How to make a async.each loop break and return a specific value / error when a condition is met. For instance, in this case, if status exists i wish to return something from the function 'check' rather than iterating through rest of the items. Any suggestions?
const check = (condition) => {
if (condition === 100) {
async.each(items, (item, callback) => {
if (status) {
callback(Boom.conflict('Conflict'));
} else {
// some logic
callback(destroyInstance);
}
});
}
};
This looks like a duplicate of this question:
Break out of javascript nested async.each loop but continue main loop
Check out the answer, it should help you. In that case it is a nested async inside another async loop, but the answer should still work.
Edit:
The solution, as indicated in that thread but without the extra layers, is as follows:
async.each(subArray, function(item, callback) {
if (SOME_CONDITION) {
// Break out of async
var err = new Error('Broke out of async');
err.break = true;
return callback(err);
}
// continue
callback();
}, function(err) {
if (err && err.break) {
// Handle break out of async here
} else {
// Otherwise handle here
}
});
As async.each works parallely on each item in items array. It does not looks to be possible to stop the action at any condition, because the order of execution of items are not deterministic here. I guess you should use async.eachSeries instead.
So whenever the condition is met what you should do is throw an error (return callback(err)), what this will do is it will stop executing further. And if you want to send some value instead of throwing and error, You can just pretend to send an error and while checking in the main callback of async.eachSeries you just check the type of value u have got from the iteratee callback function as u can see for example:
async.eachSeries(items, function iteratee(item, callback) {
if (status) {
return callback(E/V); // error(E) or value(V) u want to send to the done function
} else {
// some logic
callback();
}
}, function done(e) {
if(e){
if(type of e === TYPE OF RETURN VALUE U WANTED) // or any other logic u can come up with
return cb(null, e);
else return cb(e);
}
else cb(null, //...);
});
Tell me if it works.
Related
so currently I'm setting up a little nodejs database and I'm basically trying to get all user info (if the user exists) and do a callback returning the user's info, if the user doesn't exist then I return null. Basically my issue here is that when I do return callback(null) or return callback(userdata) return doesn't stop code execution, resulting in 2 executions of the callback and invalid data. It could be due to the various callbacks that these return callback()'s are nested in or the foreach loop, although if it is, I'm not sure how to fix it. Any help is greatly appreciated, thank you!
Here is the code:
const getUserInfo = (sessionToken, callback) => {
return fs.readdir(__dirname + "/database/users", (err, files) => {
if(err) return callback(null);
return files.forEach((file, index) => {
return fs.readFile(__dirname + "/database/users/" + file, "UTF-8", (err, data) => {
if(err) return callback(null);
try {
const parsedData = JSON.parse(data);
if(parsedData.sessionToken === sessionToken) return callback({ email: parsedData.email, username: parsedData.username, bought: parsedData.bought });
else if(index === files.length - 1) return callback(null);
} catch {
return callback(null);
}
});
});
});
}
A little explanation for the code: this is a function that takes in a sessionToken, which is a random string of characters for the user's current session, and a callback of course. It then reads the database/users directory to get all the users, then for every user in that directory it reads the file and try's to parse the data through JSON, if it doesn't work then of course it returns null, if it does work and the sessionToken matches then it executes the callback with all the necessary user information. If we get to the end of the list and none of the sessions matched then we come to the conclusion the user doesn't exist.
Don't use a .forEach() loop. Use a plain for loop instead. .forEach() provides NO way to stop the loop. When you return from the .forEach() loop, you're just returning from the callback function, not returning from the parent function so there's no way to stop the loop or return from the parent function from within the .forEach() loop.
Using a regular for loop, you can either break or return to stop the loop.
In addition, since you're doing an asynchronous operation inside the loop, you also will need to use let data = await fs.promises.readFile() (and make the parent function async) in order to sequence your file reads one after the other. Without that, all your read operations will proceed in parallel and you will have no control over the order of completion. And, one you do that, you may as well return a promise from your function with the results, rather than use a plain callback function.
