Issue 6819 - Shadow globals in snippets

lib/snippets.js

Index: lib/snippets.js
===================================================================
--- a/lib/snippets.js
+++ b/lib/snippets.js
@@ -160,23 +160,60 @@
  * @param {string[]} libraries
  * @return {string}
  */
 function compileScript(script, libraries)
 {
   return `
     "use strict";
     {
+      let globals = {};
+
+      for (let name of typeof window != "undefined" ? Object.keys(window) : [])

[Manish Jethani, 2018/07/31 14:17:08]

Shadow globals on the window object (browser).

[(Draft), 2018/09/24 11:31:55]

Nit: I think checking if `window` is undefined first, before the loop would be
easier to grok. 

+        globals[name] = null;
+
+      for (let name of typeof global != "undefined" ? Object.keys(global) : [])

[Manish Jethani, 2018/07/31 14:17:08]

Shadow globals on the global object (Node.js, for testing).

+        globals[name] = null;
+
+      if (typeof browser != "undefined")
+      {
+        globals.browser = {

[Manish Jethani, 2018/07/31 14:17:08]

Expose only certain extension APIs.

+          runtime: {
+            getURL: browser.runtime.getURL
+          }
+        };
+      }
+
+      if (typeof document != "undefined")
+      {
+        globals.document = new Proxy(document, {

[Manish Jethani, 2018/07/31 14:17:08]

Make document object available with restricted access.

[Manish Jethani, 2018/07/31 14:17:08]

Use a Proxy object here so it is transparent to the calling code (e.g. `document
instanceof HTMLDocument` still works).

+          get(target, property)
+          {
+            if (property == "defaultView")

[Manish Jethani, 2018/07/31 14:17:08]

Prevent access to the window object.

+              return null;
+
+            let value = target[property];
+            if (typeof value == "function")
+              return value.bind(target);

[Manish Jethani, 2018/07/31 14:17:08]

Bind the function to the target so its `this` is the target.

+
+            return value;
+          }
+        });
+      }
+
       const libraries = ${JSON.stringify(libraries)};
 
       const script = ${JSON.stringify(parseScript(script))};
 
       let imports = Object.create(null);
       for (let library of libraries)
-        new Function("exports", library)(imports);
+      {
+        let func = new Function("exports", ...Object.keys(globals), library);

[Manish Jethani, 2018/07/31 14:17:08]

Pass the globals as arguments to the snippet library.

+        func(imports, ...Object.keys(globals).map(key => globals[key]));
+      }
 
       for (let [name, ...args] of script)
       {
         if (Object.prototype.hasOwnProperty.call(imports, name))
         {
           let value = imports[name];
           if (typeof value == "function")
             value(...args);