Use config.logger as a logger constructor

README.md

@@ -70,13 +70,20 @@ The following presets are supported:
 
 By default, this library uses an instance of Pino with a modified [`logMethod`](https://getpino.io/#/docs/api?id=logmethod), to give reasonable out-the-box behaviour for JSON logging. If you need logs in a different format, for example to change the message field or transform logged objects, you can provide your own instance of Pino to the library.
 
-This is done by creating a `next-logger.config.js` file in the root of your project. The file should be a CommonJS module, and your custom Pino instance should be exported in a field called `logger`. For example:
+This is done by creating a `next-logger.config.js` file in the root of your project. The file should be a CommonJS module, and a function returning your custom Pino instance should be exported in a field called `logger`. This function will be called with the library's default Pino configuration, to allow you to extend it's behaviour (or completely replace it).
+
+For example:
 
 ```js
 // next-logger.config.js
 const pino = require('pino')
 
-const logger = pino({ messageKey: 'message', mixin: () => ({ name: 'custom-pino-instance' }) })
+const logger = defaultConfig =>
+  pino({
+    ...defaultConfig,
+    messageKey: 'message',
+    mixin: () => ({ name: 'custom-pino-instance' }),
+  })
 
 module.exports = {
   logger,

lib/defaultLogger.js → lib/defaultPinoConfig.js

@@ -1,7 +1,6 @@
 const { format } = require('util')
-const pino = require('pino')
 
-module.exports = pino({
+module.exports = {
   level: 'debug',
   hooks: {
     // https://getpino.io/#/docs/api?id=logmethod
@@ -55,4 +54,4 @@ module.exports = pino({
       return method.apply(this, [mergingObject, message])
     },
   },
-})
+}

lib/logger.js

@@ -1,9 +1,9 @@
 const { cosmiconfigSync } = require('cosmiconfig')
+const pino = require('pino')
 
-const defaultLogger = require('./defaultLogger')
+const defaultPinoConfig = require('./defaultPinoConfig')
 
 let config = {}
-let logger = defaultLogger
 
 const explorerSync = cosmiconfigSync('next-logger')
 const results = explorerSync.search()
@@ -12,8 +12,13 @@ if (results && results.config) {
   config = results.config
 }
 
-if ('logger' in config && config.logger && typeof config.logger.child === 'function') {
+// Set the default logger constructor to Pino.
+let logger = pino
+
+// If logger exists in the config file, and it's a function, use it as the logger constructor.
+if ('logger' in config && typeof config.logger === 'function') {
   logger = config.logger
 }
 
-module.exports = logger
+// Call the logger constructor with the library's default Pino configuration.
+module.exports = logger(defaultPinoConfig)

tests/config/next-logger.config.js

@@ -1,6 +1,11 @@
 const pino = require('pino')
 
-const logger = pino({ messageKey: 'message', mixin: () => ({ name: 'custom-pino-instance' }) })
+const logger = defaultConfig =>
+  pino({
+    ...defaultConfig,
+    messageKey: 'message',
+    mixin: () => ({ name: 'custom-pino-instance' }),
+  })
 
 module.exports = {
   logger,

tests/defaultLogger.spec.js → tests/defaultPinoConfig.spec.js

@@ -3,11 +3,11 @@ const { promisify } = require('util')
 
 const execAsync = promisify(exec)
 
-describe('defaultLogger', () => {
+describe('defaultPinoConfig', () => {
   const logArgs = async (args, method = 'info') => {
     // This is needed to escape double quotes in arguments. For example, turning "Hello World" into \"Hello World\"
     const safeArgs = args.map(arg => String(arg).replace(/"/g, '\\"')).join(',')
-    const process = execAsync(`node -e "require('./lib/defaultLogger').${method}(${safeArgs})"`)
+    const process = execAsync(`node -e "require('pino')(require('./lib/defaultPinoConfig')).${method}(${safeArgs})"`)
     const { stdout } = await process
     return JSON.parse(stdout.trim())
   }