For a project, I am using the net module to create a 'mini web framework'
I'm having a lot of trouble dealing with this one callback
var sendFile(path) {
fs.readFile(path, config, this.handleRead.bind(this));
}
where readFile is defined as:
var handleRead = function(contentType, data, err) {
if (err) {
console.log(err); //returns properly
} else {
console.log(data); //returns properly
console.log(contentType) //returning undefined
}
So far this code works in the sense that I can catch errors and also write the data properly.
My question is : how do I send the contentType through the call back?
I've tried -
var sendFile(path) {
var contentType = ContentType['the path type']
fs.readFile(path, config, this.handleRead(contentType).bind(this));
}
But then this causes data and err to be undefined.
I'm quite new to js and am still confused about how to work with callbacks. Any input is appreciated!
.bind() lets you do more than just set the "context" (this value of a function). You can also "bind" arguments in the function.
Try:
function sendFile(path) {
var contentType = ContentType['the path type']
fs.readFile(path, config, this.handleRead.bind(this, contentType));
}
This will pass a callback with its context set to whatever this is and its 1st parameter set to contentType. As long as this callback is called with data (and possibly err), then everything will work.
Related
I am trying to understand more on how callback functions work in JavaScript.
I found this simple example code online and after adding the imports & additional txt file it's working. However I am not sure why it is working as the parameters of the functions aren't being defined anywhere?
fs.readFile(`${__dirname}/temp.txt`, (err, data) => {
console.log(`Breed: ${data}`);
superagent.get(`https://dog.ceo/api/breed/${data}/images/random`).end((err,res) =>{
console.log(res.body)
}); })
Upon running this I get the expected result on the console with the line displaying Breed: ${data} and the next line showing the body of the res.
However, the err, data parameters have never been provided to the arrow function within readFile. Neither has the err,res parameters ever been called/provided a value for it to display a message to the console, then how are these lines displaying a message to the console?
I could have understood this working if .readFile returned a err and data value & .end returned a err and res value, however upon reading the documentation both these functions return void, thus i'm confused and help would be appreciated.
You pass a function in to readFile, and later on readFile will call your function. When it calls your function, readFile will pass in the relevant values.
You can do something similar yourself, like this:
function myReadFile(filename, callback) {
setTimeout(() => {
if (Math.random() > 0.5) {
callback(null, 'fake file');
} else {
callback('an error', null);
}
}, 1000);
}
myReadFile(`${__dirname}/temp.txt`, (err, data) => {
console.log(`Breed: ${data}`);
})
(though of course the real readFile function isn't doing timeouts or random numbers, but rather reading from a file and listening for that operation to succeed or error out)
so what I want to achieve is loading an array that holds objects into json so that I can parse it back into the program and using it anywhere in my program. I have tried the fs way of doing this but I soon found out that I cannot use the variables outside the async, callback function when I read it. I am asking if there is a better method of loading my array full of objects into json, reading it and being able to use these variables anywhere in my program. I would also like some insight as I would like to learn more about a topic I am a rookie with, thank you!
my problem is that I cannot access variables from an async callback function as it returns undefined when I call it out of the function itself.
class Blockchain {
constructor() {
this.chain = [Blockchain.getGenesis()];
console.log(Blockchain.getGenesis());
}
//further into the code with the issued function reading the data from .json to //place the objects into variables for the program to use
static getGenesis(genChain) {
fs.readFile(jsonRoute, 'utf-8', function(err, data) {
if (err) throw err;
jsonChain = JSON.parse(data);
genesis = jsonChain.blocks[0].GENESIS_DATA;
//console.log(genesis);
/*console.log(jsonChain.blocks[0].GENESIS_DATA.lastHash); //specify element of where the object is and the field that you want to read*/
/*jsonChain.blocks.push({
GENESIS_DATA
});
console.log(jsonChain);
fs.writeFile('CDSM/CDSM.json', JSON.stringify(jsonChain), 'utf-8', function(err) {
if (err) throw err;
console.log('Done!');*/
});
return genesis; //returns undefined since the variable I need to access gets //destroyed before I call it in the constructor (this is a multi module project //and need the array to "save" basically
}
}
.json file:
{"blocks":[{"GENESIS_DATA":{"timestamp":1,"lastHash":"v01d","hash":"?M=(((Position-1)=>ter)=>sen)=>non?","difficulty":20,"nonce":0,"data":[]}}]}
due to this architectural restriction I am asking if there is a different way of reading a json file and referencing the contents of it into variables.
