feature: allow the user to set the messageFormat option as a function

Readme.md

@@ -166,6 +166,22 @@ const prettifyQuery = value => {
 }
 ```
 
+`messageFormat` option allows you to customize the message output. The format can be defined by a template `string` like this:
+```js
+{
+  messageFormat: '{levelLabel} - {pid} - url:{request.url}'
+}
+```
+But this option can also be defined as a `function` with this prototype:
+```js
+{
+  messageFormat: (log, messageKey, levelLabel) => {
+    // do some log message customization
+    return customized_message;
+  }
+}
+```
+
 <a id="license"><a>
 ## License
 

lib/utils.js

@@ -190,7 +190,7 @@ function prettifyLevel ({ log, colorizer = defaultColorizer, levelKey = LEVEL_KE
  * @param {object} input.log The log object with the message to colorize.
  * @param {string} [input.messageKey='msg'] The property of the `log` that is the
  * message to be prettified.
- * @param {string} [input.messageFormat=undefined] A format string that defines how the
+ * @param {string|function} [input.messageFormat=undefined] A format string or function that defines how the
  *  logged message should be formatted, e.g. `'{level} - {pid}'`.
  * @param {function} [input.colorizer] A colorizer function that has a
  * `.message(str)` method attached to it. This function should return a colorized
@@ -201,7 +201,7 @@ function prettifyLevel ({ log, colorizer = defaultColorizer, levelKey = LEVEL_KE
  * that is the prettified message.
  */
 function prettifyMessage ({ log, messageFormat, messageKey = MESSAGE_KEY, colorizer = defaultColorizer, levelLabel = LEVEL_LABEL }) {
-  if (messageFormat) {
+  if (messageFormat && typeof messageFormat === 'string') {
     const message = String(messageFormat).replace(/{([^{}]+)}/g, function (match, p1) {
       // return log level as string instead of int
       if (p1 === levelLabel && log[LEVEL_KEY]) {
@@ -217,6 +217,10 @@ function prettifyMessage ({ log, messageFormat, messageKey = MESSAGE_KEY, colori
     })
     return colorizer.message(message)
   }
+  if (messageFormat && typeof messageFormat === 'function') {
+    const msg = messageFormat(log, messageKey, levelLabel)
+    return colorizer.message(msg)
+  }
   if (messageKey in log === false) return undefined
   if (typeof log[messageKey] !== 'string') return undefined
   return colorizer.message(log[messageKey])

test/lib/utils.public.test.js

@@ -116,6 +116,18 @@ tap.test('prettifyMessage', t => {
     t.is(str, 'localhost/test - param:  - foo')
   })
 
+  t.test('`messageFormat` supports function definition', async t => {
+    const str = prettifyMessage({
+      log: { level: 30, request: { url: 'localhost/test' }, msg: 'incoming request' },
+      messageFormat: (log, messageKey, levelLabel) => {
+        let msg = log[messageKey]
+        if (msg === 'incoming request') msg = `--> ${log.request.url}`
+        return msg
+      }
+    })
+    t.is(str, '--> localhost/test')
+  })
+
   t.end()
 })