Here's an example implementation:
const getUserInfo = async (sessionToken) => {
let files = await fs.promises.readdir(__dirname + "/database/users");
for (let file of files) {
let data = await fs.promises.readFile(__dirname + "/database/users/" + file, "UTF-8");
const parsedData = JSON.parse(data);
if (parsedData.sessionToken === sessionToken) {
return { email: parsedData.email, username: parsedData.username, bought: parsedData.bought };
}
}
return null;
}
// usage
getUserInfo(token).then(val => {
if (val) {
console.log('Got user data', val);
} else {
console.log('User data not found');
}
}).catch(err => {
console.log(err);
})
I have 2 functions that I'm running asynchronously. I'd like to write them using waterfall model. The thing is, I don't know how..
Here is my code :
var fs = require('fs');
function updateJson(ticker, value) {
//var stocksJson = JSON.parse(fs.readFileSync("stocktest.json"));
fs.readFile('stocktest.json', function(error, file) {
var stocksJson = JSON.parse(file);
if (stocksJson[ticker]!=null) {
console.log(ticker+" price : " + stocksJson[ticker].price);
console.log("changing the value...")
stocksJson[ticker].price = value;
console.log("Price after the change has been made -- " + stocksJson[ticker].price);
console.log("printing the the Json.stringify")
console.log(JSON.stringify(stocksJson, null, 4));
fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function(err) {
if(!err) {
console.log("File successfully written");
}
if (err) {
console.error(err);
}
}); //end of writeFile
} else {
console.log(ticker + " doesn't exist on the json");
}
});
} // end of updateJson
Any idea how can I write it using waterfall, so i'll be able to control this? Please write me some examples because I'm new to node.js
First identify the steps and write them as asynchronous functions (taking a callback argument)
read the file
function readFile(readFileCallback) {
fs.readFile('stocktest.json', function (error, file) {
if (error) {
readFileCallback(error);
} else {
readFileCallback(null, file);
}
});
}
process the file (I removed most of the console.log in the examples)
function processFile(file, processFileCallback) {
var stocksJson = JSON.parse(file);
if (stocksJson[ticker] != null) {
stocksJson[ticker].price = value;
fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
if (err) {
processFileCallback(error);
} else {
console.log("File successfully written");
processFileCallback(null);
}
});
}
else {
console.log(ticker + " doesn't exist on the json");
processFileCallback(null); //callback should always be called once (and only one time)
}
}
Note that I did no specific error handling here, I'll take benefit of async.waterfall to centralize error handling at the same place.
Also be careful that if you have (if/else/switch/...) branches in an asynchronous function, it always call the callback one (and only one) time.
Plug everything with async.waterfall
async.waterfall([
readFile,
processFile
], function (error) {
if (error) {
//handle readFile error or processFile error here
}
});
Clean example
The previous code was excessively verbose to make the explanations clearer. Here is a full cleaned example:
async.waterfall([
function readFile(readFileCallback) {
fs.readFile('stocktest.json', readFileCallback);
},
function processFile(file, processFileCallback) {
var stocksJson = JSON.parse(file);
if (stocksJson[ticker] != null) {
stocksJson[ticker].price = value;
fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
if (!err) {
console.log("File successfully written");
}
processFileCallback(err);
});
}
else {
console.log(ticker + " doesn't exist on the json");
processFileCallback(null);
}
}
], function (error) {
if (error) {
//handle readFile error or processFile error here
}
});
I left the function names because it helps readability and helps debugging with tools like chrome debugger.
If you use underscore (on npm), you can also replace the first function with _.partial(fs.readFile, 'stocktest.json')
First and foremost, make sure you read the documentation regarding async.waterfall.
Now, there are couple key parts about the waterfall control flow:
The control flow is specified by an array of functions for invocation as the first argument, and a "complete" callback when the flow is finished as the second argument.
The array of functions are invoked in series (as opposed to parallel).
If an error (usually named err) is encountered at any operation in the flow array, it will short-circuit and immediately invoke the "complete"/"finish"/"done" callback.
Arguments from the previously executed function are applied to the next function in the control flow, in order, and an "intermediate" callback is supplied as the last argument. Note: The first function only has this "intermediate" callback, and the "complete" callback will have the arguments of the last invoked function in the control flow (with consideration to any errors) but with an err argument prepended instead of an "intermediate" callback that is appended.
The callbacks for each individual operation (I call this cbAsync in my examples) should be invoked when you're ready to move on: The first parameter will be an error, if any, and the second (third, fourth... etc.) parameter will be any data you want to pass to the subsequent operation.
