diff --git a/lib/util.js b/lib/util.js
index b876960..3d5d243 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -18,6 +18,15 @@ function when (ee, evt) {
 }
 
 function spawnOnPort (cmd, port) {
+  if (typeof cmd === 'function') {
+    return new Promise((resolve, reject) => {
+      cmd(port, err => {
+        if (err) reject(err)
+        else resolve(0, null)
+      })
+    })
+  }
+
   const sp = execSpawn(envString(cmd, {PORT: port + ''}), {stdio: 'inherit'})
   return new Promise((resolve, reject) => {
     sp.on('exit', (code, signal) => {
diff --git a/lib/validate.js b/lib/validate.js
index a970ed9..e89b4c3 100644
--- a/lib/validate.js
+++ b/lib/validate.js
@@ -6,6 +6,9 @@ module.exports = (schema) => {
   return validate
 
   function validate (args) {
+    const onPortFn = typeof args.onPort === 'function' && args.onPort
+    if (onPortFn) delete args.onPort
+
     const privateProps = {
       workingDir: {type: 'string'}
     }
@@ -14,7 +17,10 @@ module.exports = (schema) => {
       properties: {...schema.properties, ...privateProps}
     }
     )
-    if (valid(args)) return
+    if (valid(args)) {
+      if (onPortFn) args.onPort = onPortFn
+      return
+    }
     const [{keyword, dataPath, params, message}] = valid.errors
     if (keyword === 'type') {
       const flag = dataPath.substr(
diff --git a/platform/v8.js b/platform/v8.js
index 8896485..841bbaf 100644
--- a/platform/v8.js
+++ b/platform/v8.js
@@ -137,4 +137,4 @@ function collectInliningInfo (sp) {
     cb()
   }))
   return inlined
-}
\ No newline at end of file
+}