JS implements the multiplication of two large Numbers of integers

Large Numbers, that is, Numbers beyond the maximum range of Numbers that the language can represent, where the Numbers can only be represented as strings. So how do you multiply two large Numbers? I used JS to implement the following code:

 
console.log(bigMut("567", "1234")); // 699678 
function bigMut(big, common) { 
big += ""; 
common += ""; 
if (big.length < common.length) { 
big = [common, common = big][0]; 
} 
big = big.split("").reverse(); 
var oneMutManyRes = []; 
var i = 0, 
len = big.length; 
for (; i < len; i++) { 
oneMutManyRes[oneMutManyRes.length] = oneMutMany(big[i], common) + getLenZero(i); 
} 
var result = oneMutManyRes[0]; 
for (i = 1, len = oneMutManyRes.length; i < len; i++) { 
result = bigNumAdd(result, oneMutManyRes[i]); 
} 
return result; 
} 
function getLenZero(len) { 
len += 1; 
var ary = []; 
ary.length = len; 
return ary.join("0"); 
} 
function oneMutMany(one, many) { 
one += ""; 
many += ""; 
if (one.length != 1) { 
one = [many, many = one][0]; 
} 
one = parseInt(one, 10); 
var i = 0, 
len = many.length, 
resAry = [], 
addTo = 0, 
curItem, 
curRes, 
toSave; 
many = many.split("").reverse(); 
for (; i <= len; i++) { 
curItem = parseInt(many[i] || 0, 10); 
curRes = curItem * one + addTo; 
toSave = curRes % 10; 
addTo = (curRes - curRes % 10) / 10; 
resAry.unshift(toSave); 
} 
if (resAry[0] == 0) { 
resAry.splice(0, 1); 
} 
return resAry.join(""); 
} 
function bigNumAdd(big, common) { 
big += ""; 
common += ""; 
var maxLen = Math.max(big.length, common.length), 
bAry = big.split("").reverse(), 
cAry = common.split("").reverse(), 
i = 0, 
addToNext = 0, 
resAry = [], 
fn, 
sn, 
sum; 
for (; i <= maxLen; i++) { 
fn = parseInt(bAry[i] || 0); 
sn = parseInt(cAry[i] || 0); 
sum = fn + sn + addToNext; 
addToNext = (sum - sum % 10) / 10; 
resAry.unshift(sum % 10); 
} 
if (resAry[0] == 0) { 
resAry.splice(0, 1); 
} 
return resAry.join(""); 
}