NodeJs callbacks simple example

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callbackssimplenodejs
90%

Costa asked what this would look like if we were to honor the node error callback conventions.,Asynchronous (with callback):,A callback is a function passed as an parameter to a Higher Order Function (wikipedia). A simple implementation of a callback would be:,we are creating a simple function as

var myCallback = function(data) {
   console.log('got data: ' + data);
};

var usingItNow = function(callback) {
   callback('get it?');
};

Finally use it with this next line:

usingItNow(myCallback);

Here I rewrite our example in this convention.

var myCallback = function(err, data) {
   if (err) throw err; // Check for the error and throw if it exists.
   console.log('got data: ' + data); // Otherwise proceed as usual.
};

var usingItNow = function(callback) {
   callback(null, 'get it?'); // I dont want to throw an error, so I pass null for the error argument
};

If we want to simulate an error case, we can define usingItNow like this

var usingItNow = function(callback) {
   var myError = new Error('My custom error!');
   callback(myError, 'get it?'); // I send my error as the first argument.
};

The final usage is exactly the same as in above:

usingItNow(myCallback);
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88%

For example, a function to read a file may start reading file and return the control to the execution environment immediately so that the next instruction can be executed. Once file I/O is complete, it will call the callback function while passing the callback function, the content of the file as a parameter. So there is no blocking or wait for File I/O. This makes Node.js highly scalable, as it can process a high number of requests without waiting for any function to return results.,The second example shows that the program does not wait for file reading and proceeds to print "Program Ended" and at the same time, the program without blocking continues reading the file.,Callback is an asynchronous equivalent for a function. A callback function is called at the completion of a given task. Node makes heavy use of callbacks. All the APIs of Node are written in such a way that they support callbacks.,The first example shows that the program blocks until it reads the file and then only it proceeds to end the program.

Create a text file named input.txt with the following content −

Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
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72%

In a synchronous program, you would write something along the lines of:,This works just fine and is very typical in other development environments. However, if fetchData takes a long time to load the data (maybe it is streaming it off the drive or the internet), then this causes the whole program to 'block' - otherwise known as sitting still and waiting - until it loads the data. Node.js, being an asynchronous platform, doesn't wait around for things like file I/O to finish - Node.js uses callbacks. A callback is a function called at the completion of a given task; this prevents any blocking, and allows other code to be run in the meantime.,Node.js is a trademark of Joyent, Inc. and is used with its permission. Please review the Trademark List and Trademark Guidelines of the OpenJS Foundation.,The typical convention with asynchronous functions (which almost all of your functions should be):

In a synchronous program, you would write something along the lines of:

function processData() {
   var data = fetchData();
   data += 1;
   return data;
}
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65%

NodeJS has asynchronous callbacks and commonly supplies two parameters to your functions sometimes conventionally called err and data. An example with reading a file text., Multithreading , Multithreading ,The function you send in to a function as a callback may be called zero times, once, or multiple times. It all depends on implementation.

Callback functions can be synchronous or asynchronous. Since Asynchronous callback functions may be more complex here is a simple example of a synchronous callback function.

// a function that uses a callback named `cb` as a parameter
function getSyncMessage(cb) {
   cb("Hello World!");
}

console.log("Before getSyncMessage call");
// calling a function and sending in a callback function as an argument.
getSyncMessage(function(message) {
   console.log(message);
});
console.log("After getSyncMessage call");
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75%

How to wait for a promise to finish before returning the variable of a function?,Explanation: fs library is loaded to handle file-system related operations. The readFile() function is asynchronous and control return immediately to the next instruction in the program while the function keep running in the background. A callback function is passed which gets called when the task running in the background are finished.Output:,Explanation: fs library is loaded to handle file-system related operations. The readFileSync() function is synchronous and blocks execution until finished. The function blocks the program until it reads the file and then only it proceeds to end the programOutput:,Example 2: Code for reading a file asynchronously (non-blocking code) in Node.js. Create a text file inputfile1.txt with the following content.

Example 1: Code for reading a file synchronously (blocking code) in Node.js. Create a text file inputfile1.txt with the following content:

Hello Programmer!!!
   Learn NodeJS with GeeksforGeeks

Example 2: Code for reading a file asynchronously (non-blocking code) in Node.js. Create a text file inputfile1.txt with the following content.

Hello Programmer!!!
   Learn NodeJS with GeeksforGeeks
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40%

An example of a callback is the following:,Let’s see an example with two callbacks:,The functions setInterval and setTimeout callbacks are very simple. They don’t provide any parameters on the callback functions. But, if we are reading from the file system or network, we can get the response as a callback parameter.,Modularize: convert your anonymous callbacks into named functions.

123456
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22%

Let’s take a look at these simple async operations. They do nothing special, just fire a timer and call a function once the timer finished.,In previous node versions, where .allSettled is not available, we can implement our own version in just a few lines:,There have been native promises in javascript since 2014, receiving an important boost in performance in Node.js 8. We will make use of them in our functions to make them non-blocking – without the traditional callbacks. The following example will call the modified version of both our previous functions in such a manner:,Node.js at Scale is a collection of articles focusing on the needs of companies with bigger Node.js installations and advanced Node developers. Chapters:

Let’s take a look at these simple async operations. They do nothing special, just fire a timer and call a function once the timer finished.

function fastFunction(done) {
   setTimeout(function() {
      done()
   }, 100)
}

function slowFunction(done) {
   setTimeout(function() {
      done()
   }, 300)
}
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60%

Here is a simple, yet bold, example of a callback function.,When fs.readFile is done fetching the file funFileName, it executes the callback function, which handles the error if an error is thrown and logs the retrieved file to the console.,A Callback is simply a function passed as an argument to another function which will then use it (call it back).,In this second example, file will be undefined when we try to log it, because fs.readFile won’t be done fetching before we get to the console.log().

Here is a simple, yet bold, example of a callback function.

fs.readFile(funFileName, function(err, file) {
   if (err) handleError(err);
   console.log("file: ", file)
})
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