Add path utilities isAncestor and isDescendant (#4836)

* Add path utilities isAncestor and isDescendant

* Create ancestry.js

packages/babel-traverse/src/path/ancestry.js

@@ -175,6 +175,20 @@ export function getAncestry() {
   return paths;
 }
 
+/**
+ * A helper to find if `this` path is an ancestor of @param maybeDescendant
+ */
+export function isAncestor(maybeDescendant) {
+  return !!maybeDescendant.findParent((parent) => parent === this);
+}
+
+/**
+ * A helper to find if `this` path is a descendant of @param maybeAncestor
+ */
+export function isDescendant(maybeAncestor) {
+  return maybeAncestor.isAncestor(this);
+}
+
 export function inType() {
   let path = this;
   while (path) {

packages/babel-traverse/test/ancestry.js

@@ -0,0 +1,65 @@
+let traverse = require("../lib").default;
+let assert = require("assert");
+let parse = require("babylon").parse;
+
+describe("path/ancestry", function () {
+  describe("isAncestor", function () {
+    let ast = parse("var a = 1; 'a';");
+
+    it("returns true if ancestor", function() {
+      let paths = [];
+      traverse(ast, {
+        "Program|NumericLiteral"(path) {
+          paths.push(path);
+        },
+      });
+
+      let [ programPath, numberPath ] = paths;
+
+      assert(programPath.isAncestor(numberPath));
+    });
+
+    it("returns false if not ancestor", function() {
+      let paths = [];
+      traverse(ast, {
+        "Program|NumericLiteral|StringLiteral"(path) {
+          paths.push(path);
+        }
+      });
+
+      let [ , numberPath, stringPath ] = paths;
+
+      assert(!stringPath.isAncestor(numberPath));
+    });
+  });
+
+  describe("isDescendant", function () {
+    let ast = parse("var a = 1; 'a';");
+
+    it("returns true if descendant", function() {
+      let paths = [];
+      traverse(ast, {
+        "Program|NumericLiteral"(path) {
+          paths.push(path);
+        },
+      });
+
+      let [ programPath, numberPath ] = paths;
+
+      assert(numberPath.isDescendant(programPath));
+    });
+
+    it("returns false if not descendant", function() {
+      let paths = [];
+      traverse(ast, {
+        "Program|NumericLiteral|StringLiteral"(path) {
+          paths.push(path);
+        }
+      });
+
+      let [ , numberPath, stringPath ] = paths;
+
+      assert(!numberPath.isDescendant(stringPath));
+    });
+  });
+});