Since getGenesis is asynchronous, you need to have it return a Promise. A promise is basically just a task that will be completed at some point in the future, and it's gonna wait for the result and do something with it.
class Blockchain {
constructor() {
this.chain = [Blockchain.getGenesis()];
console.log(Blockchain.getGenesis());
}
static getGenesis(genChain) {
return new Promise(function(resolve, reject) {
fs.readFile(jsonRoute, 'utf-8', function(err, data) {
if(err) return reject(err);
const jsonChain = JSON.parse(data);
resolve(jsonChain.blocks[0].GENESIS_DATA);
});
});
}
}
Then call it like this:
Blockchain.getGenesis(genChain).then(function(genesisData) {
console.log(genesisData); // Genesis block data is here.
}, function(err) {
// This only runs if there was an error.
console.log(err);
});
resolve and reject are simply two functions you can call to tell the promise about the data when it's complete. If the request succeeded then you call resolve with the data. If there is an error then you call reject with the error data.
The Promise class exposes the then method, which simply binds a success and error callback to be executed when the promise is complete.
So I am reading about callbacks because I´m learning backend development on Node.js, and in several webs they say this good practice about writing callbacks with error argument as its first argument:
For example:
fs.readFile('/foo.txt', function(err, data) {
// If an error occurred, handle it (throw, propagate, etc)
if(err) {
console.log('Unknown Error');
return;
}
// Otherwise, log the file contents
console.log(data);
});
Ok sure, I think I understand it clearly what is happening. If once the module fs finishes reading the file "foo.text" there is an error, then the callback function executes console.log("Uknown error") but how Javascript / Node knows that the variable err corresponds to an error in the code??
Because If i name it error instead of err , I imagine it also works right? And what If put it in the second argument? I imagine then it wouldn´t work. Is that it? If its why it is called a good practice if there is no other way to put the error argument but in the first place.
but how Javascript / Node knows that the variable err corresponds to an error in the code??
By convention. The way readFile (and other Node.js callback-style functions) are written, they call their callback with the error as the first argument, or null as the first argument. The name of that parameter in the function signature is irrelevant (you can call it anything you like; err, e, and error are all common). It's the fact it's the first parameter that matters, because it will receive the first argument when called.
In these modern times, though, things are moving away from Node.js callback-style APIs and toward APIs using Promises, which make the error and success paths much more distinct. Then async/await syntax is layered on top of promises to make it possible to write asynchronous code using the standard logical flow control structures.
Node.js callback style (like your code):
const fs = require("fs");
// ...
fs.readFile('/foo.txt', function(err, data) {
// If an error occurred, handle it (throw, propagate, etc)
if (err) {
// It failed
console.log(err);
return;
}
// It worked
console.log(data);
});
With promises via the fs.promises API:
const fsp = require("fs").promises;
// ...
fsp.readFile('/foo.txt')
.then(data => {
// It worked
console.log(data);
})
.catch(err => {
console.log(err);
});
Of course, you may not handle errors at that level; you might instead return the result of calling then so that the caller can chain off it and handle errors (or pass it off to its caller, etc.).
With async/await (this must be inside an async function, although in modules top-level await is coming):
const fsp = require("fs").promises;
// ...inside an `async` function:
try {
const data = await fsp.readFile('/foo.txt');
} catch (err) {
console.log(err);
}
And again, you might not handle errors at that level; you might let them propagate and have the caller handle them (or the caller might let them propgate to its caller to handle them, etc.).
Not all of Node.js's API has promises yet. You can wrap a single callback-style API function with promises via util.promisify; you can wrap an entire API via various npm modules like promisify.
I am completely new to JavaScript, and I am having a trouble with using fs.readFile() function.
I am required to read a file, and determine whether I have to call successFn (when err boolean is false) or errorFn (if err boolean is true).
