Welcome to this lesson on writing table data to text files. Text files are a common method for storing and sharing data due to their simplicity and compatibility with various systems. In this lesson, we will learn how to write formatted table data into a text file using tabs as delimiters with TypeScript.

By the end of this lesson, you will be able to take structured data and write it to a text file using TypeScript, leveraging its type safety and static typing advantages. Let’s dive into this skill, building on what you've learned about parsing tables from text and CSV files.

Before we proceed, let's briefly recall two key concepts: arrays in TypeScript and basic file handling using the fs module.

• Arrays: In TypeScript, arrays allow you to store a collection of items with strict type definitions, such as rows in a table. This step is crucial as it helps organize data into a structured format, ensuring type safety.

• File Handling: We previously covered basic reading operations. Now, we move on to writing data. The fs module in Node.js works seamlessly with TypeScript by allowing you to define types for file operations and handle data with type accuracy.

These foundational concepts will guide us in writing table data into a text file with enhanced error prevention and clarity.

Let's begin by structuring our table data using an array of arrays. This format helps us represent rows and columns in a simple and organized manner. Here's an example of how we might represent some tabular data, including type annotations:

TypeScript
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1const data: (string | number)[][] = [
2  ['Name', 'Age', 'Occupation'],
3  ['John', 28, 'Engineer'],
4  ['Alice', 34, 'Doctor'],
5  ['Bob', 23, 'Artist'],
6];

Each inner array represents a row of our table, with columns such as 'Name', 'Age', and 'Occupation'. The type annotations (string | number)[][] ensure that we have an array of arrays containing either strings or numbers, maintaining type safety and simplifying data management.

TypeScript's static typing enhances JavaScript's join method by enforcing explicit type conversion, ensuring data consistency when joining array elements into a single string with a specified separator:

TypeScript
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1const row: (string | number)[] = ['John', 28, 'Engineer'];
2const joinedString: string = row.map(String).join('\t');
3
4console.log(joinedString);  // Outputs: John    28    Engineer

• row.map(String): Converts each element to a string, making them compatible with join, which is critical for non-string elements.
• join('\t'): Joins converted elements into a single tab-delimited string.

This approach ensures numerical or other non-string data types are safely converted and joined without errors, enhanced by TypeScript’s static type checks.

Let's now write the data to a text file with tabs as delimiters, incorporating TypeScript's static typing for enhanced reliability:

TypeScript
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1import * as fs from 'fs';
2const outputFilePath: string = 'output.txt';
3
4const content: string = data.map(row => row.join('\t')).join('\n');
5
6fs.writeFileSync(outputFilePath, content, 'utf-8');
7
8console.log(`Data written to ${outputFilePath} using tab delimiters.`);

• import * as fs from 'fs': Imports the Node.js fs module for file handling, compatible with TypeScript.
• data.map(row => row.join('\t')): Iterates over each row in the data array. For each row, it uses the join('\t') method to concatenate the elements of the row into a single string, placing a tab character ('\t') between each element.
• join('\n'): Separates each row with a newline character, creating the final multi-line string.
• fs.writeFileSync(outputFilePath, content, 'utf-8'): Writes the content to the specified file path synchronously using UTF-8 encoding, with type safety protections.

TypeScript’s type system enhances error prevention, ensuring your file operations are more reliable and robust.

Let's consider another example of writing the table data. Imagine you want to create a file with employees older than 25. To write only this subset of the table data to a text file, you can use the filter function in TypeScript. This allows you to selectively write rows that meet specific criteria. Here's how to implement this with our example data:

TypeScript
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1const filteredData: (string | number)[][] = data.filter(row => typeof row[1] === 'number' && row[1] > 25);
2
3const filteredContent: string = filteredData.map(row => row.join('\t')).join('\n');
4
5fs.writeFileSync('filtered_output.txt', filteredContent, 'utf-8');
6
7console.log('Filtered data written to filtered_output.txt using tab delimiters.');

• data.filter(row => typeof row[1] === 'number' && row[1] > 25): Filters the rows to include only those where the age (second column) is a number and greater than 25.
• filteredData.map(row => row.join('\t')): Joins the elements of each filtered row into a tab-delimited string.
• fs.writeFileSync('filtered_output.txt', filteredContent, 'utf-8'): Writes the filtered content to a new file filtered_output.txt.

By applying the filter function, you focus only on the data that fits specific criteria, showcasing the power and flexibility of TypeScript in managing your data outputs efficiently.

In this lesson, you learned how to effectively structure, write, and verify data stored in a text file using TypeScript. We started by preparing data using arrays with type annotations and then utilized Node.js's file-handling techniques to write this data using tabs as delimiters. TypeScript's enhanced error checking and static typing improve development workflows and help minimize runtime errors.

Now, we encourage you to engage with practice exercises that follow this lesson. Apply what you've learned to reinforce your understanding and gain confidence. Keep experimenting with different data structures, adapt them, and ensure your outputs meet your expectations. Well done on progressing through this important lesson leveraging TypeScript’s advantages.