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

docs/errors.md

@@ -345,4 +345,16 @@ If you're only using named AMD modules as part of a script loaded by System.impo
 
 The reason for this is that SystemJS generally identifies modules by their URLs - one URL per module. Import Maps are the primary method to alias a bare specifier to a URL, but it's also possible to identify modules by a name without specifying a URL for each module, by creating named System.register or named AMD modules.
 
-When the SystemJS amd.js extra's `define` function is given a named AMD module, the module is identified by the URL of the currently executing script, if the script was loaded via `System.import()`. However, if the script was not loaded by `System.import()` or if there are multiple defines in the same script, then the module(s) will not have a URL associated with them and therefore will not be accessible by any identifier. To solve this problem, the named-register.js tracks all named modules (including named AMD modules and named System.register modules) by name, rather than by URL. This makes it possible to have one script that registers multiple named modules.
+When the SystemJS amd.js extra's `define` function is given a named AMD module, the module is identified by the URL of the currently executing script, if the script was loaded via `System.import()`. However, if the script was not loaded by `System.import()` or if there are multiple defines in the same script, then the module(s) will not have a URL associated with them and therefore will not be accessible by any identifier. To solve this problem, the named-register.js tracks all named modules (including named AMD modules and named System.register modules) by name, rather than by URL. This makes it possible to have one script that registers multiple named modules.
+
+## W7
+
+### Import assertion failed
+
+When using import assetion like
+
+```js
+import { foo } from 'foo.js' assert { type: 'json' };
+```
+
+User should ensure the server response header contains the correct MIME 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

@@ -1,4 +1,5 @@
 import { resolveUrl } from '../common.js';
+import {errMsg} from '../err-msg.js';
 
 /*
  * Loads JSON, CSS, Wasm module types based on file extension
@@ -9,35 +10,32 @@ 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(;|$)/;
-
-  var fetch = systemJSPrototype.fetch;
-  systemJSPrototype.fetch = function (url, options) {
-    return fetch(url, options)
-    .then(function (res) {
-      if (options.passThrough)
-        return res;
-
-      if (!res.ok)
-        return res;
-      var contentType = res.headers.get('content-type');
-      if (jsonContentType.test(contentType))
-        return res.json()
+  var fallback = '_fallback';
+  var contentTypes = [
+    [/^application\/json(;|$)/, '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 +46,11 @@ 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 +63,46 @@ 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 mime = res.headers.get('content-type');
+        var contentType
+        for (var i = 0; i < contentTypes.length; i--) {
+          var rule = contentTypes[i];
+          if (rule[0].test(mime)) {
+            contentType = rule[1]
+            break
+          }
+        }
+
+        // ensure type match
+        if (assertType && contentType){
+          if(contentType === fallback){
+            contentType = assertType
+          }
+          if (assertType !== contentType){
+            console.warn(process.env.SYSTEM_PRODUCTION ? errMsg('W7') : errMsg('W7', `Module ${url} has an unexpected content type of "${mime}": expected module type ${assertType} got ${contentType}.`));
+            contentType = undefined
+          }
+        }
+
+        var loader = loaders[contentType];
+        if (loader) return loader(res, url, options);
+        return res;
+      });
   };
 })(typeof self !== 'undefined' ? self : global);

test/browser/core.js

@@ -227,6 +227,22 @@ suite('SystemJS Standard Tests', function() {
     });
   });
 
+  test('should load a.txt as css module by assert', function () {
+    return System.import('fixturesbase/css-modules/a.txt', {assert: {type: 'css'}}).then(function (m) {
+      assert.ok(m);
+      assert.ok(isCSSStyleSheet(m.default));
+      document.adoptedStyleSheets = document.adoptedStyleSheets.concat(m.default);
+    });
+  });
+
+  test('should load a as css module by assert', function () {
+    return System.import('fixturesbase/css-modules/a', {assert: {type: 'css'}}).then(function (m) {
+      assert.ok(m);
+      assert.ok(isCSSStyleSheet(m.default));
+      document.adoptedStyleSheets = document.adoptedStyleSheets.concat(m.default);
+    });
+  });
+
   test('should support application/javascript css module override', function () {
     return System.import('fixturesbase/css-modules/javascript.css').then(function (m) {
       assert.ok(m);

test/fixtures/css-modules/a

@@ -0,0 +1,6 @@
+.hello {
+  background-color: peru;
+}
+body {
+  background-color: lightblue;
+}

test/fixtures/css-modules/a.txt

@@ -0,0 +1,6 @@
+.hello {
+  background-color: peru;
+}
+body {
+  background-color: lightblue;
+}