Reuse objects and functions with style

Usage no npm install needed!

<script type="module">
  import reusify from '';



npm version Build Status Coverage Status

Reuse your objects and functions for maximum speed. This technique will make any function run ~10% faster. You call your functions a lot, and it adds up quickly in hot code paths.

$ node benchmarks/createNoCodeFunction.js
Total time 53133
Total iterations 100000000
Iteration/s 1882069.5236482036

$ node benchmarks/reuseNoCodeFunction.js
Total time 50617
Total iterations 100000000
Iteration/s 1975620.838848608

The above benchmark uses fibonacci to simulate a real high-cpu load. The actual numbers might differ for your use case, but the difference should not.

The benchmark was taken using Node v6.10.0.

This library was extracted from fastparallel.


var reusify = require('reusify')
var fib = require('reusify/benchmarks/fib')
var instance = reusify(MyObject)

// get an object from the cache,
// or creates a new one when cache is empty
var obj = instance.get()

// set the state
obj.num = 100

// reset the state.
// if the state contains any external object
// do not use delete operator (it is slow)
// prefer set them to null
obj.num = 0

// store an object in the cache

function MyObject () {
  // you need to define this property
  // so V8 can compile MyObject into an
  // hidden class = null
  this.num = 0

  var that = this

  // this function is never reallocated,
  // so it can be optimized by V8
  this.func = function () {
    if (null) {
      // do nothing
    } else {
      // calculates fibonacci

The above example was intended for synchronous code, let's see async:

var reusify = require('reusify')
var instance = reusify(MyObject)

for (var i = 0; i < 100; i++) {
  getData(i, console.log)

function getData (value, cb) {
  var obj = instance.get()

  obj.value = value
  obj.cb = cb

function MyObject () { = null
  this.value = null

  var that = this = function () {
    asyncOperation(that.value, that.handle)

  this.handle = function (err, result) {
    that.cb(err, result)
    that.value = null
    that.cb = null

Also note how in the above examples, the code, that consumes an istance of MyObject, reset the state to initial condition, just before storing it in the cache. That's needed so that every subsequent request for an instance from the cache, could get a clean instance.


It is faster because V8 doesn't have to collect all the functions you create. On a short-lived benchmark, it is as fast as creating the nested function, but on a longer time frame it creates less pressure on the garbage collector.

Other examples

If you want to see some complex example, checkout middie and steed.


Thanks to Trevor Norris for getting me down the rabbit hole of performance, and thanks to Mathias Buss for suggesting me to share this trick.