// 加法
    0.1 + 0.2 = 0.30000000000000004
    0.7 + 0.1 = 0.7999999999999999
    0.2 + 0.4 = 0.6000000000000001

    // 減法
    1.5 - 1.2 = 0.30000000000000004
    0.3 - 0.2 = 0.09999999999999998
    
    // 乘法
    19.9 * 100 = 1989.9999999999998
    0.8 * 3 = 2.4000000000000004
    35.41 * 100 = 3540.9999999999995

    // 除法
    0.3 / 0.1 = 2.9999999999999996
    0.69 / 10 = 0.06899999999999999

npm install mathjs

import * as math from 'mathjs';
export default {
    // 加
	add(num1,num2){
		return math.add(math.bignumber(num1),math.bignumber(num2));
	},
	// 乘
	multiply(num1,num2){
		return math.multiply(math.bignumber(num1),math.bignumber(num2));
	},
	// 減
	subtract(num1,num2){
		return math.subtract(math.bignumber(num1),math.bignumber(num2));
	},
	// 除
	divide(num1,num2){
		return math.divide(math.bignumber(num1),math.bignumber(num2));
	}
}

import math from './math.js'
let addNum = math.add(3,2); // 5
let mulNum = math.multiply(3,2); // 6
let subNum = math.subtract(3,2); // 1 
let divNum = math.divide(3,2); // 1.5

<script type="text/javascript">
// functions and constants
console.log('mathjs BigNumber eg')
print(math.add(math.bignumber(0.1), math.bignumber(0.2))) // 加法 BigNumber, 0.3
print(math.subtract(math.bignumber(1), math.bignumber(0.9))) // 減法 BigNumber, 0.1
print(math.multiply(math.bignumber(4.10), math.bignumber(100))) // 乘法 BigNumber, 0.1
print(math.ceil(math.bignumber(6.10)/ math.bignumber(0.1)));   //除法 向上取整 61
print(math.floor(math.bignumber(6.10)/ math.bignumber(0.1)));   //除法 向下取整 61
print(math.round(math.bignumber(6.10)/ math.bignumber(0.1)));   // 除法 四舍五入 61
/**
 * Helper function to output a value in the console. Value will be formatted.
 * @param {*} value
 */
function print (value) {
  const precision = 14
  console.log(math.format(value, precision))
}
</script>