improve compatibility check

scripts/build-funcs.js

@@ -28,9 +28,7 @@ import { checkCompatibility } from '../src/helpers';
  * @returns {string}
  */
 function prepareCompatibilityGuard() {
-    const compatibilityGuard = checkCompatibility;
-    const compatibilityGuardString = attachDependencies(compatibilityGuard);
-    return `${compatibilityGuardString}\n${compatibilityGuard.name}()`;
+    return `${checkCompatibility.toString()}\n${checkCompatibility.name}()`;
 }
 
 /**
@@ -40,10 +38,8 @@ function prepareCompatibilityGuard() {
  * @returns {string}
  */
 const getScriptletFunctionsString = () => {
-    const compatibilityGuardString = prepareCompatibilityGuard();
-
     function wrapInFunc(name, code) {
-        return `function ${name}(source, args){\n${compatibilityGuardString}\n${code}\n}`;
+        return `function ${name}(source, args){\n${prepareCompatibilityGuard()}\n${code}\n}`;
     }
     // we require scriptlets list dynamically, because scriptletsList file can be not built in the
     // moment of this script execution

src/helpers/check-compatibility.js

@@ -0,0 +1,25 @@
+/**
+ * Checks if current browser is supported
+ * To be injected to every scriptlet at build time
+ *
+ * @throws error on unsupported browser
+ */
+export function checkCompatibility() {
+    const isBrowserSupported = () => {
+        // arr.every() is not used because missing global method
+        // would throw ReferenceError when added to array
+        return typeof fetch !== 'undefined'
+            && typeof Proxy !== 'undefined'
+            && typeof Reflect !== 'undefined'
+            && typeof Response !== 'undefined'
+            && typeof Array.prototype.includes !== 'undefined'
+            && typeof String.prototype.includes !== 'undefined'
+            && typeof String.prototype.startsWith !== 'undefined'
+            && typeof String.prototype.endsWith !== 'undefined'
+            && typeof Element.prototype.attachShadow !== 'undefined';
+    };
+
+    if (!isBrowserSupported()) {
+        throw new Error('Browser is not supported by AdGuard Scriptlets.');
+    }
+}

src/helpers/index.js

@@ -31,4 +31,4 @@ export * from './parse-flags';
 export * from './parse-keyword-value';
 export * from './random-id';
 export * from './throttle';
-export * from './is-browser-supported';
+export * from './check-compatibility';