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# thunky
Delay the evaluation of a paramless async function and cache the result (see [thunk](http://en.wikipedia.org/wiki/Thunk_%28functional_programming%29)).
```
npm install thunky
```
[](http://travis-ci.org/mafintosh/thunky)
## Example
Let's make a simple function that returns a random number 1 second after it is called for the first time
``` js
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.
``` js
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
``` js
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
``` js
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