[feature/#67-support-no-transform] Add supports for "Cache-Control: n…

…o-transform"

index.js

@@ -20,6 +20,11 @@ const encodingMethods = {
   deflate: zlib.createDeflate
 }
 
+/**
+* Regex to match no-transform directive in a cache-control header
+*/
+const NO_TRANSFORM_REGEX = /(?:^|,)\s*?no-transform\s*?(?:,|$)/
+
 /**
  * Compress middleware.
  *
@@ -48,6 +53,11 @@ module.exports = (options = {}) => {
     // forced compression or implied
     if (!(ctx.compress === true || filter(ctx.response.type))) return
 
+    // Don't compress for Cache-Control: no-transform
+    // https://tools.ietf.org/html/rfc7234#section-5.2.1.6
+    const cacheControl = ctx.response.get('Cache-Control')
+    if (cacheControl && NO_TRANSFORM_REGEX.test(cacheControl)) return
+
     // identity
     const encoding = ctx.acceptsEncodings('gzip', 'deflate', 'identity')
     if (!encoding) ctx.throw(406, 'supported encodings: gzip, deflate, identity')