Fast modular exponentiation in Java Script

fme.js

/**
 * Fast modular exponentiation for a ^ b mod n
 * @returns {number}
 */
var fastModularExponentiation = function(a, b, n) {
  a = a % n;
  var result = 1;
  var x = a;

  while(b > 0){
    var leastSignificantBit = b % 2;
    b = Math.floor(b / 2);

    if (leastSignificantBit == 1) {
      result = result * x;
      result = result % n;
    }

    x = x * x;
    x = x % n;
  }
  return result;
};

var assert = function(actual, expected){
  if (actual != expected){
    throw new Error('Assertion failed');
  }
};

assert(fastModularExponentiation(12, 53, 7), 3);
assert(fastModularExponentiation(7, 12, 10), 1);
assert(fastModularExponentiation(3, 51, 13), 1);

@devildelta that is because Javascript uses a floating point representation for numbers, in general you should not use Javascript for large arithmetic.

@y-richie-y @devildelta Now that chrome and node.js support BigInt natively, you can use javascript as well.
Simply append an n to every number literal to use BigInt:

const modExp = function (a, b, n) {
    a = a % n;
    var result = 1n;
    var x = a;
    while (b > 0) {
        var leastSignificantBit = b % 2n;
        b = b / 2n;
        if (leastSignificantBit == 1n) {
            result = result * x;
            result = result % n;
        }
        x = x * x;
        x = x % n;
    }
    return result;
};
let sum = 0n;
const digit_window = 10n**10n;
for(let i = 1n;i<=1000n;i++){
    sum = (sum + modExp(i,i,digit_window)) % digit_window
}
console.log(sum);
// 9110846700n