Universal polyfill for Math.fround, does not depend on Float32Array.

fround.js

// NOTE: This implementation is incorrect (it doesn't break ties to even).  See comments below for discussion.

Math.fround = Math.fround || function(arg) {
  arg = Number(arg);
  // Return early for ±0 and NaN.
  if (!arg) return arg;
  var sign = arg < 0 ? -1 : 1;
  if (sign < 0) arg = -arg;
  // Compute the exponent (8 bits, signed).
  var exp = Math.floor(Math.log(arg) / Math.LN2);
  var powexp = Math.pow(2, Math.max(-126, Math.min(exp, 127)));
  // Handle subnormals: leading digit is zero if exponent bits are all zero.
  var leading = exp < -127 ? 0 : 1;
  // Compute 23 bits of mantissa, inverted to round toward zero.
  var mantissa = Math.round((leading - arg / powexp) * 0x800000);
  if (exp > 0 && mantissa <= -0x800000) return sign * Infinity;
  return sign * powexp * (leading - mantissa / 0x800000);
};

fround_test.js

it('should handle terminating numbers', function() {                                                                                                                     [5/32762]
  // These cases all have terminating binary representations (i.e. integers
  // or rationals with powers of two in the denominator).
  assertPositiveZero(Math.fround(0));
  assertNegativeZero(Math.fround(-0));
  assertEquals(1, Math.fround(1));
  assertEquals(-1, Math.fround(-1));
  assertEquals(0.5, Math.fround(0.5));
  assertEquals(0.25, Math.fround(0.25));
  assertEquals(-0.75, Math.fround(-0.75));
  assertEquals(3, Math.fround(3));
  assertEquals(-20.375, Math.fround(-20.375));
  assertEquals(101.3125, Math.fround(101.3125));
  assertEquals(1 << 22, Math.fround(1 << 22));
});

it('should handle large numbers', function() {
  assertEquals(1 << 30, Math.fround(1 << 30));
  assertEquals(2 ** 127, Math.fround(2 ** 127));
  assertEquals(-(2 ** 127), Math.fround(-(2 ** 127)));
  assertEquals(1.875 * (2 ** 127), Math.fround(1.875 * (2 ** 127)));
  assertEquals(-1.9375 * (2 ** 127), Math.fround(-1.9375 * (2 ** 127)));
  assertEquals('a', Infinity, Math.fround(2 ** 128));
  assertEquals(-Infinity, Math.fround(-(2 ** 128)));
  const maxFloat = 3.4028234663852886e38;
  assertEquals(maxFloat, Math.fround(3.4028235e38));
  assertEquals(Infinity, Math.fround(3.4028236e38));
  assertEquals('b', Infinity, Math.fround(Infinity));
  assertEquals(-maxFloat, Math.fround(-3.4028235e38));
  assertEquals(-Infinity, Math.fround(-3.4028236e38));
  assertEquals(-Infinity, Math.fround(-Infinity));
});

it('should handle small numbers', function() {
  // Smallest normal float32
  assertEquals(1.015625 * 2 ** -126, Math.fround(1.015625 * 2 ** -126));
  assertEquals(-1.015625 * 2 ** -126, Math.fround(-1.015625 * 2 ** -126));
  // Subnormal numbers
  assertEquals(1.015625 * 2 ** -127, Math.fround(1.015625 * 2 ** -127));
  assertEquals(1.875 * 2 ** -128, Math.fround(1.875 * 2 ** -128));
  // Numbers exactly between two floats round toward zero
  const minFloat = 2 ** -149;
  assertEquals(12 * minFloat, Math.fround(12.5 * minFloat));
  assertEquals(13 * minFloat, Math.fround(12.5 * (1 + EPSILON) * minFloat));
  assertEquals(-12 * minFloat, Math.fround(-12.5 * minFloat));
  assertEquals(-13 * minFloat, Math.fround(-12.5 * (1 + EPSILON) * minFloat));
  // Smallest non-zero float32
  assertEquals(minFloat, Math.fround(minFloat));
  assertEquals(-minFloat, Math.fround(-minFloat));
  assertEquals(minFloat, Math.fround((1 + EPSILON) / 2 * minFloat));
  assertEquals(-minFloat, Math.fround(-(1 + EPSILON) / 2 * minFloat));
  assertPositiveZero(Math.fround(minFloat / 2));
  assertNegativeZero(Math.fround(-minFloat / 2));
  // Edge cases around mantissa === -0x800000
  assertEquals(2 ** -125, Math.fround(2 ** -125));
  assertEquals(2 ** -124, Math.fround(2 ** -124));
});

it('should handle non-numbers', function() {
  assertEquals(1, Math.fround(noCheck('1')));
  assertExactlyNaN(Math.fround(noCheck([1, 2])));
  assertExactlyNaN(Math.fround(noCheck('a')));
  assertExactlyNaN(Math.fround(NaN));
});

Thanks for finding that - I added a comment to the top of the gist to hopefully direct anyone to your comment.