[js] allow duck typing for locators

Fixes #3056

Author: jleyba

javascript/node/selenium-webdriver/lib/by.js

@@ -259,6 +259,14 @@ function check(locator) {
   if (locator instanceof By || typeof locator === 'function') {
     return locator;
   }
+
+  if (locator
+      && typeof locator === 'object'
+      && typeof locator.using === 'string'
+      && typeof locator.value === 'string') {
+    return new By(locator.using, locator.value);
+  }
+
   for (let key in locator) {
     if (locator.hasOwnProperty(key) && By.hasOwnProperty(key)) {
       return By[key](locator[key]);

javascript/node/selenium-webdriver/test/lib/by_test.js

@@ -88,6 +88,27 @@ describe('by', function() {
   });
 
   describe('checkedLocator', function() {
+    it('accepts a By instance', function() {
+      let original = by.By.name('foo');
+      let locator = by.checkedLocator(original);
+      assert.strictEqual(locator, original);
+    });
+
+    it('accepts custom locator functions', function() {
+      let original = function() {};
+      let locator = by.checkedLocator(original);
+      assert.strictEqual(locator, original);
+    });
+
+    // See https://github.com/SeleniumHQ/selenium/issues/3056
+    it('accepts By-like objects', function() {
+      let fakeBy = {using: 'id', value: 'foo'};
+      let locator = by.checkedLocator(fakeBy);
+      assert.strictEqual(locator.constructor, by.By);
+      assert.equal(locator.using, 'id');
+      assert.equal(locator.value, 'foo');
+    });
+
     it('accepts class name', function() {
       let locator = by.checkedLocator({className: 'foo'});
       assert.equal('css selector', locator.using);