module-types check assert type, allowed fallback to specified type.

docs/module-types.md

@@ -12,12 +12,22 @@ SystemJS supports loading modules that are in the following formats:
 | [AMD](https://github.com/amdjs/amdjs-api/wiki/AMD) | [AMD extra](/dist/extras/amd.js) | [AMD extra](/dist/extras/amd.js) | * |
 | [UMD](https://github.com/umdjs/umd) | [AMD extra](/dist/extras/amd.js) | [AMD extra](/dist/extras/amd.js) | * |
 
-### File Extension Limitations
+### Module type detection
 
 When loading JSON modules, CSS modules and Web Assembly modules, the browser specifications require interpreting these modules based on checking their MIME type. Since SystemJS has to choose upfront whether to append a script element (for JS modules) or make a fetch request (for a JSON/CSS/Wasm module), it needs to know the module type upfront at resolution time.
 
 Instead of reading the MIME type, the file extension is thus used specifically for the JSON, CSS and Web Assembly module cases.
 
+If module type is specified by import, the mime is acceptable as the module type, will use the specified module type.
+
+e.g.
+
+```js
+System.import('https://example.com/my-module.txt', {assert: {type: 'css'}});
+```
+
+This will force resolve the module as a CSS module, even if the file extension is `.txt` and the MIME type is `text/plain`.
+
 ## JSON Modules
 
 [JSON modules](https://github.com/whatwg/html/pull/4407) support importing a JSON file as the default export.

src/extras/module-types.js

@@ -9,35 +9,31 @@ import { resolveUrl } from '../common.js';
 
   var moduleTypesRegEx = /^[^#?]+\.(css|html|json|wasm)([?#].*)?$/;
   var _shouldFetch = systemJSPrototype.shouldFetch.bind(systemJSPrototype)
-  systemJSPrototype.shouldFetch = function (url) {
-    return _shouldFetch(url) || moduleTypesRegEx.test(url);
+  systemJSPrototype.shouldFetch = function (url, parent, meta) {
+    var assertType = ((meta || {}).assert || {}).type;
+    return _shouldFetch(url, parent, meta) || moduleTypesRegEx.test(url) || !!loaders[assertType];
   };
 
-  var jsonContentType = /^application\/json(;|$)/;
-  var cssContentType = /^text\/css(;|$)/;
-  var wasmContentType = /^application\/wasm(;|$)/;
-
-  var fetch = systemJSPrototype.fetch;
-  systemJSPrototype.fetch = function (url, options) {
-    return fetch(url, options)
-    .then(function (res) {
-      if (options.passThrough)
-        return res;
-
-      if (!res.ok)
-        return res;
-      var contentType = res.headers.get('content-type');
-      if (jsonContentType.test(contentType))
-        return res.json()
+  var contentTypes = [
+    [/^application\/json(;|$)/, 'json'],
+    [/^text\/css(;|$)/, 'css'],
+    [/^application\/wasm(;|$)/, 'webassembly'],
+    [/^text\/plain(;|$)/, '_fallback'],
+    [/^application\/octet-stream(;|$)/, '_fallback'],
+  ]
+  var loaders = {
+    'json': function (res) {
+      return res.json()
         .then(function (json) {
           return new Response(new Blob([
             'System.register([],function(e){return{execute:function(){e("default",' + JSON.stringify(json) + ')}}})'
           ], {
             type: 'application/javascript'
           }));
-        });
-      if (cssContentType.test(contentType))
-        return res.text()
+        })
+    },
+    'css': function (res, url) {
+      return res.text()
         .then(function (source) {
           source = source.replace(/url\(\s*(?:(["'])((?:\\.|[^\n\\"'])+)\1|((?:\\.|[^\s,"'()\\])+))\s*\)/g, function (match, quotes, relUrl1, relUrl2) {
             return 'url(' + quotes + resolveUrl(relUrl1 || relUrl2, url) + quotes + ')';
@@ -48,11 +44,11 @@ import { resolveUrl } from '../common.js';
             type: 'application/javascript'
           }));
         });
-      if (wasmContentType.test(contentType))
-        return (WebAssembly.compileStreaming ? WebAssembly.compileStreaming(res) : res.arrayBuffer().then(WebAssembly.compile))
+    },
+    'webassembly': function (res, url) {
+      return (WebAssembly.compileStreaming ? WebAssembly.compileStreaming(res) : res.arrayBuffer().then(WebAssembly.compile))
         .then(function (module) {
-          if (!global.System.wasmModules)
-            global.System.wasmModules = Object.create(null);
+          if (!global.System.wasmModules) global.System.wasmModules = Object.create(null);
           global.System.wasmModules[url] = module;
           // we can only set imports if supported (eg early Safari doesnt support)
           var deps = [];
@@ -65,15 +61,40 @@ import { resolveUrl } from '../common.js';
                 setterSources.push('function(m){i[' + key + ']=m}');
               }
             });
-          return new Response(new Blob([
-            'System.register([' + deps.join(',') + '],function(e){var i={};return{setters:[' + setterSources.join(',') +
-            '],execute:function(){return WebAssembly.instantiate(System.wasmModules[' + JSON.stringify(url) +
-            '],i).then(function(m){e(m.exports)})}}})'
-          ], {
+          return new Response(new Blob(['System.register([' + deps.join(',') + '],function(e){var i={};return{setters:[' + setterSources.join(',') + '],execute:function(){return WebAssembly.instantiate(System.wasmModules[' + JSON.stringify(url) + '],i).then(function(m){e(m.exports)})}}})'], {
             type: 'application/javascript'
           }));
         });
-      return res;
-    });
+    }
+  }
+
+  var fetch = systemJSPrototype.fetch;
+  systemJSPrototype.fetch = function (url, options) {
+    return fetch(url, options)
+      .then(function (res) {
+        if (options.passThrough) return res;
+
+        if (!res.ok) return res;
+
+        var assertType = ((options.meta || {}).assert || {}).type;
+        var contentTypeHeader = res.headers.get('content-type');
+        var contentType = contentTypes.find(function (rule) {
+          return rule[0].test(contentTypeHeader)
+        }).map(function (rule) {
+          return rule[1]
+        })
+        if (assertType){
+          if(contentType === '_fallback'){
+            contentType = assertType
+          }
+          if (assertType !== contentType){
+            throw new Error('Module type assertion failed. Expected ' + assertType + ' but got ' + contentType)
+          }
+        }
+
+        var loader = loaders[contentType];
+        if (loader) return loader(res, url, options);
+        return res;
+      });
   };
 })(typeof self !== 'undefined' ? self : global);