js 处理大数相减

        function sub(num1, num2) {
            if(num1 === num2) return '0'
            function lt(num1, num2) {
                if (num1.length < num2.length) {
                    return true
                } else if (num1.length === num2.length) {
                    return num1 < num2
                } else {
                    return false
                }
            }
            let isMinus = false
            if (lt(num1, num2)) {
                [num1, num2] = [num2, num1]
                isMinus = true
            }

            let len = Math.max(num1.length, num2.length)
            num1 = num1.padStart(len, 0)
            num2 = num2.padStart(len, 0)

            let flag = 0,
                result = '',
                temp
            for (let i = len - 1; i >= 0; i--) {
                temp = parseInt(num1[i]) - flag - parseInt(num2[i])
                if (temp < 0) {
                    result = (10 + temp) + result
                    flag = 1
                } else {
                    result = temp + result
                    flag = 0
                }
            }
            result = (isMinus ? '-' : '') + result.replace(/^0+/, '')
            return result
        }
        // 传入字符串格式
        console.log(sub('9999999999011112','9999999999011111'
)) // 1
    console.log(sub('9999999999011112','9999999999011113'
)) // -1