string to number without using Number or parseFloat

str2num.test.js

/**
 * @param {string} str
 */
function str2num (str) {
  const [m, e = ''] = str.split('e')
  const [i, f = ''] = m.split('.')
  let result = 0
  // handle int
  let sign = 1
  for (let x of i) {
    if (x === '+') {
      continue
    } else if (x === '-') {
      sign *= -1
      continue
    } else {
      result *= 10
      result += c2n(x)
    }
  }

  // handle fraction
  if (f) {
    result += str2num(f) / (10 ** f.length)
  }

  // handle exponent
  if (e) {
    let exponent
    let sign = 1
    if (e[0] === '-') {
      sign = -1
      exponent = str2num(e.slice(1))
    } else if (e[0] === '+') {
      exponent = str2num(e.slice(1))
    } else {
      exponent = str2num(e)
    }
    if (sign === -1) {
      result /= 10 ** exponent
    } else {
      result *= 10 ** exponent
    }
  }

  // handle sign
  result *= sign
  return result
}

function c2n (char) {
  const n = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][char]
  if (n == null) throw new Error(`unknown char: "${char}"`)
  return n
}

function assert (input) {
  let output = str2num(input)
  let expect = Number(input)
  if (output !== expect) {
    throw new Error(`input ${input}, got ${output}, while expect ${expect}`)
  }
}

void function test () {
  assert('13.4e-7')
  assert('.4e+7')
  assert('-.2e+1')
  assert('+.6e+0')
  assert('0')
  assert('-0e-0')
  assert('0e0')
  assert('123')
  assert('2e1')
  assert('2e-0')
  assert('.1')
  console.log('All right.')
}()