77 lines
2.0 KiB
Markdown
77 lines
2.0 KiB
Markdown
# 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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npm install 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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## License
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MIT |