go's defer statement in JavaScript

Downloads in past


0.5.13 years ago7 years agoMinified + gzip package size for golike-defer in KB


Node compatibility PackagePhobia
Package Version Build Status Latest Commit
go's defer statement in JavaScript

defer() is a function decorator which injects a $defer() function as a first parameter.
This injected function can be used to register deferreds, functions which will be executed at the end of the decorated function execution, no matter how it ended (via a return or a throw).
Note: the deferreds are executed in the reverse order of their declarations.


Installation of the npm package:
> npm install --save golike-defer


import { defer } from 'golike-defer'

const fn = defer(
  // Works both with sync and async functions
  async function($defer, ...args) {
    $defer(() => {
      console.log("always called at the end of the function");

    $defer.onFailure(() => {
      console.log("called at the end of the function only on failure");

    $defer.onSuccess(() => {
      console.log("called at the end of the function only on success");

Context and arguments can be passed to the deferred function:
  • $defer(cb): called without context nor arguments
  • $defer(cb, arg1, arg2): called with arguments arg1 and arg2
  • $defer.call(thisArg, cb): called with context thisArg
  • $defer.call(thisArg, 'method'): thisArg.method called with context thisArg


import { defer } from 'golike-defer'
import fs from 'fs'

const readFileSync = defer(($defer, path) => {
  const fd = fs.openSync(path, 'r')

  // The file will be automatically closed at the end of the function,
  // whether it succeed or failed.
  $defer(fs.closeSync, fd)

  const { size } = fs.statSync(path)

  const buffer = Buffer.allocUnsafe(size)
  fs.readSync(fd, buffer, 0, buffer.length)

  return buffer

// Helper to promisify a function call.
const fromCallback = fn => new Promise((resolve, reject) => {
  fn((error, result) => error ? reject(error) : resolve(result))

const readFile = defer(async ($defer, path) => {
  const fd = await fromCallback(cb => fs.open(path, 'r', cb))

  // The file will be automatically closed at the end of the function,
  // whether it succeed or failed.
  $defer(() => fromCallback(cb => fs.close(fd, cb)))

  const { size } = await fromCallback(cb => fs.stat(path, cb))

  const buffer = Buffer.allocUnsafe((size)
  await fromCallback(cb => fs.read(fd, buffer, 0, buffer.length, cb))

  return buffer

On error

Exceptions (or rejected promises) thrown in deferred are caught and printed on the console.
This can be customized with the onError() method:
const myDefer = defer.onError(error => {

const fn = myDefer(($defer, arg1, arg2) => {
  // ...


# Install dependencies
> npm

# Run the tests
> npm test

# Continuously compile
> npm run dev

# Continuously run the tests
> npm run dev-test

# Build for production (automatically called by npm install)
> npm run build


Contributions are very welcomed, either on the documentation or on the code.
You may:
you've encountered;
  • fork and create a pull request.


ISC © Julien Fontanet