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Delay the evaluation of a paramless async function and cache the result
JavaScript
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README.md
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README.md

thunky

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);
});

License

MIT

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