pruncalc/node_modules/d3-format/src/formatDecimal.js

export default function(x) {
  return Math.abs(x = Math.round(x)) >= 1e21
      ? x.toLocaleString("en").replace(/,/g, "")
      : x.toString(10);
}

// Computes the decimal coefficient and exponent of the specified number x with
// significant digits p, where x is positive and p is in [1, 21] or undefined.
// For example, formatDecimalParts(1.23) returns ["123", 0].
export function formatDecimalParts(x, p) {
  if (!isFinite(x) || x === 0) return null; // NaN, ±Infinity, ±0
  var i = (x = p ? x.toExponential(p - 1) : x.toExponential()).indexOf("e"), coefficient = x.slice(0, i);

  // The string returned by toExponential either has the form \d\.\d+e[-+]\d+
  // (e.g., 1.2e+3) or the form \de[-+]\d+ (e.g., 1e+3).
  return [
    coefficient.length > 1 ? coefficient[0] + coefficient.slice(2) : coefficient,
    +x.slice(i + 1)
  ];
}