Modulo in JavaScript - large number

Question

I try to calculate with JS' modulo function, but don't get the right result (which should be 1). Here is a hardcoded piece of code.

var checkSum = 210501700012345678131468;
alert(checkSum % 97);

Result: 66

Whats the problem here?

Regards, Benedikt

Answer 1

A bunch of improvements to Benedikt's version: cRest += '' + cDivident; is a bugfix; parseInt(divisor) makes it possible to pass both arguments as strings; check for empty string at the end makes it always return numerical values; added var statements so it's not using global variables; converted foreach to old-style for so it works in browsers with older Javascript; fixed the cRest == 0; bug (thanks @Dan.StackOverflow).

function modulo(divident, divisor) {
  let cDivident = '';
  let cRest = '';

  for (let i in divident) {
    let cChar = divident[i];
    let cOperator = cRest + '' + cDivident + '' + cChar;

    if (cOperator < parseInt(divisor)) {
      cDivident += '' + cChar;
    } else {
      cRest = cOperator % divisor;
      if (cRest == 0) {
        cRest = '';
      }
      cDivident = '';
    }
  }
  cRest += '' + cDivident;
  if (cRest == '') {
    cRest = 0;
  }
  return cRest;
}