fix: give an empty request body the shape express gives it

src/middlewares.js

@@ -181,10 +181,6 @@ function createBodyParser(defaultType, beforeReturn) {
             }
 
             const length = req.headers['content-length'];
-            // skip reading empty body
-            if(length == '0') {
-                return next();
-            }
 
             if(options.simpleType) {
                 const semicolonIndex = type.indexOf(';');
@@ -204,6 +200,13 @@ function createBodyParser(defaultType, beforeReturn) {
                 }
             }
 
+            // an empty body still has to produce this parser's empty value the way express does -
+            // {} for json and urlencoded, '' for text, an empty Buffer for raw - rather than leaving
+            // req.body as the placeholder object. there is nothing to read, so run the tail directly
+            if(length == '0') {
+                return beforeReturn(req, res, next, options, Buffer.alloc(0));
+            }
+
             // skip reading too large body
             if(length && +length > options.limit) {
                 return next(new Error('Request entity too large'));

tests/tests/middlewares/empty-body-parsers.js

@@ -0,0 +1,49 @@
+// must give an empty body the same shape express does, for every body parser
+
+const express = require("express");
+
+const app = express();
+
+app.post('/json', express.json(), (req, res) => {
+    res.json({ type: typeof req.body, isBuffer: Buffer.isBuffer(req.body), value: JSON.stringify(req.body) });
+});
+
+app.post('/urlencoded', express.urlencoded({ extended: false }), (req, res) => {
+    res.json({ type: typeof req.body, keys: Object.keys(req.body).length });
+});
+
+app.post('/text', express.text(), (req, res) => {
+    res.json({ type: typeof req.body, length: req.body.length });
+});
+
+app.post('/raw', express.raw(), (req, res) => {
+    res.json({ isBuffer: Buffer.isBuffer(req.body), length: req.body.length });
+});
+
+function empty(path, contentType) {
+    return fetch(`http://localhost:13333${path}`, {
+        method: 'POST',
+        body: '',
+        headers: { 'Content-Type': contentType }
+    }).then(r => r.text());
+}
+
+app.listen(13333, async () => {
+    console.log('Server is running on port 13333');
+
+    console.log(await empty('/json', 'application/json'));
+    console.log(await empty('/urlencoded', 'application/x-www-form-urlencoded'));
+    console.log(await empty('/text', 'text/plain'));
+    console.log(await empty('/raw', 'application/octet-stream'));
+
+    // a non-empty body must still parse the same way afterwards
+    const after = await fetch('http://localhost:13333/json', {
+        method: 'POST',
+        body: '{"a":1}',
+        headers: { 'Content-Type': 'application/json' }
+    });
+    console.log(await after.text());
+
+    await new Promise(resolve => setTimeout(resolve, 100));
+    process.exit(0);
+});