The first goal is to get your code working almost verbatim alongside the introduction of async.waterfall. I decided to remove all your console.log statements and simplified your error handling. Here is the first iteration (untested code):
var fs = require('fs'),
async = require('async');
function updateJson(ticker,value) {
async.waterfall([ // the series operation list of `async.waterfall`
// waterfall operation 1, invoke cbAsync when done
function getTicker(cbAsync) {
fs.readFile('stocktest.json',function(err,file) {
if ( err ) {
// if there was an error, let async know and bail
cbAsync(err);
return; // bail
}
var stocksJson = JSON.parse(file);
if ( stocksJson[ticker] === null ) {
// if we don't have the ticker, let "complete" know and bail
cbAsync(new Error('Missing ticker property in JSON.'));
return; // bail
}
stocksJson[ticker] = value;
// err = null (no error), jsonString = JSON.stringify(...)
cbAsync(null,JSON.stringify(stocksJson,null,4));
});
},
function writeTicker(jsonString,cbAsync) {
fs.writeFile('stocktest.json',jsonString,function(err) {
cbAsync(err); // err will be null if the operation was successful
});
}
],function asyncComplete(err) { // the "complete" callback of `async.waterfall`
if ( err ) { // there was an error with either `getTicker` or `writeTicker`
console.warn('Error updating stock ticker JSON.',err);
} else {
console.info('Successfully completed operation.');
}
});
}
The second iteration divides up the operation flow a bit more. It puts it into smaller single-operation oriented chunks of code. I'm not going to comment it, it speaks for itself (again, untested):
var fs = require('fs'),
async = require('async');
function updateJson(ticker,value,callback) { // introduced a main callback
var stockTestFile = 'stocktest.json';
async.waterfall([
function getTicker(cbAsync) {
fs.readFile(stockTestFile,function(err,file) {
cbAsync(err,file);
});
},
function parseAndPrepareStockTicker(file,cbAsync) {
var stocksJson = JSON.parse(file);
if ( stocksJson[ticker] === null ) {
cbAsync(new Error('Missing ticker property in JSON.'));
return;
}
stocksJson[ticker] = value;
cbAsync(null,JSON.stringify(stocksJson,null,4));
},
function writeTicker(jsonString,cbAsync) {
fs.writeFile('stocktest.json',jsonString,,function(err) {
cbAsync(err);
});
}
],function asyncComplete(err) {
if ( err ) {
console.warn('Error updating stock ticker JSON.',err);
}
callback(err);
});
}
The last iteration short-hands a lot of this with the use of some bind tricks to decrease the call stack and increase readability (IMO), also untested:
var fs = require('fs'),
async = require('async');
function updateJson(ticker,value,callback) {
var stockTestFile = 'stocktest.json';
async.waterfall([
fs.readFile.bind(fs,stockTestFile),
function parseStockTicker(file,cbAsync) {
var stocksJson = JSON.parse(file);
if ( stocksJson[ticker] === null ) {
cbAsync(new Error('Missing ticker property in JSON.'));
return;
}
cbAsync(null,stocksJson);
},
function prepareStockTicker(stocksJson,cbAsync) {
stocksJson[ticker] = value;
cbAsync(null,JSON.stringify(stocksJson,null,4));
},
fs.writeFile.bind(fs,stockTestFile)
],function asyncComplete(err) {
if ( err ) {
console.warn('Error updating stock ticker JSON.',err);
}
callback(err);
});
}
Basically nodejs (and more generally javascript) functions that require some time to execute (be it for I/O or cpu processing) are typically asynchronous, so the event loop (to make it simple is a loop that continuously checks for tasks to be executed) can invoke the function right below the first one, without getting blocked for a response. If you are familiar with other languages like C or Java, you can think an asynchronous function as a function that runs on another thread (it's not necessarily true in javascript, but the programmer shouldn't care about it) and when the execution terminates this thread notifies the main one (the event loop one) that the job is done and it has the results.
As said once the first function has ended its job it must be able to notify that its job is finished and it does so invoking the callback function you pass to it. to make an example:
var callback = function(data,err)
{
if(!err)
{
do something with the received data
}
else
something went wrong
}
asyncFunction1(someparams, callback);
asyncFunction2(someotherparams);
the execution flow would call: asyncFunction1, asyncFunction2 and every function below until asyncFunction1 ends, then the callback function which is passed as the last parameter to asyncFunction1 is called to do something with data if no errors occurred.
