Fix/follow up 25863

packages/gatsby/src/commands/develop.ts

@@ -163,6 +163,9 @@ let isRestarting
 const REGEX_IP = /^(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])$/
 
 module.exports = async (program: IProgram): Promise<void> => {
+  // In some cases, port can actually be a string. But our codebase is expecting it to be a number.
+  // So we want to early just force it to a number to ensure we always act on a correct type.
+  program.port = parseInt(program.port + ``, 10)
   const developProcessPath = slash(require.resolve(`./develop-process`))
 
   try {

packages/gatsby/src/utils/detect-port-in-use-and-prompt.ts

@@ -5,11 +5,10 @@ import prompts from "prompts"
 export const detectPortInUseAndPrompt = async (
   port: number
 ): Promise<number> => {
-  let foundPort = typeof port === `string` ? parseInt(port, 10) : port
-  const detectedPort = await detectPort(foundPort).catch((err: Error) =>
+  const detectedPort = await detectPort(port).catch((err: Error) =>
     report.panic(err)
   )
-  if (foundPort !== detectedPort) {
+  if (port !== detectedPort) {
     report.log(`\nSomething is already running at port ${port}`)
     const response = await prompts({
       type: `confirm`,
@@ -18,11 +17,11 @@ export const detectPortInUseAndPrompt = async (
       initial: true,
     })
     if (response.newPort) {
-      foundPort = detectedPort
+      port = detectedPort
     } else {
       throw new Error(`USER_REJECTED`)
     }
   }
 
-  return foundPort
+  return port
 }