1. function bigSum(a, b) {
    2.     // 取两个字符串最大长度，方便后续长度较小的前补位
    3.     const maxLen = Math.max(a.length, b.length)
    4.     // 两数前补位：'0'；如：'2'.padStart(4, '0') // '0002'
    5.     a = a.padStart(maxLen, '0')
    6.     b = b.padStart(maxLen, '0')
    8.     let sum = '' // 最终结果
    9.     let carry = 0 // 进位
    11.     for(let i = maxLen - 1; i >= 0; i--) {
    12.         const total = parseInt(a[i]) + parseInt(b[i]) + carry
    13.         carry = Math.floor(total / 10) // 向下取整，获取进位数
    14.         sum = total % 10 + sum
    15.     }
    16.     // 两数相加超过最大字符串长度
    17.     if (carry !== 0) {
    18.         sum = '' + carry + sum
    19.         console.log('两数相加超过最大字符串长度')
    20.     }
    21.     return sum
    22. }
    24. // let a = "9007199254740991";
    25. // let b = "12345678999999999";
    26. // bigSum(a, b) // "21352878254740990"
    28. let a = "1";
    29. let b = "999999999999";
    30. bigSum(a, b) // "1000000000000"