We often fall into what is called "Callback Hell" when we write some asynchronous processes with Node.js, which is a problem the hierarchy of the code becomes too deep.
There are many countermeasures and it looks a little messy, so I will sort out them in several posts. This time, I will introduce the way of returning Promise objects which hold the running state in an asynchronous process instead of making a callback an argument.
// An example of a function returning Promise. var promiseFunction = function(message, timeout) { // Pass a function which has resolve and reject as arguments // when creating a Promise object. // * resolve: a function returning an object when the process succeeds // * reject: a function returning an error object when the process fails return new Promise(function(resolve, reject){ setTimeout(function() { console.log(message); resolve(message); // Call reject function when an error occurs. // reject(error); }, timeout); }); }; // The usage of a Promise instance var promise = promiseFunction('promise', 100); // then: write post-processing promise.then(function(result){ console.log("result: " + JSON.stringify(result)); // catch: write error processing }).catch(function(error){ console.log("error: " + JSON.stringify(error)); });
Result
$ node promise.js promise result: "promise"
