Introduction

One of the most common tasks in backend development is reading data from files. Applications frequently read configuration files, log files, JSON data, text files, CSV files, and user-uploaded documents.

Unlike JavaScript running in a web browser, Node.js can directly interact with the computer’s file system through its built-in File System (fs) module. This allows developers to read files without installing any external libraries.

For automation engineers, file reading is an essential skill. Test automation frameworks often read data from CSV, JSON, XML, or text files to perform data-driven testing, load configuration values, compare expected results, generate reports, and validate application behavior.

---

What is File Reading?

File reading is the process of accessing the contents of a file stored on the computer.

Node.js provides several methods to read files, including:

• Reading an entire file

• Reading files asynchronously

• Reading files synchronously

• Reading text files

• Reading JSON files

• Reading binary files

---

Why Read Files?

Reading files allows applications to:

• Load configuration settings

• Read user data

• Process CSV files

• Read JSON data

• Display stored content

• Generate reports

• Perform data-driven testing

---

The File System (fs) Module

Node.js includes a built-in module called fs (File System).

Before reading files, import the module:

const fs = require("fs");

No installation is required because it is built into Node.js.

---

Asynchronous File Reading

The most commonly used method is fs.readFile().

Syntax

fs.readFile(path, encoding, callback);

Parameters

Parameter	Description
path	File path
encoding	File encoding (for example, "utf8")
callback	Function executed after reading the file

---

Example 1: Read a Text File

Assume there is a file named sample.txt containing:

Welcome to Node.js File Handling.

Read the file:

const fs = require("fs");

fs.readFile("sample.txt", "utf8", (error, data) => {
    if (error) {
        console.log(error);
        return;
    }

    console.log(data);
});

Sample Output

Welcome to Node.js File Handling.

---

Understanding the Callback

The callback receives two parameters:

(error, data)

• error contains an error if reading fails.

• data contains the file contents if reading succeeds.

---

Example 2: Handle File Not Found

const fs = require("fs");

fs.readFile("missing.txt", "utf8", (error, data) => {
    if (error) {
        console.log("File not found.");
        return;
    }

    console.log(data);
});

Sample Output

File not found.

---

Synchronous File Reading

Node.js also provides fs.readFileSync().

Unlike readFile(), it waits until the file is completely read.

Syntax

const data = fs.readFileSync(path, encoding);

---

Example 3: Read a File Synchronously

const fs = require("fs");

const data = fs.readFileSync("sample.txt", "utf8");

console.log(data);

Sample Output

Welcome to Node.js File Handling.

---

Asynchronous vs Synchronous Reading

Feature	readFile()	readFileSync()
Blocking	No	Yes
Performance	Faster for large applications	Slower because execution waits
Uses Callback	Yes	No
Recommended	Yes	Only when necessary

---

Reading JSON Files

Suppose user.json contains:

{
    "name": "John",
    "age": 25
}

Read the JSON file:

const fs = require("fs");

fs.readFile("user.json", "utf8", (error, data) => {
    if (error) {
        console.log(error);
        return;
    }

    const user = JSON.parse(data);

    console.log(user.name);
});

Sample Output

John

---

Real-World Example

Read a configuration file.

config.json

{
    "browser": "chrome",
    "url": "https://example.com"
}

const fs = require("fs");

fs.readFile("config.json", "utf8", (error, data) => {
    if (error) {
        console.log(error);
        return;
    }

    const config = JSON.parse(data);

    console.log(config.browser);

    console.log(config.url);
});

Sample Output

chrome

https://example.com

---

Automation Testing Example

Reading files is widely used in automation frameworks.

Playwright Example

Read browser configuration.

const fs = require("fs");

const config = JSON.parse(
    fs.readFileSync("config.json", "utf8")
);

console.log(config.browser);

---

Selenium Example

Read test data.

const fs = require("fs");

const users = fs.readFileSync("users.txt", "utf8");

console.log(users);

---

Cypress Example

Load fixture data.

const fs = require("fs");

const data = fs.readFileSync("testData.json", "utf8");

console.log(JSON.parse(data));

---

API Testing Example

Read request payload.

const fs = require("fs");

const payload = fs.readFileSync("request.json", "utf8");

console.log(payload);

---

Data-Driven Testing Example

Read employee data.

const fs = require("fs");

const employees = fs.readFileSync("employees.json", "utf8");

console.log(JSON.parse(employees));

---

Common Mistakes

Forgetting to Import fs

Incorrect:

fs.readFile("sample.txt");

Correct:

const fs = require("fs");

---

Forgetting the Encoding

Incorrect:

fs.readFile("sample.txt", (error, data) => {
    console.log(data);
});

Without an encoding, the output is a Buffer object instead of readable text.

Correct:

fs.readFile("sample.txt", "utf8", (error, data) => {
    console.log(data);
});

---

Ignoring Errors

Always check the error parameter before using the file data.

---

Best Practices

• Prefer fs.readFile() for most applications because it is asynchronous.

• Use fs.readFileSync() only when blocking behavior is acceptable.

• Always specify the correct file encoding (such as "utf8" for text files).

• Handle file read errors gracefully.

• Parse JSON using JSON.parse() after reading.

• Keep configuration and test data in separate files.

• Use relative or absolute file paths carefully.

---

Conclusion

Reading files is one of the most important capabilities of Node.js. Using the built-in fs module, developers can easily read text files, JSON files, configuration files, and other resources.

For automation engineers, file reading is fundamental for data-driven testing, configuration management, API payloads, report validation, and automation frameworks.

Mastering file reading is the first step toward effective file handling in Node.js.

---

Frequently Asked Questions (FAQs)

Which module is used to read files in Node.js?

The built-in fs (File System) module.

---

How do I import the fs module?

const fs = require("fs");

---

What is the difference between readFile() and readFileSync()?

• readFile() is asynchronous and does not block execution.

• readFileSync() is synchronous and blocks execution until the file is read.

---

Why should I specify "utf8" when reading a text file?

Without an encoding, Node.js returns a Buffer instead of a readable string.

---

How do I read a JSON file?

Read the file as text and then use JSON.parse().

const data = fs.readFileSync("user.json", "utf8");

const user = JSON.parse(data);

---

Why is file reading important in automation testing?

Automation engineers use file reading to load test data, configuration files, API payloads, expected results, reports, and environment-specific settings.

---

Key Takeaways

• Node.js uses the built-in fs module for file handling.

• fs.readFile() reads files asynchronously.

• fs.readFileSync() reads files synchronously.

• Always specify "utf8" when reading text files.

• Handle file read errors properly.

• Use JSON.parse() when reading JSON files.

• Asynchronous reading is recommended for most applications.

• File reading is widely used in Playwright, Selenium, Cypress, API testing, and Node.js applications.

• Store configuration and test data in external files for better maintainability.

• Mastering file reading is essential for backend development and automation testing.