feat(mixin): add mixin config option

docs/api.md

@@ -96,6 +96,27 @@ logger.foo('hi')
 logger.info('hello') // Will throw an error saying info in not found in logger object
 ```
 
+#### `mixin` (Function):
+
+Default: `undefined`
+
+If provided, the `mixin` function is called each time one of the active
+logging methods is called. The function must return an object. The
+properties of the returned object will be added to the logged JSON.
+
+```js
+let n = 0
+const logger = pino({
+  mixin () {
+    return { line: ++n }
+  }
+})
+logger.info('hello')
+// {"level":30,"time":1573664685466,"pid":78742,"hostname":"x","line":1,"msg":"hello","v":1}
+logger.info('world')
+// {"level":30,"time":1573664685469,"pid":78742,"hostname":"x","line":2,"msg":"hello","v":1}
+```
+
 #### `redact` (Array | Object):
 
 Default: `undefined`

lib/proto.js

@@ -11,6 +11,7 @@ const {
   setLevelSym,
   getLevelSym,
   chindingsSym,
+  mixinSym,
   asJsonSym,
   messageKeySym,
   writeSym,
@@ -115,14 +116,15 @@ function bindings () {
 function write (_obj, msg, num) {
   const t = this[timeSym]()
   const messageKey = this[messageKeySym]
+  const mixin = this[mixinSym]
   const objError = _obj instanceof Error
   var obj
 
   if (_obj === undefined || _obj === null) {
-    obj = {}
+    obj = mixin ? mixin() : {}
     obj[messageKey] = msg
   } else {
-    obj = Object.assign({}, _obj)
+    obj = Object.assign(mixin ? mixin() : {}, _obj)
     if (msg) {
       obj[messageKey] = msg
     } else if (objError) {

lib/symbols.js

@@ -6,6 +6,7 @@ const levelValSym = Symbol('pino.levelVal')
 const useLevelLabelsSym = Symbol('pino.useLevelLabels')
 const changeLevelNameSym = Symbol('pino.changeLevelName')
 const useOnlyCustomLevelsSym = Symbol('pino.useOnlyCustomLevels')
+const mixinSym = Symbol('pino.mixin')
 
 const lsCacheSym = Symbol('pino.lsCache')
 const chindingsSym = Symbol('pino.chindings')
@@ -37,6 +38,7 @@ module.exports = {
   getLevelSym,
   levelValSym,
   useLevelLabelsSym,
+  mixinSym,
   lsCacheSym,
   chindingsSym,
   parsedChindingsSym,

pino.js

@@ -29,6 +29,7 @@ const {
   messageKeySym,
   useLevelLabelsSym,
   changeLevelNameSym,
+  mixinSym,
   useOnlyCustomLevelsSym
 } = symbols
 const { epochTime, nullTime } = time
@@ -71,6 +72,7 @@ function pino (...args) {
     customLevels,
     useLevelLabels,
     changeLevelName,
+    mixin,
     useOnlyCustomLevels
   } = opts
 
@@ -92,6 +94,7 @@ function pino (...args) {
   const timeSliceIndex = time().indexOf(':') + 1
 
   if (useOnlyCustomLevels && !customLevels) throw Error('customLevels is required if useOnlyCustomLevels is set true')
+  if (mixin && typeof mixin !== 'function') throw Error(`Unknown mixin type "${typeof mixin}" - expected "function"`)
 
   assertDefaultLevelFound(level, customLevels, useOnlyCustomLevels)
   const levels = mappings(customLevels, useOnlyCustomLevels)
@@ -110,6 +113,7 @@ function pino (...args) {
     [formatOptsSym]: formatOpts,
     [messageKeySym]: messageKey,
     [serializersSym]: serializers,
+    [mixinSym]: mixin,
     [chindingsSym]: chindings
   }
   Object.setPrototypeOf(instance, proto)

test/mixin.test.js

@@ -0,0 +1,106 @@
+'use strict'
+
+const os = require('os')
+const { test } = require('tap')
+const { sink, once } = require('./helper')
+const pino = require('../')
+
+const { pid } = process
+const hostname = os.hostname()
+const level = 50
+const name = 'error'
+
+test('mixin object is included', async ({ ok, same }) => {
+  let n = 0
+  const stream = sink()
+  const instance = pino({
+    mixin () {
+      return { hello: ++n }
+    }
+  }, stream)
+  instance.level = name
+  instance[name]('test')
+  const result = await once(stream, 'data')
+  ok(new Date(result.time) <= new Date(), 'time is greater than Date.now()')
+  delete result.time
+  same(result, {
+    pid,
+    hostname,
+    level,
+    msg: 'test',
+    hello: 1,
+    v: 1
+  })
+})
+
+test('mixin object is new every time', async ({ plan, ok, same }) => {
+  plan(6)
+
+  let n = 0
+  const stream = sink()
+  const instance = pino({
+    mixin () {
+      return { hello: n }
+    }
+  }, stream)
+  instance.level = name
+
+  while (++n < 4) {
+    const msg = `test #${n}`
+    stream.pause()
+    instance[name](msg)
+    stream.resume()
+    const result = await once(stream, 'data')
+    ok(new Date(result.time) <= new Date(), 'time is greater than Date.now()')
+    delete result.time
+    same(result, {
+      pid,
+      hostname,
+      level,
+      msg,
+      hello: n,
+      v: 1
+    })
+  }
+})
+
+test('mixin object is not called if below log level', async ({ ok }) => {
+  const stream = sink()
+  const instance = pino({
+    mixin () {
+      ok(false, 'should not call mixin function')
+    }
+  }, stream)
+  instance.level = 'error'
+  instance.info('test')
+})
+
+test('mixin object + logged object', async ({ ok, same }) => {
+  let n = 0
+  const stream = sink()
+  const instance = pino({
+    mixin () {
+      return { hello: ++n }
+    }
+  }, stream)
+  instance.level = name
+  instance[name]({ foo: 42 })
+  const result = await once(stream, 'data')
+  ok(new Date(result.time) <= new Date(), 'time is greater than Date.now()')
+  delete result.time
+  same(result, {
+    pid,
+    hostname,
+    level,
+    foo: 42,
+    hello: 1,
+    v: 1
+  })
+})
+
+test('mixin not a function', async ({ throws }) => {
+  const stream = sink()
+  throws(function () {
+    pino({ mixin: 'not a function' }, stream)
+  })
+})