To create a new file or replace an existing one: fs.writeFile
To write to a file in JS you all you need is a built-in Node.js package “fs”:
const fs = require("fs");
const path = "/path/to/file.txt";
const data = "Sample text data";
function callback(error) {
// error parameter will only be passed if anything goes wrong
if (error) throw error;
else console.log("Success");
}
fs.writeFile(path, data, callback);This simple approach lets you create a file and put a primitive variable into it.
To add to the content of an existing file: fs.appendFile
What if instead of rewriting the content of a file every time you need to add some info to the end of the file? To solve this problem you might wanna take a look at appendFile method:
const fs = require("fs");
const path = "/path/to/file.txt";
const data = "Some extra data";
function callback(error) {
...
}
fs.appendFile(path, data, callback);As you can see, appendFile shares the interface with writeFile. The only difference – the file will not be fully overwritten.
If the file doesn’t exist yet, appendFile will create a new one.
How to put an object into a file in JavaScript
The above methods work excellent while we’re working with primitive variables, such as strings or numbers. But what if you would need to save a more complex entity to a file – an object or an array?
Don’t worry, there is a simple solution: JSON.stringify
const fs = require("fs");
const path = "/path/to/file.json";
const rawData = {
name: "John Doe",
address: "221B Baker Street"
};
function callback(error) {
...
}
const data = JSON.stringify(rawData);
fs.writeFile(path, data, callback);To convert the json data back to an object, you can use another built-in method JSON.parse.
Important note for beginner JavaScript developers: any file operations are only available on the server side, so to write to a file using js, you would need a running Node.js server.
There is no such api in the browser.
Learn more about I/O operations in JavaScript: WSchools
