thunky

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

npm install thunky

Example

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

1var thunky = require('thunky')
2
3var test = thunky(function (callback) { // the inner function should only accept a callback
4  console.log('waiting 1s and returning random number')
5  setTimeout(function () {
6    callback(Math.random())
7  }, 1000)
8})
9
10test(function (num) {  // inner function is called the first time we call test
11  console.log(num) // prints random number
12})
13
14test(function (num) {  // subsequent calls waits for the first call to finish and return the same value
15  console.log(num) // prints the same random number as above
16})

Lazy evaluation

Thunky makes it easy to implement a lazy evaluation pattern.

1var getDb = thunky(function (callback) {
2  db.open(myConnectionString, callback)
3})
4
5var queryDb = function (query, callback) {
6  getDb(function (err, db) {
7    if (err) return callback(err)
8    db.query(query, callback)
9  })
10}
11
12queryDb('some query', function (err, result) { ... } )
13
14queryDb('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

1var fails = thunky(function (callback) {
2  console.log('returning an error')
3  callback(new Error('bad stuff'))
4})
5
6fails(function (err) { // inner function is called
7  console.log(err)
8});
9
10fails(function (err) { // inner function is called again as it returned an error before
11  console.log(err)
12})

Promise version

A promise version is available as well

1var thunkyp = require('thunky/promise')
2
3var ready = thunkyp(async function () {
4  // ... do async stuff
5  return 42
6})
7
8// same semantics as the callback version
9await ready()

License

MIT