So, to make 2 or more asynchronous functions execute one after another only when they ended you have to call them inside their callback functions:
function asyncTask1(data, function(result1, err)
{
if(!err)
asyncTask2(data, function(result2, err2)
{
if(!err2)
//call maybe a third async function
else
console.log(err2);
});
else
console.log(err);
});
result1 is the return value from asyncTask1 and result2 is the return value for asyncTask2. You can this way nest how many asynchronous functions you want.
In your case if you want another function to be called after updateJson() you must call it after this line:
console.log("File successfully written");
I am using Javascript, webdriverio (v2.1.2) to perform some data extraction from an internal site. So the idea is
Authenticate
Open the required URL, when authenticated
In the new page, search for an anchor tag having specific keyword
Once found, click on the anchor tag
Below is what I have tried and it works (last two points). I had to use Q and async to achieve it. I was hoping to use only Q to achieve it. Can someone help me, on how to achieve it using Q only ??
var EmployeeAllocationDetails = (function () {
'use stricy';
/*jslint nomen: true */
var Q = require('Q'),
async = require('async'),
_ead_name = 'Employee Allocation Details',
goToEadFromHome;
goToEadFromHome = function (browserClient) {
browserClient.pause(500);
var deferred = Q.defer();
browserClient.elements('table.rmg td.workListTD div.tab2 div.contentDiv>a', function (err, results) {
if (err) {
deferred.reject(new Error('Unable to get EAD page. ' + JSON.stringify(err)));
} else {
async.each(results.value, function (oneResult, callback) {
console.log('Processing: ' + JSON.stringify(oneResult));
browserClient.elementIdText(oneResult.ELEMENT, function (err, result) {
if (err) {
if (err.message.indexOf('referenced element is no longer attached to the DOM') > -1 ){
callback();
} else {
callback('Error while processing :' + JSON.stringify(oneResult) + '. ' + err);
}
} else if(!result){
console.log('result undefined. Cannot process: ' + JSON.stringify(oneResult));
callback();
} else if(result.value.trim() === _ead_name){
deferred.resolve(oneResult);
callback();
}
});
}, function (err) {
// if any of the processing produced an error, err would equal that error
if( err ) {
// One of the iterations produced an error.
// All processing will now stop.
console.log('A processing failed to process. ' + err);
} else {
console.log('All results have been processed successfully');
}
}); //end of async.each
}
});
return deferred.promise;
};
return {
launchEad : goToEadFromHome
}
})();
module.exports = EmployeeAllocationDetails;
Related Github Issue link https://github.com/webdriverio/webdriverio/issues/123
I think you should use async. I think your code is great. It runs everything in parallel and it handles error well.
If
If you want to remove async, there are several options:
use Q flow control
copy paste async's implementation
implement it yourself
If you try to use Q's flow control it will look something like this (pseudo-code):
var getTextActions = [];
function createAction(element){
return function(){
return element.getText();
}
}
each(elements, function(element){ getTextActions.push( createAction(element) ) });
Q.all(getTextActions).then(function(results) {
... iterate all results and resolve promise with first matching element..
} );
note this implementation has worse performance. It will first get the text from all elements, and only then try to resolve your promise. You implementation is better as it all runs in parallel.
I don't recommend implementing it yourself, but if you still want to, it will look something like this (pseudo-code):
var elementsCount = elements.length;
elements.each(function(element){
element.getText(function(err, result){
elementsCount --;
if ( !!err ) { logger.error(err); /** async handles this much better **/ }
if ( isThisTheElement(result) ) { deferred.resolve(result); }
if ( elementsCount == 0 ){ // in case we ran through all elements and didn't find any
deferred.resolve(null); // since deferred is resolved only once, calling this again if we found the item will have no effect
}
})
})
if something is unclear, or if I didn't hit the spot, let me know and I will improve the answer.
I have many calls to a service at the end of which i want to write to a file my final collection when all the callbacks of the service have returned.
is there there a way to be sure that all callbacks are done ?
for (id in idsCollection) {
object.callService(id, function (res) {
collection.push(res);
});
}
filewriter.writetoFile("filename.json", JSon.Stringify(collection));
EDIT : just for the record i'm using cheerio with nodeJS.
