Allow json to be called multiple times

README.md

@@ -97,6 +97,7 @@ npm start
 - Use `require('micro').json`.
 - Buffers and parses the incoming body and returns it.
 - Exposes an `async` function that can be run with  `await`.
+- Can be called multiple times, as it caches the raw request body the first time.
 - `limit` is how much data is aggregated before parsing at max. Otherwise, an `Error` is thrown with `statusCode` set to `413` (see [Error Handling](#error-handling)). It can be a `Number` of bytes or [a string](https://www.npmjs.com/package/bytes) like `'1mb'`.
 - If JSON parsing fails, an `Error` is thrown with `statusCode` set to `400` (see [Error Handling](#error-handling))
 - Example:

lib/index.js

@@ -40,12 +40,21 @@ async function run(req, res, fn, onError) {
   }
 }
 
+// maps requests to buffered raw bodies so that
+// multiple calls to `json` work as expected
+const rawBodyMap = new WeakMap()
+
 async function json(req, {limit = '1mb'} = {}) {
   try {
     const type = req.headers['content-type']
     const length = req.headers['content-length']
     const encoding = typer.parse(type).parameters.charset
-    const str = await getRawBody(req, {limit, length, encoding})
+
+    let str = rawBodyMap.get(req)
+    if (!str) {
+      str = await getRawBody(req, {limit, length, encoding})
+      rawBodyMap.set(req, str)
+    }
 
     try {
       return JSON.parse(str)