Deal with prettifier function returning undefined

bin.js

@@ -24,12 +24,14 @@ args
   .example('cat log | pino-pretty -t', 'To convert Epoch timestamps to ISO timestamps use the -t option')
   .example('cat log | pino-pretty -t "SYS:yyyy-mm-dd HH:MM:ss"', 'To convert Epoch timestamps to local timezone format use the -t option with "SYS:" prefixed format string')
   .example('cat log | pino-pretty -l', 'To flip level and time/date in standard output use the -l option')
-  .example('cat log | pino-pretty -s \'msg == `hello world`\'', 'Only prints messages with msg equals to \'hello world\'')
+  .example('cat log | pino-pretty -s "msg == \'hello world\'"', 'Only prints messages with msg equals to \'hello world\'')
 
 const opts = args.parse(process.argv)
 const pretty = prettyFactory(opts)
 const prettyTransport = through.obj(function (chunk, enc, cb) {
-  process.stdout.write(pretty(chunk.toString()))
+  const line = pretty(chunk.toString())
+  if (line === undefined) return cb()
+  process.stdout.write(line)
   cb()
 })
 

test/basic.test.js

@@ -419,5 +419,14 @@ test('basic prettifier tests', (t) => {
     log.info({ foo: { bar: true } }, 'foo')
   })
 
+  t.test('handles `undefined` return values', (t) => {
+    t.plan(2)
+    const pretty = prettyFactory({search: 'msg == \'hello world\''})
+    let formatted = pretty(`{"msg":"nope", "time":${epoch}, "level":30, "v":1}`)
+    t.is(formatted, undefined)
+    formatted = pretty(`{"msg":"hello world", "time":${epoch}, "level":30, "v":1}`)
+    t.is(formatted, `[${epoch}] INFO: hello world\n`)
+  })
+
   t.end()
 })