Create an array. Push something onto the array each time you set up a callback. Pop something off it each time the callback runs. Check to see if the array is empty inside the callback function. If it is empty, then all the callbacks are done.
I typically use the node-async library for this sort of thing. It makes it easy to do exactly what you're talking about:
async.each(yourArray,
function(element, next) {
// this callback gets called for each element in your array
element.doSomething(function(returnValue){
next(returnValue) // call next when you're done
}
}, function(err, returnValues) {
// when all the elements in the array are processed, this is called
if (err) return console.log(err);
console.log(returnValues) // this is an array of the returnValues
});
})
You could simply count them. In your case it seems you already know how many callbacks there are going to be.
var remaining = idsCollection.length; // assuming array
for (id in idsCollection) {
object.callService(id, function (res) {
collection.push(res);
remaining -= 1; // decrement by 1 per callback
// here you can check if remaining === 0 (all done)
});
}
you can use nimble lib http://caolan.github.io/nimble/.
nimble paralel example
var _ = require('nimble');
_.parallel([
function (callback) {
setTimeout(function () {
console.log('one');
callback();
}, 25);
},
function (callback) {
setTimeout(function () {
console.log('two');
callback();
}, 0);
}
], function(){
console.log('done')
});
output
> two
> one
> done
I see many answers here, but I hope that this solution may still help someone.
Create a promise for each callback to be extinguished as such:
function funcToLoop(arg){
return new Promise((resolve, reject) => {
try{
funcWithCallback(arg, (cbArg) => {
// do your stuff
resolve(cbArg)
});
} catch (e) {
reject(e)
}
});
}
Then, you can create a loop as a async function and handle eventual results/states/etc here:
async function mainLoop(array){
let results = [];
for (let arg of array){
results.push(await funcToLoop(arg))
}
// handle results
}
... or you can have a sync function, collect the promises and handle them:
function mainLoop(array){
let promises = [];
for (let arg of array){
promises.push(funcToLoop(arg))
}
Promise.all(promises).then(()=>{
// handle promises
})
}
Claudio
jQuery.Deferred() objects might be what you are looking for.
OR if you are using HTML5 you can use promises .
Here is how to create promises
var promise = new Promise(function(resolve, reject) {
// do a thing, possibly async, then…
if (/* everything turned out fine */) {
resolve("Stuff worked!");
}
else {
reject(Error("It broke"));
}
});
And here is how to use them
promise.then(function(result) {
console.log(result); // "Stuff worked!"
}, function(err) {
console.log(err); // Error: "It broke"
});
Check this link for more info
If you're using jQuery, you can use $.when
Example:
exmCall1 = $.getJson(..);
exmCall2 = $.getJson(..);
$.when(exmCall1, exmCall2).done(function (exmCall1Ret, exmCall2Ret) {
//do stuff
});
You can read the actual documentation here: http://api.jquery.com/jquery.when/
Or do some hardcode:
var running;
for (id in idsCollection) {
object.callService(id, function (res) {
collection.push(res);
running += 1;
});
}
var loop = setInterval(function() {
if(running >= idsCollection.length) {
filewriter.writetoFile("filename.json", JSon.Stringify(collection));
clearInterval(loop);
}
, 500);
As a novice in Javascript, I'm confused on which could be the best way to differentiate between the result computed by an asynchronous function, and any exception/error.
If I'm right, you cannot use try-catch in this scenario, as the called function
ends before the callback, and it is this latter who actually may throw an exception.
Well.
I've seen so far some library functions expecting a callback like: function(err, result).
So, one have to test err before using result.
Also I tried myself to return either the actual result or an Error object.
Here, the callback is of the form function(result)
and you have to test result instanceof Error before using it.
It follows an example of this:
function myAsyncFunction ( callBack ) {
async_library_function( "some data", function (err, result) {
if (err) { callBack ( new Error ("my function failed") ); return; }
callBack ( some calculation with result );
});
} // myFunction ()
//
// calling myFunction
//
myAsyncFunction ( function (result) {
if (result instanceof Error ) { log ("some error occurred"); return; }
log ("Alright, this is the result: " + result);
});
What is the best (maybe the common) way to do this?
Thanks.
There are three main approaches that I've been using myself:
Having an "error" parameter passed to the callback.
