精确加减乘除

/**

 * 解决 js 浮点运算时精度丢失问题

 * plus 加

 * minus 减

 * times 乘

 * divide 除

 */

/**

 * 把错误的数据转正

 * strip(0.09999999999999998)=0.1

 */

function strip(num, precision = 12) {

  return +parseFloat(num.toPrecision(precision))

}

/**

 * Return digits length of a number

 * @param {*number} num Input number

 */

function digitLength(num) {

  // Get digit length of e

  const eSplit = num.toString().split(/[eE]/)

  const len = (eSplit[0].split('.')[1] || '').length - +(eSplit[1] || 0)

  return len > 0 ? len : 0

}

/**

 * 把小数转成整数，支持科学计数法。如果是小数则放大成整数

 * @param {*number} num 输入数

 */

function float2Fixed(num) {

  if (num.toString().indexOf('e') === -1) {

    return Number(num.toString().replace('.', ''))

  }

  const dLen = digitLength(num)

  return dLen > 0 ? strip(num * Math.pow(10, dLen)) : num

}

/**

 * 精确加法

 */

Math.plus = function (num1, num2, ...others) {

  if (others.length > 0) {

    return Math.plus(Math.plus(num1, num2), others[0], ...others.slice(1))

  }

  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)))

  return (Math.times(num1, baseNum) + Math.times(num2, baseNum)) / baseNum

}

/**

 * 精确减法

 */

Math.minus = function (num1, num2, ...others) {

  if (others.length > 0) {

    return Math.minus(Math.minus(num1, num2), others[0], ...others.slice(1))

  }

  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)))

  return (Math.times(num1, baseNum) - Math.times(num2, baseNum)) / baseNum

}

/**

 * 精确乘法

 */

Math.times = function (num1, num2, ...others) {

  if (others.length > 0) {

    return Math.times(Math.times(num1, num2), others[0], ...others.slice(1))

  }

  const num1Changed = float2Fixed(num1)

  const num2Changed = float2Fixed(num2)

  const baseNum = digitLength(num1) + digitLength(num2)

  const leftValue = num1Changed * num2Changed

  return leftValue / Math.pow(10, baseNum)

}

/**

 * 精确除法

 */

Math.divide = function (num1, num2, ...others) {

  if (others.length > 0) {

    return Math.divide(Math.divide(num1, num2), others[0], ...others.slice(1))

  }

  const num1Changed = float2Fixed(num1)

  const num2Changed = float2Fixed(num2)

  return Math.times(num1Changed / num2Changed, Math.pow(10, digitLength(num2) - digitLength(num1)))

}