add feature to potentially provide more types

Author: laysent

README.md

@@ -88,6 +88,24 @@ plugins: [
 ]
 ```
 
+In case you need more fine control of the `as` atribute, you could also provide a function here.
+When using it, entry name will be provided as the parameter, and function itself should return a
+string for `as` attribute:
+
+```javascript
+plugins: [
+  new HtmlWebpackPlugin(),
+  new PreloadWebpackPlugin({
+    rel: 'preload',
+    as(entry) {
+      if (/\.css$/.test(entry)) return 'style';
+      if (/\.png$/.test(entry)) return 'image';
+      return 'script';
+    }
+  })
+]
+```
+
 By default, the plugin will assume async script chunks will be preloaded. This is the equivalent of:
 
 ```js

index.js

@@ -76,6 +76,8 @@ class PreloadPlugin {
             let asValue;
             if (!options.as) {
               asValue = entry.match(/\.css$/) ? 'style' : 'script';
+            } else if (typeof options.as === 'function') {
+              asValue = options.as(entry);
             } else {
               asValue = options.as;
             }

test/spec.js

@@ -190,6 +190,37 @@ describe('PreloadPlugin preloads normal chunks', function() {
     });
     compiler.outputFileSystem = new MemoryFileSystem();
   });
+
+  it('use custom as attribute based on return value of callback', (done) => {
+    const compiler = webpack({
+      entry: path.join(__dirname, 'fixtures', 'file.js'),
+      output: {
+        path: OUTPUT_DIR,
+        filename: 'bundle.js',
+        chunkFilename: 'chunk.[chunkhash].css',
+        publicPath: '/',
+      },
+      plugins: [
+        new HtmlWebpackPlugin(),
+        new PreloadPlugin({
+          rel: 'preload',
+          as(entry) {
+            if (entry.indexOf('/chunk') === 0) return 'style';
+            return 'script';
+          },
+          include: 'all',
+        }),
+      ],
+    }, function(err, result) {
+      expect(err).toBeFalsy();
+      expect(JSON.stringify(result.compilation.errors)).toBe('[]');
+      const html = result.compilation.assets['index.html'].source();
+      expect(html).toContain('<link rel="preload" as="style" href="/chunk');
+      expect(html).toContain('<link rel="preload" as="script" href="/bundle.js"');
+      done();
+    });
+    compiler.outputFileSystem = new MemoryFileSystem();
+  });
 });
 
 describe('PreloadPlugin prefetches normal chunks', function() {