toFixed.js

toFixed = (value, fractionDigits) => {
  const f =
    fractionDigits === undefined
    ? 0
    : +fractionDigits

  console.log('1. f:', f)

  if (f < 0 || f > 20) {
    console.error('2. throw a RangeError')
    throw new RangeError('fractionDigits must be between 0 and 20')
  }

  let x = +value
  console.log('3. x:', x)

  if (isNaN(x)) {
    console.log('4. x is NaN')
    return 'NaN'
  }

  let s = ''
  console.log('5. s:', s)

  if (x < 0) {
    s = '-'
    x = -x
    console.log('6. s:', s, 'x:', x)
  }

  let m
  if (x >= 10e21) {
    m = x.toString()
    console.log(`7. m:`, m)
  } else {
    console.group('8.')

    const n0 = x * 10 ** f
    const n1 = Math.floor(n0)
    const n2 = Math.ceil(n0)

    const v1 = Math.abs(n1 / 10 ** f - x)
    const v2 = Math.abs(n2 / 10 ** f - x)

    console.log('8.a. n0:', n0)
    console.log('n1:', n1, 'v1:', v1)
    console.log('n2:', n2, 'v2:', v2)

    let n
    if (v1 < v2) {
      n = n1
    } else if (v2 < v1) {
      n = n2
    } else if (n1 < n2) {
      n = n2
    } else {
      n = n1
    }

    console.log('n:', n)

    if (n === 0) {
      m = '0'
    } else {
      m = n.toString()
    }

    if (f != 0) {
      let k = m.length

      if (k <= f) {
        let z = '0'.repeat(f + 1 - k)

        m = z + m

        k = f + 1
      }

      let a = m.slice(0, k - f)
      let b = m.slice(k - f)

      m = a + '.' + b
    }

    console.groupEnd()
  }

  return s + m
}