98 lines
2.4 KiB
Markdown
98 lines
2.4 KiB
Markdown
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# thunky
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Delay the evaluation of a paramless async function and cache the result (see [thunk](http://en.wikipedia.org/wiki/Thunk_%28functional_programming%29)).
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```
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npm install thunky
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```
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[](http://travis-ci.org/mafintosh/thunky)
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## Example
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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
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``` js
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var thunky = require('thunky')
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var test = thunky(function (callback) { // the inner function should only accept a callback
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console.log('waiting 1s and returning random number')
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setTimeout(function () {
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callback(Math.random())
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}, 1000)
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})
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test(function (num) { // inner function is called the first time we call test
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console.log(num) // prints random number
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})
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test(function (num) { // subsequent calls waits for the first call to finish and return the same value
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console.log(num) // prints the same random number as above
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})
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```
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## Lazy evaluation
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Thunky makes it easy to implement a lazy evaluation pattern.
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``` js
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var getDb = thunky(function (callback) {
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db.open(myConnectionString, callback)
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})
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var queryDb = function (query, callback) {
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getDb(function (err, db) {
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if (err) return callback(err)
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db.query(query, callback)
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})
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}
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queryDb('some query', function (err, result) { ... } )
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queryDb('some other query', function (err, result) { ... } )
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```
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The first time `getDb` is called it will try do open a connection to the database.
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Any subsequent calls will just wait for the first call to complete and then call your callback.
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A nice property of this pattern is that it *easily* allows us to pass any error caused by `getDb` to the `queryDb` callback.
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## Error → No caching
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If the thunk callback is called with an `Error` object as the first argument it will not cache the result
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``` js
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var fails = thunky(function (callback) {
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console.log('returning an error')
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callback(new Error('bad stuff'))
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})
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fails(function (err) { // inner function is called
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console.log(err)
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});
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fails(function (err) { // inner function is called again as it returned an error before
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console.log(err)
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})
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```
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## Promise version
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A promise version is available as well
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``` js
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var thunkyp = require('thunky/promise')
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var ready = thunkyp(async function () {
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// ... do async stuff
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return 42
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})
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// same semantics as the callback version
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await ready()
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```
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## License
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MIT
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