Added onChild callback with tests and documentation

docs/api.md

@@ -508,6 +508,18 @@ when using the `transport` option. In this case an `Error` will be thrown.
 
 * See [pino.transport()](#pino-transport)
 
+#### `onChild` (Function)
+
+The `onChild` function is a callback that will be called on each creation of a new child and use the child instance as it's first argument.
+```js
+const parent = require('pino')({ onChild: (instance) => {
+  // Exceute call back code for each newly created child.
+}})
+// `onChild` will now be executed with the new child.
+parent.child(bindings)
+```
+
+
 <a id="destination"></a>
 ### `destination` (SonicBoom | WritableStream | String | Object)
 

lib/proto.js

@@ -141,7 +141,7 @@ function child (bindings, options) {
   instance[chindingsSym] = asChindings(instance, bindings)
   const childLevel = options.level || this.level
   instance[setLevelSym](childLevel)
-
+  this.onChild(instance)
   return instance
 }
 

lib/tools.js

@@ -301,8 +301,9 @@ function createArgsNormalizer (defaultOptions) {
     if ('onTerminated' in opts) {
       throw Error('The onTerminated option has been removed, use pino.final instead')
     }
-    const { enabled } = opts
+    const { enabled, onChild } = opts
     if (enabled === false) opts.level = 'silent'
+    if (!onChild) opts.onChild = noop
     if (!stream) {
       if (!hasBeenTampered(process.stdout)) {
         // If process.stdout.fd is undefined, it means that we are running

pino.d.ts

@@ -79,6 +79,12 @@ interface LoggerExtras<Options = LoggerOptions> extends EventEmitter {
      */
     child<ChildOptions extends pino.ChildLoggerOptions>(bindings: pino.Bindings, options?: ChildOptions): pino.Logger<Options & ChildOptions>;
 
+    /**
+     * A callback that will run on each creation of a new child.
+     * @param child: The newly created child logger instance.
+     */
+    onChild<ChildOptions extends pino.ChildLoggerOptions>(child: pino.Logger<Options & ChildOptions>): void;
+
     /**
      * Registers a listener function that is triggered when the level is changed.
      * Note: When browserified, this functionality will only be available if the `events` module has been required elsewhere

pino.js

@@ -102,7 +102,8 @@ function pino (...args) {
     formatters,
     hooks,
     depthLimit,
-    edgeLimit
+    edgeLimit,
+    onChild
   } = opts
 
   const stringifySafe = configure({
@@ -175,7 +176,8 @@ function pino (...args) {
     [chindingsSym]: chindings,
     [formattersSym]: allFormatters,
     [hooksSym]: hooks,
-    silent: noop
+    silent: noop,
+    onChild
   })
 
   Object.setPrototypeOf(instance, proto())

test/basic.test.js

@@ -725,3 +725,13 @@ test('throws if prettyPrint is passed in as an option', async (t) => {
     })
   }, new Error('prettyPrint option is no longer supported, see the pino-pretty package (https://github.com/pinojs/pino-pretty)'))
 })
+
+test('Should invoke `onChild` with the newly created child', async ({ equal }) => {
+  let innerChild
+  const child = pino({
+    onChild: (instance) => {
+      innerChild = instance
+    }
+  }).child({ foo: 'bar' })
+  equal(child, innerChild)
+})