When I err is false, however, I am trying to call an "extractor" function as a parameter -- extractor separates the data read into each word.
I am required to call fs.readFile, so I am required to use parameters and one of the parameters is fileName.
The grader is going to test my code using his own files, so I am not supposed to define fileName, yet I am being asked to define it (I understand why I am being asked to do it, but I feel that there should be a way to get around this issue.)
Also, JavaScript syntax is very not intuitive, so I might be making some mistakes with the syntax, but here is my code, and please let me know how I can fix my issue:
function readAndExtractWith(extractor) {
fs.readFile(fileName, 'utf-8', (err, data) => {
if (err) {
errorFn(err);
}
else {
data = extractor();
successFn(data);
}
return extractor;
});
}
fileName is not defined because you never define it. Since you are using that in a function you need to either pass the variable as an argument, specify it inside the function, or define it globally.
var fileName = "stackoverflow.jpg";
// when executing the function pass the fileName parameter
readAndExtractWith(extractor, fileName);
function readAndExtractWith(extractor, fileName) {
fs.readFile(fileName, 'utf-8', (err, data) => {
if (err) {
errorFn(err);
}
else {
data = extractor();
successFn(data);
}
return extractor;
});
}
I am struggling with getting my head around how to overcome and handle the async nature of Node.JS. I have done quite a bit of reading on it and tried to make Node do what I want by either using a message passing solution or callback functions.
My problem is I have a object where I want to constructor to load a file and populate an array. Then I want all calls to this function use that loaded data. So I need the original call to wait for the file to be loaded and all subsequent calls to use the already loaded private member.
My issue is that the function to load load the data and get the data is being executed async even if it return a function with a callback.
Anyways, is there something simple I am missing? Or is there an easier pattern I could use here? This function should return part of the loaded file but returns undefined. I have checked that the file is actually being loaded, and works correctly.
function Song() {
this.verses = undefined;
this.loadVerses = function(verseNum, callback) {
if (this.verses === undefined) {
var fs = require('fs'),
filename = 'README.md';
fs.readFile(filename, 'utf8', function(err, data) {
if (err) {
console.log('error throw opening file: %s, err: %s', filename, err);
throw err;
} else {
this.verses = data;
return callback(verseNum);
}
});
} else {
return callback(verseNum);
}
}
this.getVerse = function(verseNum) {
return this.verses[verseNum + 1];
}
}
Song.prototype = {
verse: function(input) {
return this.loadVerses(input, this.getVerse);
}
}
module.exports = new Song();
Update:
This is how I am using the song module from another module
var song = require('./song');
return song.verse(1);
"My issue is that the function to load the data and get the data is being executed async even if it return a function with a callback."
#AlbertoZaccagni what I mean by that scentence is that this line return this.loadVerses(input, this.getVerse); returns before the file is loaded when I expect it to wait for the callback.
That is how node works, I will try to clarify it with an example.
function readFile(path, callback) {
console.log('about to read...');
fs.readFile(path, 'utf8', function(err, data) {
callback();
});
}
console.log('start');
readFile('/path/to/the/file', function() {
console.log('...read!');
});
console.log('end');
You are reading a file and in the console you will likely have
start
about to read...
end
...read!
You can try that separately to see it in action and tweak it to understand the point. What's important to notice here is that your code will keep on running skipping the execution of the callback, until the file is read.
Just because you declared a callback does not mean that the execution will halt until the callback is called and then resumed.
So this is how I would change that code:
function Song() {
this.verses = undefined;
this.loadVerses = function(verseNum, callback) {
if (this.verses === undefined) {
var fs = require('fs'),
filename = 'README.md';
fs.readFile(filename, 'utf8', function(err, data) {
if (err) {
console.log('error throw opening file: %s, err: %s', filename, err);
throw err;
} else {
this.verses = data;
return callback(verseNum);
}
});
} else {
return callback(verseNum);
}
}
}
Song.prototype = {
verse: function(input, callback) {
// I've removed returns here
// I think they were confusing you, feel free to add them back in
// but they are not actually returning your value, which is instead an
// argument of the callback function
this.loadVerses(input, function(verseNum) {
callback(this.verses[verseNum + 1]);
});
}
}
module.exports = new Song();
To use it:
var song = require('./song');
song.verse(1, function(verse) {
console.log(verse);
});
I've ignored
the fact that we're not treating the error as first argument of the callback
the fact that calling this fast enough will create racing conditions, but I believe this is another question
[Collected into an answer and expanded from my previous comments]
TL;DR You need to structure your code such that the result of any operation is only used inside that operation's callback, since you do not have access to it anywhere else.
