Fix crash in non-web builds, emit warning instead

Closes #87

examples/non-web-build/README.md

@@ -0,0 +1,3 @@
+# Non-Web build
+
+Ensure the plugin doesn't crash in a build for a non-web target.

examples/non-web-build/test.js

@@ -0,0 +1,17 @@
+var expect = require('expect');
+var webpackVersion = Number(
+  require('webpack/package.json').version.split('.')[0]
+);
+
+module.exports.skip = function skip() {
+  // No reliable way to detect JsonWebTemplate usage on Webpack < 4.
+  return webpackVersion < 4;
+};
+
+module.exports.check = function check(stats) {
+  expect(stats.compilation.errors).toEqual([]);
+  expect(stats.compilation.warnings.length).toEqual(1);
+  expect(stats.compilation.warnings[0].message).toMatch(
+    /This plugin is not useful for non-web targets/
+  );
+};

examples/non-web-build/webpack.config.js

@@ -0,0 +1,19 @@
+var SriPlugin = require('webpack-subresource-integrity');
+var HtmlWebpackPlugin = require('html-webpack-plugin');
+
+module.exports = {
+  entry: {
+    index: './index.js'
+  },
+  target: 'node',
+  output: {
+    crossOriginLoading: 'anonymous'
+  },
+  plugins: [
+    new SriPlugin({
+      hashFuncNames: ['sha256', 'sha384'],
+      enabled: true
+    }),
+    new HtmlWebpackPlugin()
+  ]
+};

jmtp.js

@@ -73,8 +73,15 @@ WebIntegrityJsonpMainTemplatePlugin.prototype.apply = function apply(
   }
 
   if (mainTemplate.hooks) {
-    mainTemplate.hooks.jsonpScript.tap('SriPlugin', jsonpScriptPlugin);
-    mainTemplate.hooks.localVars.tap('SriPlugin', localVarsPlugin);
+    if (mainTemplate.hooks.jsonpScript && mainTemplate.hooks.localVars) {
+      mainTemplate.hooks.jsonpScript.tap('SriPlugin', jsonpScriptPlugin);
+      mainTemplate.hooks.localVars.tap('SriPlugin', localVarsPlugin);
+    } else {
+      this.sriPlugin.warnOnce(
+        this.compilation,
+        'This plugin is not useful for non-web targets.'
+      );
+    }
   } else {
     mainTemplate.plugin('jsonp-script', jsonpScriptPlugin);
     mainTemplate.plugin('local-vars', localVarsPlugin);