formatNumber.js

function formatNumber (num) {
  
  if (typeof num !== 'number') {
    console.error('Incorrect value was passed. Please enter a valid number')
    return
  }
  
  let isNegativeNum
  let isFloatingNum
  let absoluteNum
  let decimalNum
  
  let formatedString = ''
  let count = 1
  
  // Negative Numbers
  if (num < 0) {
    absoluteNum = Math.abs(num)
    isNegativeNum = true
  }
  
  absoluteNum = absoluteNum || num
  
  // Floating point numbers
  if (!Number.isInteger(absoluteNum)) {
    decimalNum = absoluteNum - Math.floor(absoluteNum)
    decimalNum = decimalNum.toString().slice(1)
    
    absoluteNum = Math.floor(absoluteNum)
    isFloatingNum = true
  }
  
  // Add the commas to the number
  let iteratableAbsoluteNum = absoluteNum.toString()
  let len = iteratableAbsoluteNum.length
  for (let i = len - 1; i >= 0; i--) {
    formatedString = iteratableAbsoluteNum.charAt(i) + formatedString    
    if (count % 3 === 0 && i != 0) {
      count = 0
      formatedString = ',' + formatedString      
    }
    count++
  }
  
  if (isNegativeNum) {
    formatedString = '-' + formatedString
  }
  
  if (isFloatingNum) {
    formatedString = formatedString + decimalNum.toString()
  }
  
  console.log(formatedString)
}

formatNumber(100000) // "100,000"
formatNumber(-1000) // "-1,000"
formatNumber(-10.10) // "-10.09999999999999964"
formatNumber(10.10) // "10.09999999999999964"