And while assigning it to an external global variable will certainly work as expected, do so will only occur after the callback has fired, which happens at a time you cannot predict.
Commentary
Callbacks do not return values because by their very nature, they are executed sometime in the future.
Once you pass a callback function into a controlling asynchronous function, it will be executed when the surrounding function decides to call it. You do not control this, and so waiting for a returned result won't work.
Your example code, song.verse(1); cannot be expected to return anything useful because it is called immediately and since the callback hasn't yet fired, will simply return the only value it can: null.
I'm afraid this reliance on asynchronous functions with passed callbacks is an irremovable feature of how NodeJS operates; it is at the very core of it.
Don't be disheartened though. A quick survey of all the NodeJS questions here shows quite clearly that this idea that one must work with the results of async operations only in their callbacks is the single greatest impediment to anyone understanding how to program in NodeJS.
For a truly excellent explanation/tutorial on the various ways to correctly structure NodeJS code, see Managing Node.js Callback Hell with Promises, Generators and Other Approaches.
I believe it clearly and succinctly describes the problem you face and provides several ways to refactor your code correctly.
Two of the features mentioned there, Promises and Generators, are programming features/concepts, the understanding of which would I believe be of great use to you.
Promises (or as some call them, Futures) is/are a programming abstraction that allows one to write code a little more linearly in a if this then that style, like
fs.readFileAsync(path).then(function(data){
/* do something with data here */
return result;
}).catch(function(err){
/* deal with errors from readFileAsync here */
}).then(function(result_of_last_operation){
/* do something with result_of_last_operation here */
if(there_is_a_problem) throw new Error('there is a problem');
return final_result;
})
.catch(function(err){
/* deal with errors when there_is_a_problem here */
}).done(function(final_result){
/* do something with the final result */
});
In reality, Promises are simply a means of marshaling the standard callback pyramid in a more linear fashion. (Personally I believe they need a new name, since the idea of "a promise of some value that might appear in the future" is not an easy one to wrap one's head around, at least it wasn't for me.)
Promises do this by (behind the scenes) restructuring "callback hell" such that:
asyncFunc(args,function callback(err,result){
if(err) throw err;
/* do something with the result here*/
});
becomes something more akin to:
var p=function(){
return new Promise(function(resolve,reject){
asyncFunc(args,function callback(err,result){
if(err) reject(err)
resolve(result);
});
});
});
p();
where any value you provide to resolve() becomes the only argument to the next "then-able" callback and any error is passed via rejected(), so it can be caught by any .catch(function(err){ ... }) handlers you define.
Promises also do all the things you'd expect from the (somewhat standard) async module, like running callbacks in series or in parallel and operating over the elements of an array, returning their collected results to a callback once all the results have been gathered.
But you will note that Promises don't quite do what you want, because everything is still in callbacks.
(See bluebird for what I believe is the simplest and thus, best Promises package to learn first.)
(And note that fs.readFileAsync is not a typo. One useful feature of bluebird is that it can be made to add this and other Promises-based versions of fs's existing functions to the standard fs object. It also understands how to "promisify" other modules such as request and mkdirp).
Generators are the other feature described in the tutorial above, but are only available in the new, updated but not yet officially released version of JavaScript (codenamed "Harmony").
Using generators would also allow you to write code in a more linear manner, since one of the features it provides is the ability of waiting on the results of an asynchronous operation in a way that doesn't wreak havoc with the JavaScript event-loop. (But as I said, it's not a feature in general use yet.)
You can however use generators in the current release of node if you'd like, simply add "--harmony" to the node command line to tell it to turn on the newest features of the next version of JavaScript.