fix: write file directly from the plugin

.gitignore

@@ -4,3 +4,4 @@ spec/webpack-out
 yarn.lock
 .nyc_output
 coverage
+spec/output/

lib/plugin.js

@@ -94,20 +94,16 @@ ManifestPlugin.prototype.apply = function(compiler) {
 
     var json = JSON.stringify(cache, null, 2);
 
-    compilation.assets[outputName] = {
-      source: function() {
-        return json;
-      },
-      size: function() {
-        return json.length;
-      }
-    };
-
-    if (this.opts.writeToFileEmit) {
-      var outputFolder = compilation.options.output.path;
-      var outputFile = path.join(outputFolder, this.opts.fileName);
+    var outputFolder = compilation.options.output.path;
+    var outputFile = path.join(outputFolder, this.opts.fileName);
 
+    const shouldUseNodeFs = compiler.outputFileSystem.constructor.name !== 'MemoryFileSystem' ||
+                                      this.opts.writeToFileEmit;
+    if (shouldUseNodeFs) {
       fse.outputFileSync(outputFile, json);
+    } else {
+      compiler.outputFileSystem.mkdirpSync(path.dirname(outputFile));
+      compiler.outputFileSystem.writeFileSync(outputFile, json);
     }
 
     compileCallback();

spec/plugin.integration.spec.js

@@ -0,0 +1,53 @@
+var fs = require('fs');
+var path = require('path');
+
+var _ = require('lodash');
+var webpack = require('webpack');
+
+var ManifestPlugin = require('../index.js');
+
+function webpackConfig (webpackOpts, opts) {
+  return _.merge({
+    plugins: [
+      new ManifestPlugin(opts.manifestOptions)
+    ]
+  }, webpackOpts);
+}
+
+function webpackCompile(config, cb) {
+  var compiler = webpack(config);
+
+  compiler.run(function(err, stats){
+    expect(err).toBeFalsy();
+    expect(stats.hasErrors()).toBe(false);
+
+    cb(stats);
+  });
+};
+
+describe('ManifestPlugin using real fs', function () {
+  describe('basic behavior', function () {
+    it('outputs a manifest of one file', function (done) {
+      webpackCompile({
+        context: __dirname,
+        output: {
+          filename: '[name].js',
+          path: path.join(__dirname, 'output/single-file')
+        },
+        entry: './fixtures/file.js',
+        plugins: [
+          new ManifestPlugin()
+        ]
+      }, function() {
+        var manifest = JSON.parse(fs.readFileSync(path.join(__dirname, 'output/single-file/manifest.json')))
+
+        expect(manifest).toBeDefined();
+        expect(manifest).toEqual({
+          'main.js': 'main.js'
+        });
+
+        done();
+      });
+    });
+  });
+});

spec/plugin.spec.js

@@ -62,7 +62,7 @@ function webpackCompile(webpackOpts, opts, cb) {
   var fs = compiler.outputFileSystem = new MemoryFileSystem();
 
   compiler.run(function(err, stats){
-    var manifestFile = JSON.parse( fs.readFileSync(manifestPath).toString() );
+    var manifestFile = JSON.parse(fs.readFileSync(manifestPath).toString());
 
     expect(err).toBeFalsy();
     expect(stats.hasErrors()).toBe(false);