Writing files with javascript

JavaScript Read and Write to Text File

In this tutorial, I will tell you about how you can read and write to text file using JavaScript. As we know JavaScript does not have the ability to access the user’s file system so for this we need to run the project on a server. To implement this we use node.js.

In node.js these are one library fs (File-System) which is used to manage all read and write operations. By using the fs module we can read and write files in both synchronous and asynchronous way.

There are many ways in which we can read and write data to file. Lets have a look on each of them one by one.

JavaScript Read and Write to Text File

Method 1: Using Node.js

First is by using writefile and readFile method in node.js environment.

This is used to write content to file. Its syntax is below:

It has three parameters path, data, callback.

Path: The path is the location of Text File. If the file is to be generated in the same folder as that of the program, then provide the name of the file only. If the file does not exist then the new file will be created automatically.

Data: Second is Data in This parameter we need to pass info that is required to write in the file.

Callback: Last one is the Callback Function in this we pass error string. If the operation fails to write the data, an error shows the fault.

To run above code run this command:

>node index.js

It is used to read the content of the file. its syntax is:

It also has three parameters path, callback, options.

path is a location of Text File. If both file and program are in a similar folder, then directly give the file name of the text file.

Second option which specifies the data is to be gathered from the file and it is optional. If you pass nothing then it returns raw buffer.

The last one is Callback function which has two more parameters (error, txtString). If at any instance it fails to load or read the file then it returns error or exception otherwise it returns all data of the file.

To run this program enter below command:

>node index.js

Method 2: Using ActiveXObject

Another method is by using a web page ActiveX objects but this method is mainly work in Internet Explorer only.

This ActiveXObject is used to return an instance of a file system library which is used with the CreateTextFile method. This JS method which generates a file defined functions and states a TextStream object to read from or write to the file. It also returns a boolean value on the basis of this we can find out that we can overwrite to the current file or not. So, if you want to add the data to the created file you can use writeline method which is used to add text string to the file.

Using ActiveX objects, the following should be included in your code to read a file:

The ActiveX object contains three things libraryname, classname, location. So, classname is the instance of a class that needs to be created. libraryname is a mandatory field and it is the sign of the app giving the object.

To open a new file:

Источник

How to Write to a File Using JavaScript, With Examples

This article will show you how to write to files from JavaScript – both from the web browser and Node.js environments. With examples on how to write, append, update, delete and rename files.

JavaScript code is usually run from one of two environments – within a web browser when viewing a web page or in a Node.js environment which allows JavaScript to be executed outside of a web browser – commonly used for building back-end services and web apps.

Write to a File From the Browser with JavaScript

First, how to do it from the browser. Modern JavaScript includes built-in tools for this. The steps are as follows:

• Create a file using the JavaScript Blob object to represent the file
• Create a URL for the new object
• Provide a link which the user can click to tell the browser to download the Blob object from the URL as a file

The user can then click the link, and they will be presented with the standard save dialog from their web browser, allowing them to save the generated file wherever they wish.

Here’s how the code looks as a re-usable function you can copy and paste to use yourself:

// A global variable should be defined to hold the URL for the file to be downloaded // This is good practice as if many links are being generated or the link is being regularly updated, you don't want to be creating new variables every time, wasting memory var textFileUrl = null; // Function for generating a text file URL containing given text function generateTextFileUrl(txt) < let fileData = new Blob([txt], ); // If a file has been previously generated, revoke the existing URL if (textFileUrl !== null) < window.URL.revokeObjectURL(textFile); >textFileUrl = window.URL.createObjectURL(fileData); // Returns a reference to the global variable holding the URL // Again, this is better than generating and returning the URL itself from the function as it will eat memory if the file contents are large or regularly changing return textFileUrl; >; // Generate the file download URL and assign it to the link // Wait until the page has loaded! Otherwise the download link element will not exist window.addEventListener("load", function()< document.getElementById('downloadLink').href = generateTextFileUrl('Hello world!'); >);

You’ll need the following link in your HTML to display a download button:

This solution does not attempt to emulate clicking on the generated links or any of that nonsense. Browsers will usually block it, and you shouldn’t be trying to sneak downloads past a user without them expressly clicking a link to save the file themselves anyway.

Write to a File From Node.js

If you’re working in a Node.js environment, all of the tools for managing files are available in the built-in fs library.

Reading or writing files takes time and can halt execution. Previously, it was best to use callbacks to perform file operations once a file has been successfully read or written to.

Now, however, Promises provide a standardized approach for asynchronous tasks. So these examples will use Promises instead of callbacks to keep things modern.

Here’s how to write a text file from Node.js:

// Import the promise-based version of the fs library const fs = require('fs').promises; // Define some text to be written to a file var textData = "Hello world!"; try < // Write text to the given file name // await tells JavaScript to wait for the asyncronous function (Promise) to resolve before continuing await fs.writeFile('myFile.txt', textData); >catch (error) < // Output any errors for inspection console.log(error); >

Appending to a File from Node.js

Following on from above, it’s simple to use the fs library to append data to a file:

const fs = require(‘fs’).promises; try < await fs.appendFile('myFile.txt', textData); >catch (error)

Updating a File

To update a file, you can either append to it using appendFile or overwrite it using writeFile, as outlined above.

Renaming a File

const fs = require(‘fs’).promises; try < await fs.rename('oldFileName.txt', 'newFileName.txt'); >catch (error)

Deleting a File

const fs = require(‘fs’).promises; try < await fs.unlink('myFile.txt'); >catch (error)

Источник

How to Write a File in JavaScript

To write a file in JavaScript, you can use the “writeFile()” method of the fs(File-System) module. The fs.writeFile() is a Node.js method used to write the specified data to a file asynchronously. By default, the file would be replaced if it existed.

Syntax

writeFile(Path, Data, callback)

Parameters

The writeFile() method accepts three parameters: path, data, and callback.

• path: This is the location of the Text file. If you want to generate the file at the same location where your program is stored, then, in this case, provide the file’s name only.
• data: In this, we need to provide the content that needs to be stored in the file.
• callback: This function argues (err) that if the operation fails to write data in the file, an error log will be shown to you.

Example

Create an app.js file and add the following code inside it.

const fs = require('fs') let data = "How to write a file in Javascript." fs.writeFile('output.txt', data, (err) =>  if (err) throw err; >);

It will create an output.txt file that has the following content in it.

How to write a file in Javascript.

Explanation

1. To use the module in Node.js, use the require() function.
2. We created a variable named fs using const. One can also use let and other keywords for the same.
3. Then another variable data is created using let. Here a plain text file is stored.
4. A function called writeFile() is created, which generates the texts within the data variable.

You can see that we wrote a file using the fs.writeFile() function.

Niva Shah is a Software Engineer with over eight years of experience. She has developed a strong foundation in computer science principles and a passion for problem-solving.

Leave a Comment Cancel reply

We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept All”, you consent to the use of ALL the cookies. However, you may visit «Cookie Settings» to provide a controlled consent.

Privacy Overview

This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience.

Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously.

Cookie	Duration	Description
cookielawinfo-checkbox-analytics	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category «Analytics».
cookielawinfo-checkbox-functional	11 months	The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category «Functional».
cookielawinfo-checkbox-necessary	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category «Necessary».
cookielawinfo-checkbox-others	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category «Other.
cookielawinfo-checkbox-performance	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category «Performance».
viewed_cookie_policy	11 months	The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.

Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features.

Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors.

Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.

Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. These cookies track visitors across websites and collect information to provide customized ads.

Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet.

Источник