function nFormatter(num, digits) {
  const lookup = [
    { value: 1, symbol: "" },
    { value: 1e3, symbol: "k" },
    { value: 1e6, symbol: "M" },
    { value: 1e9, symbol: "G" },
    { value: 1e12, symbol: "T" },
    { value: 1e15, symbol: "P" },
    { value: 1e18, symbol: "E" }
  ];
  const rx = /\.0+$|(\.[0-9]*[1-9])0+$/;
  var item = lookup.slice().reverse().find(function(item) {
    return num >= item.value;
  });
  return item ? (num / item.value).toFixed(digits).replace(rx, "$1") + item.symbol : "0";
}

/*
 * Tests
 */
const tests = [
  { num: 0, digits: 1 },
  { num: 12, digits: 1 },
  { num: 1234, digits: 1 },
  { num: 100000000, digits: 1 },
  { num: 299792458, digits: 1 },
  { num: 759878, digits: 1 },
  { num: 759878, digits: 0 },
  { num: 123, digits: 1 },
  { num: 123.456, digits: 1 },
  { num: 123.456, digits: 2 },
  { num: 123.456, digits: 4 }
];
tests.forEach(function(test) {
  console.log("nFormatter(" + test.num + ", " + test.digits + ") = " + nFormatter(test.num, test.digits));
});