Node.js Callback Functions

Node.js callback function: Asynchronicity is one of the basic factors that make Node.js popular. Callback is the implementation of function asynchronicity. Generally, in Node.js, most functions that handle resources have callback variants.

When a resource is called for an asynchronous function for a task, the control is immediately allowed to continue executing the subsequent statements after the function. The task on the resource will start in parallel. This can help Node.js continues to execute other tasks while this function is using the resource. Once the task using the resource is completed, Node.js will use the callback function to resume. Call Callback functions have parameters: data objects, result objects, and/or error objects that contain task-related information.

Blocking function：In contrast to asynchronous functions, synchronous functions will execute blocks until the resource is completed. Therefore, synchronous functions are also called blocking functions.

Node.js nested callback functions　: If multiple operations are performed on resources sequentially and must be performed asynchronously, callback functions can be nested.

Now, when reading (task) files (resources), we will see three cases compared with callback functions and blocking functions.

• Node.js blocking function example

• Node.js callback function example

• Node.js Nested Callback Function Example

Blocking function example

The following is an example of a Node.js script that synchronously reads the sample.txt file.

　var fs = require('fs');　
　
//　Read the file sample.txt
var　data　=　fs.readFileSync('sample.txt');　
console.log("Reading file completed: ")　+　, new Date().toISOString());　
　
console.log("After　readFileSync　statement　:　"　+　, new Date().toISOString());

When the above program runs

Terminal Output

　arjun@arjun-VPCEH26EN:~/nodejs$ node read-file-sync.js　
Reading file completed:　2017-10-19T12:21:40.103Z
After　readFileSync　statement　:　2017-10-19T12:21:40.105Z

The after readFileSync statement always executes after the file reading is completed. fs.readFileSync blocks the execution flow.

Node.js callback function example

The following is an example of a Node.js script, which usesNode.js callback functionAsynchronously read the sample.txt file.

　var fs = require('fs');　
　
//　Read the file sample.txt
fs.readFile('sample.txt',　
　　　　//　Callback function called when the file reading is completed
　　　　function(err,　data)　{　　
　　　　　　　　if (err) throw err;　
　　　　　　　　//　Data is a buffer that contains the content of the file
　　　　　　　　console.log("Reading file completed: ")　+　, new Date().toISOString());　
　});　
　
console.log("After readFile asynchronously: ");　+　, new Date().toISOString());

When the above program runs

Terminal Output

arjun@arjun-VPCEH26EN:~/nodejs$ node read-file-async.js　
After readFile asynchronously:　2017-10-19T12:25:36.982Z
Reading file completed:　2017-10-19T12:25:36.987Z

You may notice that even after executing console.log in the asynchronous statement of readFile, it takes about5Milliseconds. file. fs.readFile('sample.txt', callback function{..}) does not block the execution but starts a new process in parallel with the main control flow to read the file (performing a task on resources).

Node.js Nested Callback Function Example

To demonstrate nested callback functions in Node.js, we will consider the plan of renaming a file and then using an asynchronous function to delete it.

　var fs = require('fs');　
　
fs.rename('sample.txt', 'sample_old.txt',　
　　　　//　First Callback Function
　　　　function (err) {　
　　　　　　　　if (err) throw err;　
　　　　　　　　console.log('File Renamed.');　
　　　　　　　　fs.unlink('sample_old.txt',　
　　　　　　　　　　　　//　Second Callback Function
　　　　　　　　　　　　function (err) {　
　　　　　　　　　　　　　　　　if (err) throw err;　
　　　　　　　　　　　　　　　　console.log('File Deleted.');　
　　　　　　　　　　　　}　
　　　　　　　　);　　
　　　　}　
　);

When the above Node.js example is run with node

Terminal Output

　arjun@arjun-VPCEH26EN:~/nodejs$ node nodejs-nested-callback.js　
File Renamed.　
File Deleted.

Conclusion:

In this Node.js tutorial – Node.js Callback Functions, we learned about the execution flow of callback functions and how they achieve non-blocking, as well as how to combine nested callbacks with examples.