Add PORT env var to be used (#952)

* Add PORT env var to be used

* Format

* Create few-carpets-sing.md

* Update few-carpets-sing.md

2 files changed, 42 insertions, 7 deletions

diff --git a/.changeset/few-carpets-sing.md b/.changeset/few-carpets-sing.md
new file mode 100644
index 000000000..3b938a3b2
--- /dev/null
+++ b/.changeset/few-carpets-sing.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Allow dev server port to be set by `PORT` environment variable
diff --git a/packages/astro/src/cli.ts b/packages/astro/src/cli.ts
index accdb7978..74d1e4302 100644
--- a/packages/astro/src/cli.ts
+++ b/packages/astro/src/cli.ts
@@ -35,14 +35,44 @@ interface CLIState {
   };
 }
 
+type TypeOf = 'string' | 'boolean' | 'number';
+// Validates property types of object
+const validateOptions = (opts: Record<string, any>, specs: Record<string, TypeOf | [(v: any) => unknown, TypeOf]>) =>
+  Object.entries(specs).reduce<Record<string, any>>((options, [k, spec]) => {
+    const v = opts[k];
+
+    if (typeof spec === 'string' && typeof v === spec) {
+      options[k] = v;
+    } else if (Array.isArray(spec)) {
+      const [coercion, test] = spec;
+      const result = coercion(v);
+      if (typeof result === test) {
+        options[k] = result;
+      }
+    }
+
+    return options;
+  }, {});
+
 /** Determine which action the user requested */
 function resolveArgs(flags: Arguments): CLIState {
-  const options: CLIState['options'] = {
-    projectRoot: typeof flags.projectRoot === 'string' ? flags.projectRoot : undefined,
-    site: typeof flags.site === 'string' ? flags.site : undefined,
-    sitemap: typeof flags.sitemap === 'boolean' ? flags.sitemap : undefined,
-    port: typeof flags.port === 'number' ? flags.port : undefined,
-    config: typeof flags.config === 'string' ? flags.config : undefined,
+  const { PORT } = process.env;
+
+  // Merge options (Flags take priority)
+  const options = {
+    ...validateOptions(
+      { port: PORT },
+      {
+        port: [parseInt, 'number'],
+      }
+    ),
+    ...validateOptions(flags, {
+      projectRoot: 'string',
+      site: 'string',
+      sitemap: 'boolean',
+      port: 'number',
+      config: 'string',
+    }),
   };
 
   if (flags.version) {
@@ -101,7 +131,7 @@ function mergeCLIFlags(astroConfig: AstroConfig, flags: CLIState['options']) {
 }
 
 /** Handle `astro run` command */
-async function runCommand(rawRoot: string, cmd: (a: AstroConfig, options: any) => Promise<void>, options: CLIState['options']) {
+async function runCommand(rawRoot: string, cmd: (a: AstroConfig, opts: any) => Promise<void>, options: CLIState['options']) {
   try {
     const projectRoot = options.projectRoot || rawRoot;
     const astroConfig = await loadConfig(projectRoot, options.config);