pass merging object to mixins (pinojs#926)

* pass merging object to mixins

* add tests for passing mixin context

* improve mixin context tests

* add usage of mixin argument to documentation

Author: madelinecameron

docs/api.md

@@ -99,7 +99,7 @@ logger.info('hello') // Will throw an error saying info in not found in logger o
 Default: `undefined`
 
 If provided, the `mixin` function is called each time one of the active
-logging methods is called. The function must synchronously return an
+logging methods is called. The first and only parameter is the value `mergeObject` or an empty object. The function must synchronously return an
 object. The properties of the returned object will be added to the
 logged JSON.
 
@@ -124,7 +124,7 @@ const mixin = {
 }
 
 const logger = pino({
-    mixin() {
+    mixin({ description }) {
         return mixin
     }
 })

lib/proto.js

@@ -147,9 +147,9 @@ function write (_obj, msg, num) {
   var obj
 
   if (_obj === undefined || _obj === null) {
-    obj = mixin ? mixin() : {}
+    obj = mixin ? mixin({}) : {}
   } else {
-    obj = Object.assign(mixin ? mixin() : {}, _obj)
+    obj = Object.assign(mixin ? mixin(_obj) : {}, _obj)
     if (!msg && objError) {
       msg = _obj.message
     }

test/mixin.test.js

@@ -101,3 +101,36 @@ test('mixin not a function', async ({ throws }) => {
     pino({ mixin: 'not a function' }, stream)
   })
 })
+
+test('mixin can use context', async ({ ok }) => {
+  const stream = sink()
+  const instance = pino({
+    mixin (context) {
+      ok(context !== null && context !== undefined, 'context should be defined')
+      return Object.assign({
+        error: context.message,
+        stack: context.stack
+      })
+    }
+  }, stream)
+  instance.level = name
+  instance[name]({
+    message: '123',
+    stack: 'stack'
+  }, 'test')
+})
+
+test('mixin works without context', async ({ ok }) => {
+  const stream = sink()
+  const instance = pino({
+    mixin (context) {
+      ok(context !== null && context !== undefined, 'context is still defined w/o passing mergeObject')
+
+      return {
+        something: true
+      }
+    }
+  }, stream)
+  instance.level = name
+  instance[name]('test')
+})