concat-reduce-1.js

// really slow: ~50s
try {
  return ret.concat.apply([], ret);
} catch (e) {
  if (e instanceof RangeError && ret.reduce) {
    return ret.reduce(function(a, b) {
      return a.concat(b);
    }, [])
  } else {
    throw e;
  }
}

concat-reduce-2.js

// also slow: ~45s
try {
  return ret.concat.apply([], ret);
} catch (e) {
  if (e instanceof RangeError) {
    var flat = [];
    for (var i = 0, il = ret.length; i < il; ++i) {
      flat = flat.concat(ret[i]);
    }
    return flat;
  } else {
    throw e;
  }
}

concat-reduce-3.js

// much better! ~2ms w/ native isArray, ~15ms w/out
try {
  return ret.concat.apply([], ret);
} catch (e) {
  if (e instanceof RangeError) {
    var flat = [], isArray = Array.isArray || function(obj) {
      return toString.call(obj) == '[object Array]';
    };
    for (var i = 0, il = ret.length; i < il; ++i) {
      var item = ret[i];
      if (isArray(item)) {
        flat = flat.concat(item);
      } else {
        flat.push(item);
      }
    }
    return flat;
  } else {
    throw e;
  }
}