@logtail/core

Logtail JavaScript client by Better Stack - Logging core library

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@logtail/core

This is an NPM package that provides core logging functionality.

It's used by the Node and browser logging packages.

You typically wouldn't require this package directly, unless you're building a custom logger.

The Base class

The Base class provides core features that are extended by loggers.

For example - you could create a custom logger that implements its own sync method, for getting data over to Logtail.com

import { Base } from "@logtail/core";
import { ILogtailOptions, ILogtailLog } from "@logtail/types";

class CustomLogger extends Base {
  // Constructor must take a Logtail.com source token, and (optional) options
  public constructor(
    sourceToken: string,
    options?: Partial<ILogtailOptions>
  ) {
    // Make sure you pass the source token to the parent constructor!
    super(sourceToken, options);

    // Create a custom sync method
    this.setSync(async (logs: ILogtailLog[]) => {
      // Sync the `log` somehow ... `this._sourceToken` contains your Logtail source token

      // ....

      // Finally, return the log... which will resolve our initial `.log()` call
      return logs;
    });
  }
}

Logging

Logging to Logtail is simple - just call the .log() function with a string message:

// Simple log message (defaults to the 'info' log level)
logtail.log("Hello Logtail!");

// Or, add custom context keys to pass along with the log
logtail.log("Once more, with context", {
  httpRequest: {
    "user-agent": {
      browser: "Chrome",
    },
  },
});

There are four levels of logging, each corresponding to a function:

Function	Log level	Example
.debug()	Debug - logs to report/diagnose system events	Currently logged in session object, during development
.info()	Info - data/events where no action is required; for information only	User successfully logged in
.warn()	Warning - advisory messages that something probably needs fixing, but not serious enough to cause an error	SQL query ran slower than expected
.error()	Error - something went wrong	Couldn't connect to database

By default, .log() logs at the 'info' level. You can use the above explicit log levels instead by calling the relevant function with your log message.

All log levels return a Promise that will resolve once the log has been synced with Logtail.com:

// Will resolve when synced with Logtail.com (or reject if there's an error)
logtail.log("some log message").then((log) => {
  // `log` is the transformed log, after going through middleware
});

Middleware

You can add your own middleware functions, which act as transforms on the ILogtailLog log object.

This is useful for augmenting the log prior to syncing with Logtail, or even pushing the log to another service.

Here's what a middleware function looks like:

import { ILogtailLog } from "@logtail/types";

// In this example function, we'll add custom 'context' meta to the log
// representing the currently logged in user.
//
// Note: a middleware function is any function that takes an `ILogtailLog`
// and returns a `Promise<ILogtailLog>`
async function addCurrentUser(log: ILogtailLog): Promise<ILogtailLog> {
  return {
    ...log, // <-- copy the existing log
    user: {
      // ... and add our own `context` data
      id: 1000,
      name: "John",
    },
  };
}

Then just attach to the Logtail instance with .use:

logtail.use(addCurrentUser);

You can add any number of pipeline functions to your logger instance, and they'll run in order.

Middleware functions run before the final sync to Logtail.com. Pipeline functions should return a Promise<ILogtailLog>, making it possible to augment logs with asynchronous data from external sources.

Note: If an exception is thrown anywhere in the pipeline chain, the log won't be synced. Wrap an async try/catch block around your call to .log|info|debug|warn|error() or tack on a .catch() to ensure your errors are handled.

Removing middleware

If you wish to remove middleware, pass in the original middleware function to .remove():

// `addCurrentUser` will no longer be used to transform logs
logtail.remove(addCurrentUser);

This will remove the middleware function from all future calls to .log|info|debug|warn|error().

To re-add middleware, pass it to .use()

LICENSE

ISC