => (
        wrapAsVariableInStr( normalizeDollarVar(matchedDollarVar) )
      )
      line = matchedTemplateStr.replace(/(\$[^\W]+)/g, replaceDollarVar)
      line = wrapWithConsoleLog(line)
      return line
    }
    let fn = arguments.callee.caller.toString()
    fn = fn.replace(/return debug\.fn/g, '')
    fn = fn.replace(/echo`(.*)`/g, toLog)
    eval('var ___fn = '+fn)
    return ___fn()
  }
})

var b = 20

function foo () {
  return debug.fn

  var a = 10

  echo`hello $a+$b=30`

  return 5
}

// Show that we can get a result back from the function still
var result = foo()

// Log the return value from the function call
console.log(result)

Use return debug.fn at the beginning of any function you would like to debug inside.
Wherever you use echo to inspect inside that function should work now because we replace it.

This works by using arguments.callee.caller to get all the code of the function, that the current function was called inside of.
This program alters the original functions code and runs the altered version of the function respecting return values.

Some problems I can see are:

• Make the regex better to include $this.something for matching.
• Make it work to carry this over to the newly executing function.
• Regex replacements should take into account possible ; at end of code lines.
• In strict mode arguments.callee.caller is not available so I have written an alternative...

echo  = function() { /* noop */ }

debug = {
  fn: function(prevFn) {
      const toLog = (full, matchedTemplateStr, offset) => {
      const wrapWithConsoleLog  = (line)      => ("console.log('" + line + "')")
      const wrapAsVariableInStr = (variable)  => ("'+" + variable + "+'")
      const normalizeDollarVar  = (dollarVar) => (Array.from(dollarVar).splice(1).toString())
      const replaceDollarVar = (full, matchedDollarVar, offset) => (
        wrapAsVariableInStr( normalizeDollarVar(matchedDollarVar) )
      )
      line = matchedTemplateStr.replace(/(\$[^\W]+)/g, replaceDollarVar)
      line = wrapWithConsoleLog(line)
      return line
    }
    let fn = prevFn.toString()
    fn = fn.replace(/return debug\.fn\.bind\(this, foo\)\(\)/g, '')
    fn = fn.replace(/echo`(.*)`/g, toLog)
    eval(`var ___fn = ${fn}.bind(this)`)
    return ___fn()
  }
}

var b = 20

function foo () {
  return debug.fn.bind(this, foo)()
  var a = 10

  echo`hello $a+$b=30`
  return 5
}

var result = foo()

console.log(result)