Checks if input javascript files will instantly crash on IE11

ie11checker.js

#!/usr/bin/env node
const fs = require('fs')
const acorn = require("acorn")
const glob = require('glob')

const getNextCharPosition = (idx, str, offset = 1, matchChar = ' ') => {
  let i = idx
  while (str[i] !== matchChar) {
    i += offset
  }
  return i
}

const outputs = process.argv.slice(2).map(path => glob.sync(path)).flat()
console.log('Checking files: ')
// IE11 supports let and const, but ES5 doesn't. So we rename all instances of let and const.
// This results in a lot of false positives ('let me in' => 'var me in'), but this shouldn't matter
// because we're only parsing the code, not evaluating it.
outputs.forEach(file => {
  console.log(' ', file)
  const code = fs.readFileSync(file, { encoding: 'utf8' })
    .replace(/const |let /g, 'var ')
  try {
    acorn.parse(code, { ecmaVersion: 5, ranges: true })
  } catch (e) {
    const { pos, loc: { line, column } } = e
    console.error(`Error @ ${file} - ${line}:${column}`)
    console.error(
      code.substring(
        getNextCharPosition(pos, code, -1, '\n') + 1,
        getNextCharPosition(pos, code)
      )
    )
    return process.exit(1)
  }
})
console.log('Pass')