thunky

delay the evaluation of a paramless async function and cache the result

thunky

Delay the evaluation of a paramless async function and cache the result (see thunk).

npm install thunky

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 
});

Thunky makes it easy to implement a lazy evaluation pattern.

var getDb = thunky(function(callback) {
    db.open(myConnectionString, callback);
});
 
var queryDb = function(querycallback) {
    getDb(function(errdb) {
        if (err) return callback(err);
        db.query(query, callback);
    });
};
 
queryDb('some query', function(errresult) { ... } );
 
queryDb('some other query', function(errresult) { ... } );

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.

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);
});

MIT