代码

function formatDecimal(num, decimal) {
  num = num.toString()
  let index = num.indexOf('.')
  if (index !== -1) {
    num = num.substring(0, decimal + index + 1)
  } else {
    num = num.substring(0)
  }
  return parseFloat(num).toFixed(decimal)
}

• formatDecimal(23.45678, 2) —— 23.45
• formatDecimal(23.4999999, 2) —— 23.49
• formatDecimal(23.80, 2) —— 23.80
• formatDecimal(123456, 2) —— 123456.00
• formatDecimal(23.45678, 3) —— 23.456
• formatDecimal(23.45678, 4) —— 23.4567