In this lesson, we're exploring the powerful capabilities of writing to files using JavaScript with Node.js. File writing is an essential skill in programming, allowing you to store data permanently, log information, and communicate with other systems or users. We'll delve into various modes of file operations using the fs (File System) module, focusing on write (using fs.writeFileSync), append (using fs.appendFileSync), and briefly reading (using fs.readFileSync) to confirm our output.
Let's begin by exploring how we can write to a file in JavaScript using Node.js. The write mode is used to create a new file or overwrite an existing one.
First, we define the path for the file we want to write to:
JavaScript1const fs = require('fs'); 2const outputFilePath = 'output.txt';
To write to a file, we'll use fs.writeFileSync, which overwrites the file if it already exists or creates a new one.
JavaScript1fs.writeFileSync(outputFilePath, "Hello, World!\nThis is a new line of text.\n", 'utf-8'); 2console.log(`Text written to ${outputFilePath} using 'w' mode.`);
Here, we write multiple lines of text, ensuring each ends with a newline character \n to separate lines. In contrast to some other languages, JavaScript automatically closes the file, so no explicit close is needed. If the function is executed again, the previous content will be replaced, emphasizing the session-specific nature of writing.
When executed, output.txt will contain:
1Hello, World! 2This is a new line of text.
Being synchronous, writeFileSync blocks the execution of subsequent code until the file operation is complete. While this ensures consistency in single-threaded applications, it can slow down performance in scenarios requiring frequent writes.
In write mode, any existing content in the file is erased upon opening. This is convenient for starting fresh or completely overwriting existing content. However, if you intend to retain existing data while adding new content, append mode comes into play, utilizing fs.appendFileSync.
To append text to the end of the file:
JavaScript1fs.appendFileSync(outputFilePath, "Appending another line of text.\n", 'utf-8'); 2console.log(`Text appended to ${outputFilePath} using 'a' mode.`);
This appends the new line to the existing file content, preserving all original data.
Running this results in:
1Hello, World! 2This is a new line of text. 3Appending another line of text.
To verify our written and appended content, we can utilize the read mode, provided by fs.readFileSync, to ensure everything is correctly captured.
Here's how to read the file content:
JavaScript1const content = fs.readFileSync(outputFilePath, 'utf-8'); 2console.log("\nValidating file content:"); 3console.log(content);
By following these steps, the output should confirm the expected contents of your file, assuring you that your program performs as intended.
Beyond text, you can also write numbers to a file with fs.writeFileSync. While fs.writeFileSync can accept various types of data, including strings, buffers, and typed arrays, numerical data will often need to be converted if a specific format is required.
Consider this example of writing a number:
JavaScript1const number = 42; 2fs.writeFileSync(outputFilePath, `The answer to life, the universe, and everything is: ${number}\n`, 'utf-8');
In this example, the number 42 is directly inserted into the string using template literals. After execution, output.txt will contain:
1The answer to life, the universe, and everything is: 42
This approach allows you to manage and store numerical and other data effortlessly, accommodating diverse data types.
In this lesson, you've learned to write and append text to files using JavaScript's fs module via methods like fs.writeFileSync and fs.appendFileSync, while validation was performed using fs.readFileSync. These are foundational skills for data storage and management. As you proceed to practice exercises, you'll reinforce these concepts with practical applications, preparing you for deeper exploration and usage of Node.js for file manipulation tasks.