How to save logs to files instead of showing it to the console in Node.js?

Published November 2, 2020

Most of the time when we develop applications we use the console.log() method to log information directly to the console regarding the process in Node.js.

We usually do those loggings like this,

// log something to console
console.log("Hello 😃 ");

But this may not be enough in some cases as we may also want to save some of those logs for safekeeping and reporting.

In that case, we can create a custom logger that saves logs to files.

For that first we need to get the Console class from the console module like this,

// get the Console class
const { Console } = require("console");

Now we need to make a new Console logger and pass the stdout and stderr as an object to the new Console class like this,

// get the Console class
const { Console } = require("console");

// make a new logger
const myLogger = new Console({
  stdout: "", // a write stream,
  stderr: "", // a write stream
});

We have made the new logger called myLogger but we haven't passed the values to stdout and stderr.

The stdout refers to the normal log output and the stderr refers to the error outputs.

Both the stdout and stderr accepts a write stream. If you want to know more about Write streams check out How to create a write stream in Node.js?.

Let's make a write stream called normalStdout.txt where we want all of the normal logs to be saved and another write stream called errStdErr.txt where we want all of the error logs to be saved.

// get the Console class
const { Console } = require("console");
// get fs module for creating write streams
const fs = require("fs");

// make a new logger
const myLogger = new Console({
  stdout: fs.createWriteStream("normalStdout.txt"),
  stderr: fs.createWriteStream("errStdErr.txt"),
});

We made our custom logger! Yay 🚀!

Now let's test out myLogger by using the same method we were using with the normal console object like log() and error() methods.

// get the Console class
const { Console } = require("console");
// get fs module for creating write streams
const fs = require("fs");

// make a new logger
const myLogger = new Console({
  stdout: fs.createWriteStream("normalStdout.txt"),
  stderr: fs.createWriteStream("errStdErr.txt"),
});

// saving to normalStdout.txt file
myLogger.log("Hello 😃. This will be saved in normalStdout.txt file");

// saving to errStdErr.txt file
myLogger.error("Its an error ❌. This will be saved in errStdErr.txt file");

Now the normal logs will be saved to the normalStdout.txt file and the error logs will be saved to the errStdErr.txt file.

See this example live in repl.it.

Feel free to share if you found this useful 😃.


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