Lesson 2
Reading Text Files Line-by-Line with TypeScript
Introduction and Context Setting

Welcome to this lesson, where we'll explore an essential technique in text data manipulation: reading files line-by-line with TypeScript. In many real-world applications, processing data one line at a time is crucial for effective data management, especially when handling large files like logs or data streams. By the end of this lesson, you'll understand how to efficiently read and process file data line-by-line, leveraging TypeScript's capabilities with Node.js.

Opening Files

In TypeScript, when using Node.js, we handle file operations with the fs module. To open a file and read its contents, we'll utilize fs.readFileSync(). However, TypeScript adds the advantage of type safety. Here's an example:

TypeScript
1import * as fs from 'fs';
2
3const filePath: string = 'input.txt';
4const content: string = fs.readFileSync(filePath, 'utf-8');

In this example, filePath is explicitly declared as a string, indicating the location of your file, and content is a string containing the entire file's content, read synchronously.

Reading Files Line-by-Line

Reading a file line-by-line in TypeScript is straightforward. We'll use the split() method, but with explicit type annotations for clarity:

TypeScript
1import * as fs from 'fs';
2
3const filePath: string = 'input.txt';
4const content: string = fs.readFileSync(filePath, 'utf-8');
5const lines: string[] = content.split(/\r?\n/);

The regular expression /\r?\n/ is used to split the content into lines. It matches both UNIX (\n) and Windows-style (\r\n) line endings, ensuring compatibility across different text file formats. Here, lines is an array of strings, each representing a line from input.txt. For a file's content like:

Plain text
1Hello,
2world
3!

The lines array would be: ["Hello,", "world", "!"] after the split operation.

Iterating Over Lines and Cleaning Up Output

After obtaining your file lines in an array, you can iterate over them using a for...of loop.

TypeScript
1import * as fs from 'fs';
2
3const filePath: string = 'input.txt';
4const content: string = fs.readFileSync(filePath, 'utf-8');
5const lines: string[] = content.split(/\r?\n/);
6
7for (const line of lines) {
8  console.log(line.trim());
9}
  • Looping Over Lines: The for...of loop processes each line in the lines array. You may explicitly declare line as a string for clarity: for (const line: string of lines).
  • Using trim(): This method removes leading and trailing whitespace, including newline characters, from each line.

The output will display each line from input.txt without unnecessary newlines:

Plain text
1Hello,
2world
3!
Reading Integers from a File and Finding Their Sum

Let's extend our skills by reading integers from a file and calculating their sum, utilizing TypeScript's typing benefits. If numbers.txt contains:

Plain text
110
220
330
440

The following TypeScript code computes their sum:

TypeScript
1import * as fs from 'fs';
2
3const filePath: string = 'numbers.txt';
4const content: string = fs.readFileSync(filePath, 'utf-8');
5const lines: string[] = content.split(/\r?\n/);
6
7let totalSum: number = 0;
8
9for (const line of lines) {
10  const parsedNumber: number = parseInt(line.trim(), 10); // Convert each line to an integer
11  if (!isNaN(parsedNumber)) { // Ensure the line is a valid number
12    totalSum += parsedNumber; // Add the integer to totalSum
13  }
14}
15
16console.log("The sum of the numbers is:", totalSum);
  • Reading Lines: The lines from the file are read into the lines array.
  • Converting to Integers: Each trimmed line is converted to an integer using parseInt(), with the second argument 10 indicating that the numbers are in base 10.
  • Calculating Sum: Valid integers are summed up in the totalSum variable.

Executing the code, you'll see the total sum of the numbers:

Plain text
1The sum of the numbers is: 100
Summary and Practice Preparation

In this lesson, you developed the ability to read a text file line-by-line using TypeScript — a fundamental technique for efficiently processing large datasets. You've learned how to manage file I/O operations with the fs module, implement content splitting with split(), and clean data using trim(). The addition of TypeScript's typing provides robust error checking and improved code clarity.

You're now ready to tackle practice exercises, applying these concepts to further enrich your understanding. As we progress, we'll uncover more parsing techniques. Keep honing your skills with TypeScript!

Enjoy this lesson? Now it's time to practice with Cosmo!
Practice is how you turn knowledge into actual skills.