How to write to files with Node.js

In this post we’ll be covering how you can use the fs module in Node.js to write to and modify files with writeFile() and writeFileSync(). We’ll also take a look at an alternative library we can use called replace-in-file.

This post assumes you have a general knowledge of JavaScript syntax, but doesn’t expect any Node.js-specific knowledge.

If you want to learn how to write and run a Node.js script, I have covered that in my previous post on automating file renaming with Node.js.

Overwrite a file with Node.js

Using fs writeFile to write to files (asynchronous)

Let's jump straight into things with an fs writeFile() example:

const { writeFile } = require('fs');

const content = 'Hello world';
const file = '/Users/emma/src/emgoto.com/file.txt';

const callback = (err) => { /** Handle errors */ };

writeFile(file, content, callback);

This will change the contents of the file to contain the string “Hello world”.

The callback function will be executed once the file has been successfully written to, or it errors out.

Using fs writeFileSync to write to files (synchronous)

If you want an synchronous version, you can use writeFileSync:

const { writeFileSync } = require('fs');

const content = 'Hello world';
const file = '/Users/emma/src/emgoto.com/file.txt';

writeFileSync(file, content);

Read from a file, then modify part of it with Node.js

Using fs readFileSync to read from a file (synchronous)

If you want to modify a file, rather than write over its contents, you'll first need to read it. We can use readFileSync here:

const { readFileSync } = require('fs');

const content = readFileSync(file, 'utf8');
const newContent = content.replace('Hello', 'Goodbye');

writeFile(file, newContent, () => {});

After we read the file, we will have a copy of the file's contents. We can then use JavaScript's replace function to modify it, before updating the file with the new contents.

Using fs readFile to read from a file (asynchronous)

The asynchronous version of readFileSync is readFile:

const { readFile, writeFile } = require('fs');

const callback = (err, data) => {
    const newContent = data.replace('Hello', 'Goodbye');
    writeFile(file, newContent);
}

readFile(file, 'utf8', callback);

Here, when we successfully get the contents of the file (or it errors out), the callback function will be called. Then we can use writeFile to modify the contents of the file.

How to effectively use JavaScript’s replace function

In the above example, you’ll notice we were passing strings into JavaScript’s replace function:

content.replace('Hello', 'Goodbye');

This only replaces the first instance of "Hello" with "Goodbye". If you want to replace more than one instance, you’ll can make use of Regex:

content.replace(/Hello/g, 'Goodbye');

If you're rusty on your Regex (or haven’t used it before):

• Regex patterns live inside of //
• The g on the end signifies that it is “global”, which means it will find all occurrences.
• /Hello/g will find all instances of the string “Hello”

Regex can do many more cool things, but I won’t be diving into that in this post! When writing your Regex patterns, I recommend testing it with a tool like RegExr.

As well as strings and Regex, we can also pass in functions!

const addSpacesBetweenLetters = (string) => 
    string.split('').join(' ');

content.replace(/Hello/g, addSpacesBetweenLetters);

This would convert all instances of “Hello” to “H e l l o”.

Using replace-in-file to write to files

Instead of fs readFile and writeFile, there’s also a handy library we can use called replace-in-file.

To replace all instances of "Hello" with "Goodbye", you would do the following:

const replace = require('replace-in-file');

const options = {
    files: fileName,
    from: /Hello/g,
    to: 'Goodbye',
};

replace(options);

You'll notice that we no longer have to open the file and get its contents - replace-in-file will handle that for you.

The from and to variables accept strings, Regex patterns and functions.

fs writeFile vs replace-in-file

For simple use cases, fs writeFile will get the job done. There are a couple of extra features that the replace-in-file library has which make it pretty useful.

It can replace content in multiple files at once:

const replace = require('replace-in-file');

const options = {
    files: [fileName, anotherFileName],
    from: /Hello/g,
    to: 'Goodbye',
};

replace(options);

It can also replace things in bulk:

const options = {
    files: [fileName, anotherFileName],
    from: [/Hello/g, /Foo/g],
    to: ['Goodbye', 'Bar'],
}

When you're using arrays with to and from:

• The from in the 0th position will convert to the to in the 0th position
• The from in the 1st position will convert to the to in the 1st position

And so on!