formatter.ts

export function formatNumber(fmt: string, value: number) {    
  // special case for `#,##0.0###`
  // warning! you can't use `,` as a decimal delimiter in that particular case
  // also, you can use up to 8 decimal places here
  if (fmt[fmt.length - 1] === "#") {
    const dotPos = fmt.lastIndexOf(".")
    if (dotPos > -1) {

      // fractional part
      // -0.345; toFixed() is used to get rid of 0.9994999999180436 that is formatted using 0.0## into 0.999 which is wrong
      const fractPart = Number((number % 1).toFixed(8))
      // 0.0###
      const fractFmt = "0" + fmt.slice(dotPos)
      // -0.3450 or 0.3460 or -1.0 or 0.0
      const fractFormatted = SSF.format(fractFmt, fractPart)
      // we'll use formatter's rounding and check for the fractional part overflow (0 or 1)
      const absFractPartOverflow = Math.floor(Math.abs(Number(fractFormatted)))

      // int part
      // ##,##0 ; # is added to have the sign formatted when everything that's left from fmt is "0"
      const intFmt = "#" + fmt.slice(0, dotPos)
      // 123457
      const absIntPart = Math.floor(Math.abs(number)) + absFractPartOverflow
      // -123,457; 0.1 is to preserve the sign formatting
      const intFormatted = SSF.format(intFmt, Math.sign(number) * (absIntPart + 0.1))
      

      // altogether
      const fractNoZero = /\.(.+)$/.exec(fractFormatted)
      const res = intFormatted + (fractNoZero ? "." + fractNoZero[1] : "")
      return res.trim()
    } 
  }

  return SSF.format(fmt, value).trim().replace(/\.$/, "")
}