A logger abstraction for libraries

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There's a lot of logging frameworks out there; if we picked one then it'd be the wrong one for many users. Just using console.log isn't optimal for people who want a consistent logging solution.
@graphile/logger acts as an extremely lightweight zero-dependency TypeScript-native abstraction; it allows our libraries to get on with the job of logging whilst allowing library users to override how/where the logs are output.
Originally we built this for Graphile Worker, but as desire for similar facilities in our other projects grew we decided to roll it out into its own project.


Feature complete. Ignoring comments, once compiled it's only about 60 lines of code!
(This is a Node library, it's not intended to be used in the browser.)


Install via yarn add @graphile/logger or npm install @graphile/logger.

Usage for library consumers

This section is for users of a library that users @graphile/logger who want to know how to hook up their preferred logging software instead of using the console fallback. If you're a library author and you want to use @graphile/logger in your project, please instead see Usage for library authors below.

Creating a custom Logger

To create a custom logger you first need to create a log function factory - that is to say a function which returns a log function. Here's a trivially simple log function factory which logs to console:
function logFunctionFactory(scope) {
  return function logFunction(level, message, meta) {
    console.log(`${level}: ${message});

Your log function factory should conform to the LogFunctionFactory interface defined in the source code.
Once you have your log function factory you can create a logger from it:
import { Logger } from "@graphile/logger";

const logger = new Logger(logFunctionFactory);

Now you can pass your custom logger to the library you're using.

Bunyan example

Here's an example of logging with bunyan:
import { Logger, LogLevel } from "@graphile/logger";
import bunyan from "bunyan";

const bunyanLog = bunyan.createLogger({ name: "myapp" });

const logger = new Logger((scope) => {
  const scopedBunyanLog = bunyanLog.child(scope);
  return (level, message, meta) => {
    switch (level) {
      case LogLevel.ERROR:
        return scopedBunyanLog.error(`%s`, message, meta);
      case LogLevel.WARNING:
        return scopedBunyanLog.warn(`%s`, message, meta);
      case LogLevel.DEBUG:
        return scopedBunyanLog.debug(`%s`, message, meta);
      case LogLevel.INFO:
        return scopedBunyanLog.info(`%s`, message, meta);

logger.info("Hello with Bunyan", { randomNumber: { fairDiceRoll: 4 } });

Usage for library authors

This section is for library authors who want to use @graphile/logger in their library. If you're a user of a library that users @graphile/logger and you want to know how to hook up your preferred logging software, please see Usage for library consumers above.


When you're logging in your library code, you'll be dealing with a Logger instance (for creating your own Logger instance, see Usage for library consumers above). Logger has 5 methods you'll care about.
The first for methods ─ error, warn, info and debug ─ are your four logging methods. They all accept a message string and an optional meta object. It's often useful to put additional information into the meta object so that people who are doing structured (i.e. JSON) logging can filter on these special properties. We strongly recommend you only put JSON-able values into the meta, and be careful with personally identifiable information (PII) just as you would with the log message itself.
logger.info("Hello world!");
logger.error("++?????++ Out of Cheese Error. Redo From Start.");
logger.debug("The answer to the big question... 42", { meaningOfLife: 42 });

The fifth method, scope, returns a new Logger instance with a modified (generally narrower) scope.
const newLogger = logger.scope({ id: request.id });

You can use the scope to give more "ambient" information to your users, telling them about the context in which the logger is running - e.g. what's the task or request identifier that's being dealt with.


Great as a fallback when the user opts to not pass their own custom logger, this requires no setup and just logs to console; you should use this as the fallback if you don't have specific logging requirements.
import { defaultLogger } from "@graphile/logger";

function myAwesomeLibraryMethod(logger = defaultLogger) {
  logger.info("Hi with meta", { meta: true, meaning: 42 });



If you want your logs to be a little more custom, or there's particular information you want them to include from the scope, then a logger based on a custom log factory is probably what you want to use as the fallback in your library, rather than defaultLogger. For example, in Graphile Worker we have a default logger that factors the workerId, taskIdentifier and jobId into the output log messages.
You can read more about this in the source of this library (which is short and heavily documented); here's an illustrative example:
import { Logger, makeConsoleLogFactory } from "@graphile/logger";

interface LogScope {
  label?: string;
  workerId?: string;
  taskIdentifier?: string;
  jobId?: string;

const graphileWorkerDefaultLogger = new Logger<LogScope>(
    format: `[%s%s] %s: %s`,
    formatParameters(level, message, scope) {
      const taskText = scope.taskIdentifier ? `: ${scope.taskIdentifier}` : "";
      const jobIdText = scope.jobId ? `{${scope.jobId}}` : "";
      return [
        scope.label || "core",
        scope.workerId ? `(${scope.workerId}${taskText}${jobIdText})` : "",

function worker(logger: Logger<LogScope> = graphileWorkerDefaultLogger) {
  logger.info("Starting worker cluster...");

  // ...

  const workerLogger = logger.scope({
    workerId: "3b0e05e1-beed-48a6-9d2b-055e47a29b5f",
  workerLogger.info("Looking for jobs...");

  // ...

  const jobLogger = workerLogger.scope({
    taskIdentifier: "my_task",
    jobId: 84,
  jobLogger.info("Starting job...");


which would output:
[core] INFO: Starting worker cluster...
[core(3b0e05e1-beed-48a6-9d2b-055e47a29b5f)] INFO: Looking for jobs...
[core(3b0e05e1-beed-48a6-9d2b-055e47a29b5f: my_task{84})] INFO: Starting job...