Convert any number from base X to base X in Javascript

base-converter.js

function base_converter(nbasefrom, basefrom, baseto) {
	var SYMBOLS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	if (basefrom<=0 || basefrom>SYMBOLS.length || baseto<=0 || baseto>SYMBOLS.length) {
		console.log("Base unallowed");
		return null;
	}
	var i, nbaseten=0;
	if (basefrom!=10) {
		var sizenbasefrom = nbasefrom.length;
		for (i=0; i<sizenbasefrom; i++) {
			var mul, mul_ok=-1;
			for (mul=0; mul<SYMBOLS.length; mul++) {
				if (nbasefrom[i]==SYMBOLS[mul]) {
					mul_ok = 1;
					break;
				}
			}
			if (mul>=basefrom) {
				console.log("Symbol unallowed in basefrom");
				return null;
			}
			if (mul_ok==-1) {
				console.log("Symbol not found");
				return null;
			}
			var exp = (sizenbasefrom-i-1);	
			if (exp==0) nbaseten += mul;
			else nbaseten += mul*Math.pow(basefrom, exp);
		}
	} else nbaseten = parseInt(nbasefrom);
	if (baseto!=10) { 
		var nbaseto = [];
		while (nbaseten>0) {
			var mod = nbaseten%baseto;
			if (mod<0 || mod>=SYMBOLS.length) {
				console.log("Out of bounds error");
				return null;
			}
			nbaseto.push(SYMBOLS[mod]);
			nbaseten = parseInt(nbaseten/baseto);
		}
		return nbaseto.reverse().toString().replace(/,/g, '');
	} else {
		return nbaseten.toString();
	}
	return "0";
}

less precision for big integers due to limitation of parseInt

var x = "89999999179999999"; 
document.getElementById("input").innerHTML = "input : " + x;
x=base_converter(x,10,36);
document.getElementById("base36").innerHTML = "base36 : " + x;
document.getElementById("base10").innerHTML ="base10 : " + base_converter(x,36,10);

/*OUTPUT
input : 89999999179999999
base36 : OM6C4UCNKE8
base10 : 89999999180000000*/

@apanasara, did you ever make a solution for bigger strings?

Here's my solution that does not have the length limitations:

https://zamicol.github.io/BaseConverter/