formatDecimal.js

// 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, formatDecimal(1.23) returns ["123", 0].
export default function(x, p) {
  if ((i = (x = p ? x.toExponential(p - 1) : x.toExponential()).indexOf("e")) < 0) return null; // NaN, ±Infinity
  var i, 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)
  ];
}