Exercise - Read and write to files

5 minutes

You can also use the fs module in Node.js to write data to files and read data from files.

You're almost finished creating a Node.js masterpiece for Tailwind Traders. So far, your code reads any directory, finds all .json files, and creates a totals.txt file.

In this exercise, you'll complete the project by reading the .json files, adding up the store totals, and writing the grand total to the totals.txt file.

Create a method to calculate sales totals

At the top of index.js, just below the require("path") statement, create a function that will calculate the sales total. This method should take in the array of file paths that it can iterate over.
```
async function calculateSalesTotal(salesFiles) {
  let salesTotal = 0;

  // READ FILES LOOP

  return salesTotal;
}
```

Within that method, replace // READ FILES LOOP with a loop that iterates over the salesFiles array, reads the file, parses the content as JSON, and then increments the salesTotal variable with the total value from the file.

async function calculateSalesTotal(salesFiles) {
  let salesTotal = 0;
  // loop over each file path in the salesFiles array
  for (file of salesFiles) {
    // read the file and parse the contents as JSON
    const data = JSON.parse(await fs.readFile(file));
    // Add the amount in the data.total field to the salesTotal variable
    salesTotal += data.total;
  }
  return salesTotal;
}

Call the calculateSalesTotals method

In the main function, add a call to the calculateSalesTotals function just above the fs.writeFile call.

async function main() {
  const salesDir = path.join(__dirname, "stores");
  const salesTotalsDir = path.join(__dirname, "salesTotals");

  // create the salesTotal directory if it doesn't exist
  try {
    await fs.mkdir(salesTotalsDir);
  } catch {
    console.log(`${salesTotalsDir} already exists.`);
  }

  // find paths to all the sales files
  const salesFiles = await findSalesFiles(salesDir);

  // read through each sales file to calculate the sales total
  const salesTotal = await calculateSalesTotal(salesFiles);

  // write the total to the "totals.txt" file
  await fs.writeFile(path.join(salesTotalsDir, "totals.txt"), String());
}

Write the total to the totals.txt file

In the main function, modify the fs.writeFile block to write the value of the salesTotal variable to the totals.txt file.

async function main() {
  const salesDir = path.join(__dirname, "stores");
  const salesTotalsDir = path.join(__dirname, "salesTotals");

  // create the salesTotal directory if it doesn't exist
  try {
    await fs.mkdir(salesTotalsDir);
  } catch {
    console.log(`${salesTotalsDir} already exists.`);
  }

  // find paths to all the sales files
  const salesFiles = await findSalesFiles(salesDir);

  // read through each sales file to calculate the sales total
  const salesTotal = await calculateSalesTotal(salesFiles);

  // write the total to the "totals.json" file
  await fs.writeFile(
    path.join(salesTotalsDir, "totals.txt"),
    `${salesTotal}\r\n`,
    { flag: "a" }
  );
}

Press Ctrl+S or Cmd+S to save the index.js file.

Run the program

Run the program from the terminal.
```
node index.js
```
There is no output from the program. If you look in the salesTotals/totals.txt file, you'll see the total of all the sales from the sales.json file.
Run the program from the terminal again.
```
node.index.js
```
Select the index.js file.
Select the salesTotals/totals.txt file.

The totals.txt file now has a second line. Every time you run the program, the totals are added up again and a new line is written to the file.

Outstanding work! You've written a smart, robust, and handy tool that Tailwind Traders can use to process all of its stores' sales every night. In the next section, we'll review what you learned and a few tips to remember.

Got stuck?

If you got stuck during this exercise, here's the full code for this project.

const fs = require("fs").promises;
const path = require("path");

async function calculateSalesTotal(salesFiles) {
  let salesTotal = 0;
  // loop over each file path in the salesFiles array
  for (file of salesFiles) {
    // read the file and parse the contents as JSON
    const data = JSON.parse(await fs.readFile(file));
    // Add the amount in the data.total field to the salesTotal variable
    salesTotal += data.total;
  }
  return salesTotal;
}

async function findSalesFiles(folderName) {
  // this array will hold sales files as they are found
  let salesFiles = [];

  async function findFiles(folderName) {
    // read all the items in the current folder
    const items = await fs.readdir(folderName, { withFileTypes: true });

    // iterate over each found item
    for (item of items) {
      // if the item is a directory, it will need to be searched
      if (item.isDirectory()) {
        // call this method recursively, appending the folder name to make a new path
        await findFiles(path.join(folderName, item.name));
      } else {
        // Make sure the discovered file is a .json file
        if (path.extname(item.name) === ".json") {
          // store the file path in the salesFiles array
          salesFiles.push(path.join(folderName, item.name));
        }
      }
    }
  }

  await findFiles(folderName);

  return salesFiles;
}

async function main() {
  const salesDir = path.join(__dirname, "stores");
  const salesTotalsDir = path.join(__dirname, "salesTotals");

  // create the salesTotal directory if it doesn't exist
  try {
    await fs.mkdir(salesTotalsDir);
  } catch {
    console.log(`${salesTotalsDir} already exists.`);
  }

  // find paths to all the sales files
  const salesFiles = await findSalesFiles(salesDir);

  // read through each sales file to calculate the sales total
  const salesTotal = await calculateSalesTotal(salesFiles);

  // write the total to the "totals.json" file
  await fs.writeFile(
    path.join(salesTotalsDir, "totals.txt"),
    `${salesTotal}\r\n`,
    { flag: "a" }
  );
  console.log(`Wrote sales totals to ${salesTotalsDir}`);
}

main();

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