How to Replace a String in a File using Node.js

# Replace a String in a File using Node.js

To replace a string in a file in Node.js:

1. Use the fs.readFile() method to read the file.
2. Use the replace() method to replace the string with the replacement.
3. Write the result to the file using the fs.writeFile() method.

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// 👇️ if using ES6 imports uncomment next line
// import {readFile, writeFile, writeFileSync, promises as fsPromises} from 'fs';
const {readFile, writeFile} = require('fs');

readFile('./example.txt', 'utf-8', function (err, contents) {
  if (err) {
    console.log(err);
    return;
  }

  const replaced = contents.replace(/to be replaced/g, 'replacement string');

  writeFile('./example.txt', replaced, 'utf-8', function (err) {
    console.log(err);
  });
});

Note: if you use the ES6 import/export syntax, use the following import statement instead.

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import {readFile, writeFile, writeFileSync, promises as fsPromises} from 'fs';

We used the fs.readFile method to read the contents of the file.

The first parameter we passed to the method is the path to the file and the second is the encoding.

The callback function gets called with an error object or null (if there is no error) and the contents of the file.

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bobbyhadz.com

After running the node index.js command, the example.txt file looks as follows:

We used the String.replace() method to replace a string in the file.

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const replaced = contents.replace(/to be replaced/g, 'replacement string');

The forward slashes / / mark the beginning and end of the regular expression.

The g flag stands for global and replaces all occurrences of the matched string.

# Replace a String in a File ignoring the case

If you want to perform a case-insensitive match, use the i flag as well.

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// 👇️ if using ES6 imports uncomment next line
// import {readFile, writeFile, writeFileSync, promises as fsPromises} from 'fs';
const {readFile, writeFile, promises: fsPromises} = require('fs');

readFile('./example.txt', 'utf-8', function (err, contents) {
  if (err) {
    console.log(err);
    return;
  }

  // 👇️ match string case-insensitively 👇️
  const replaced = contents.replace(/to be replaced/gi, 'replacement string');

  writeFile('./example.txt', replaced, 'utf-8', function (err) {
    console.log(err);
  });
});

Note: if you use the ES6 import/export syntax, use the following import statement instead.

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import {
  readFile,
  writeFile,
  writeFileSync,
  promises as fsPromises,
} from 'fs';

The i flag stands for ignore and does a case-insensitive search in string.

The fs.writeFile() function is then used to write the updated string to the file.

Alternatively, you can use promises.

# Replace a String in a File using Promises

To replace a string in a file asynchronously:

1. Use the fsPromises.readFile() method to read the file.
2. Use the replace() method to replace the string with the replacement.
3. Write the result to the file using the fsPromises.writeFile() method.

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// 👇️ if using ES6 imports uncomment next line
// import {readFile, writeFile, writeFileSync, promises as fsPromises} from 'fs';
const {promises: fsPromises} = require('fs');

async function replaceInFile(filename, replacement) {
  try {
    const contents = await fsPromises.readFile(filename, 'utf-8');
    const replaced = contents.replace(/to be replaced/gi, replacement);

    await fsPromises.writeFile(filename, replaced);
  } catch (err) {
    console.log(err);
  }
}

replaceInFile('./example.txt', 'replacement string 🎊');

This approach achieves the same result. However, this code sample uses promises instead of callbacks.

The fsPromises.readFile() method asynchronously reads the contents of the provided file.

The method returns a promise that resolves with the contents of the file, so we have to await it or use the .then() method to get the resolved string.

The fsPromises.writeFile() method asynchronously writes data to a file replacing the file's contents if the file already exists.

# Additional Resources

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