Having an "error" callback. This is usually combined with (1).
Having some sort of global exception manager.
I'll start with the third one. The idea is to have an object that will allow dispatching errors as well as catching them globally. Something like this:
var ErrorManager = {
subscribers: [],
subscribe: function (callback) {
this.subscribers.push(callback);
},
dispatchError: function (error) {
this.subscribers.forEach(function (s) {
s.apply(window, [ error ]);
});
}
}
This is quite specific to a given situation because there's basically no easy way of tracking the origin of an error as well as it's easy to mess up with this. For example, if you need to hide a dialog box whose contents failed to load, you'd have to propagate this information (e.g. dialog box Id/element) to all the subscribers.
The above approach is good when you want to execute an action that doesn't alter (or alters an independent part) of the web application (e.g. displays a status bar or a message to a console).
The second approach basically makes a separation between successful call and a failure. For example:
$.ajax('/articles', {
success: function () { /* All is good - populating article list */ },
error: function () { /* An error occured */ }
});
In the above example, the success callback is never executed in case of a failure so if you want to have some default behavior to always trigger, you'd need to either sync between the two callbacks or have a callback that is always called (for the above example - complete).
I personally prefer the first approach - having a callback where you have an error object passed along with potential result. This eliminates problems with having to "track" the origin/context on an error as well as worrying about the clean-up procedure (default behavior). So, something like you provided:
function beginFetchArticles(onComplete) {
$.ajax('/articles', {
complete: function (xhr) {
onComplete(xhr.status != 200 ? xhr.status.toString() : null,
$.parseJSON(xhr.responseText)); /* Something a bit more secure, probably */
}
});
}
Hope this helps.
It depends vastly on your implementation. Is this a recoverable error? If it isn't, then the way you are suggesting should work just fine. If it is recoverable then you shouldn't be returning an error. You should be returning an "empty" result. Keep in mind maintainability as well. Do you want instanceof checks throughout the code? Also, I know some programmers like that JavaScript is loose with types, but you run into consistency issues when the expected object passed through can actually be unexpected. Is it a result, or an error, or even something else altogether?
That's one way to do it. Though I'd usually leave any manipulations/processing of the result to the callback function.
Another way is you can pass back both the error and result values to the callback:
callback (err, result); \\ if no error, err = null, if no result, result = null
Alternatively, you can ask for separate error and success callbacks:
function myAsyncFunction ( successCallBack, errorCallBack ) {
\* ... *\
}
And then trigger the appropriate function depending on the received response.
One approach can be like this:
function Exception(errorMessage)
{
this.ErrorMessage = errorMessage;
this.GetMessage = function()
{
return this.ErrorMessage;
}
}
function ResultModel(value, exception)
{
exception = typeof exception == "undefined"? null, exception;
this.Value = value;
this.Exception = exception;
this.GetResult = function()
{
if(exception === null)
{
return this.Value;
}
else
{
throw this.Exception;
}
}
};
And in your usage:
function myAsyncFunction ( callBack ) {
var result;
async_library_function( "some data", function (err, result) {
if (err)
{
result = new ResultModel(null, new Exception("my function failed"));
}
else
{
result = new ResultModel(some calculation with result);
}
callBack ( result );
});
}
myAsyncFunction ( function (result) {
try
{
log ("Alright, this is the result: " + result.GetResult());
}
catch(ex)
{
log ("some error occurred" + ex.GetMessage());
return;
}
});
If you want to make robust programs, you should use promises. Otherwise you have to handle 2 different kinds of errors which is pretty crazy.
Consider how to read a file as JSON without crashing the server:
var fs = require("fs");
fs.readFile("myfile.json", function(err, contents) {
if( err ) {
console.error("Cannot read file");
}
else {
try {
var result = JSON.parse(contents);
console.log(result); //Or continue callback hell here
}
catch(e) {
console.error("Invalid json");
}
}
});
With promises e.g:
var Promise = require("bluebird");
var readFile = Promise.promisify(require("fs").readFile);
readFile("myfile.json").then(JSON.parse).then(function(result){
console.log(result);
}).catch(SyntaxError, function(e){
console.error("Invalid json");
}).catch(function(e){
console.error("Cannot read file");
});
Notice also how the code grows vertically like with synchronous code instead of horizontally.