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delay the evaluation of a paramless async function and cache the result


Delay the evaluation of a paramless async function and cache the result (see thunk).

npm install thunky

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Let's make a simple function that returns a random number 1 second after it is called for the first time

var thunky = require('thunky')
var test = thunky(function (callback) { // the inner function should only accept a callback 
  console.log('waiting 1s and returning random number')
  setTimeout(function () {
  }, 1000)
test(function (num) {  // inner function is called the first time we call test 
  console.log(num) // prints random number 
test(function (num) {  // subsequent calls waits for the first call to finish and return the same value 
  console.log(num) // prints the same random number as above 

Lazy evaluation

Thunky makes it easy to implement a lazy evaluation pattern.

var getDb = thunky(function (callback) {, callback)
var queryDb = function (query, callback) {
  getDb(function (err, db) {
    if (err) return callback(err)
    db.query(query, callback)
queryDb('some query', function (err, result) { ... } )
queryDb('some other query', function (err, result) { ... } )

The first time getDb is called it will try do open a connection to the database. Any subsequent calls will just wait for the first call to complete and then call your callback.

A nice property of this pattern is that it easily allows us to pass any error caused by getDb to the queryDb callback.

Error → No caching

If the thunk callback is called with an Error object as the first argument it will not cache the result

var fails = thunky(function (callback) {
  console.log('returning an error')
  callback(new Error('bad stuff'))
fails(function (err) { // inner function is called 
fails(function (err) { // inner function is called again as it returned an error before