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); 
} www.jbxue.com
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(""); 
} www.jbxue.com
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(""); 
}