Add support for `nth-ancestor` operator in HTML filtering

Also opportunitisically converted some code to
ES6's `class`.

src/js/html-filtering.js

@@ -42,14 +42,15 @@
         }
     };
 
-    const PSelectorHasTextTask = function(task) {
+    const PSelectorHasTextTask = class {
+        constructor(task) {
             let arg0 = task[1], arg1;
             if ( Array.isArray(task[1]) ) {
                 arg1 = arg0[1]; arg0 = arg0[0];
             }
             this.needle = new RegExp(arg0, arg1);
-    };
-    PSelectorHasTextTask.prototype.exec = function(input) {
+        }
+        exec(input) {
             const output = [];
             for ( const node of input ) {
                 if ( this.needle.test(node.textContent) ) {
@@ -57,18 +58,14 @@
                 }
             }
             return output;
+        }
     };
 
-    const PSelectorIfTask = function(task) {
+    const PSelectorIfTask = class {
+        constructor(task) {
             this.pselector = new PSelector(task[1]);
-    };
-    PSelectorIfTask.prototype.target = true;
-    Object.defineProperty(PSelectorIfTask.prototype, 'invalid', {
-        get: function() {
-            return this.pselector.invalid;
         }
-    });
-    PSelectorIfTask.prototype.exec = function(input) {
+        exec(input) {
             const output = [];
             for ( const node of input ) {
                 if ( this.pselector.test(node) === this.target ) {
@@ -76,19 +73,46 @@
                 }
             }
             return output;
+        }
+        get invalid() {
+            return this.pselector.invalid;
+        }
     };
+    PSelectorIfTask.prototype.target = true;
 
-    const PSelectorIfNotTask = function(task) {
-        PSelectorIfTask.call(this, task);
+    const PSelectorIfNotTask = class extends PSelectorIfTask {
+        constructor(task) {
+            super.call(task);
             this.target = false;
+        }
     };
-    PSelectorIfNotTask.prototype = Object.create(PSelectorIfTask.prototype);
-    PSelectorIfNotTask.prototype.constructor = PSelectorIfNotTask;
 
-    const PSelectorXpathTask = function(task) {
-        this.xpe = task[1];
+    const PSelectorNthAncestorTask = class {
+        constructor(task) {
+            this.nth = task[1];
+        }
+        exec(input) {
+            const output = [];
+            for ( let node of input ) {
+                let nth = this.nth;
+                for (;;) {
+                    node = node.parentElement;
+                    if ( node === null ) { break; }
+                    nth -= 1;
+                    if ( nth !== 0 ) { continue; }
+                    output.push(node);
+                    break;
+                }
+            }
+            return output;
+        }
     };
-    PSelectorXpathTask.prototype.exec = function(input) {
+
+    const PSelectorXpathTask = class {
+        constructor(task) {
+            this.xpe = task[1];
+        }
+        exec(input) {
             const output = [];
             const xpe = docRegister.createExpression(this.xpe, null);
             let xpr = null;
@@ -107,19 +131,11 @@
                 }
             }
             return output;
+        }
     };
 
-    const PSelector = function(o) {
-        if ( PSelector.prototype.operatorToTaskMap === undefined ) {
-            PSelector.prototype.operatorToTaskMap = new Map([
-                [ ':has', PSelectorIfTask ],
-                [ ':has-text', PSelectorHasTextTask ],
-                [ ':if', PSelectorIfTask ],
-                [ ':if-not', PSelectorIfNotTask ],
-                [ ':not', PSelectorIfNotTask ],
-                [ ':xpath', PSelectorXpathTask ]
-            ]);
-        }
+    const PSelector = class {
+        constructor(o) {
             this.raw = o.raw;
             this.selector = o.selector;
             this.tasks = [];
@@ -137,17 +153,15 @@
                 }
                 this.tasks.push(pselector);
             }
-    };
-    PSelector.prototype.operatorToTaskMap = undefined;
-    PSelector.prototype.invalid = false;
-    PSelector.prototype.prime = function(input) {
+        }
+        prime(input) {
             const root = input || docRegister;
             if ( this.selector !== '' ) {
                 return root.querySelectorAll(this.selector);
             }
             return [ root ];
-    };
-    PSelector.prototype.exec = function(input) {
+        }
+        exec(input) {
             if ( this.invalid ) { return []; }
             let nodes = this.prime(input);
             for ( const task of this.tasks ) {
@@ -155,8 +169,8 @@
                 nodes = task.exec(nodes);
             }
             return nodes;
-    };
-    PSelector.prototype.test = function(input) {
+        }
+        test(input) {
             if ( this.invalid ) { return false; }
             const nodes = this.prime(input);
             const AA = [ null ];
@@ -170,7 +184,18 @@
                 if ( aa.length !== 0 ) { return true; }
             }
             return false;
+        }
     };
+    PSelector.prototype.operatorToTaskMap = new Map([
+        [ ':has', PSelectorIfTask ],
+        [ ':has-text', PSelectorHasTextTask ],
+        [ ':if', PSelectorIfTask ],
+        [ ':if-not', PSelectorIfNotTask ],
+        [ ':not', PSelectorIfNotTask ],
+        [ ':nth-ancestor', PSelectorNthAncestorTask ],
+        [ ':xpath', PSelectorXpathTask ]
+    ]);
+    PSelector.prototype.invalid = false;
 
     const logOne = function(details, exception, selector) {
         µBlock.filteringContext