js中的超过16位数字相加问题

方案一 

function sub(str1, str2) {
    // 补全0，并多补一位0
    let arr1 = null,
        arr2 = null
    if (str1.length > str2.length) {
        str1 = '0' + str1
        for (let i = str2.length; i < str1.length; i++) {
            str2 = '0' + str2
        }
        arr1 = str1.split('')
        arr2 = str2.split('')
    } else {
        str2 = '0' + str2
        for (let i = str1.length; i < str2.length; i++) {
            str1 = '0' + str1
        }
        arr1 = str1.split('')
        arr2 = str2.split('')
    }
    // 注意这里arr里边保存的还是字符串，需要转成数字后再相加
    for (let i = arr1.length - 1; i >= 0; i--) {
        let temp = parseInt(arr1[i]) + parseInt(arr2[i])
        arr1[i] = temp % 10
        // 处理得到进位的结果
        arr1[i - 1] = parseInt(arr1[i - 1]) + Math.floor(temp / 10)
    }
    if (arr1[0] === 0) {
        return (arr1.join('')).substring(1)

    } else {
        return arr1.join('')
    }
}

方案二

function sub(a,b){
    var res='', c=0;
    a = a.split('');
    b = b.split('');
    while (a.length || b.length || c){
        c += ~~a.pop() + ~~b.pop();
        res = c % 10 + res;
        c = c>9;
    }
    return res.replace(/^0+/,'');
}

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