JarCastingthunky
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
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 () {
callback(Math.random())
}, 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) {
db.open(myConnectionString, 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
console.log(err)
});
fails(function (err) { // inner function is called again as it returned an error before
console.log(err)
})
Promise version
A promise version is available as well
var thunkyp = require('thunky/promise')
var ready = thunkyp(async function () {
// ... do async stuff
return 42
})
// same semantics as the callback version
await ready()
License
MIT