You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
98 lines
2.4 KiB
98 lines
2.4 KiB
3 weeks ago
|
# 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
|