Exclude .then from property validation

lib/chai/utils/proxify.js

@@ -22,8 +22,11 @@ module.exports = function proxify (obj) {
 
   return new Proxy(obj, {
     get: function getProperty (target, property) {
-      // Don't throw error on Symbol properties such as Symbol.toStringTag
-      if (typeof property === 'string' && !Reflect.has(target, property))
+      // Don't throw error on Symbol properties such as Symbol.toStringTag, nor
+      // on .then because it's necessary for promise type-checking.
+      if (typeof property === 'string' &&
+          property !== 'then' &&
+          !Reflect.has(target, property))
         throw Error('Invalid Chai property: ' + property);
 
       return target[property];

test/expect.js

@@ -28,6 +28,11 @@ describe('expect', function () {
     err(function () {
       expect(42).to.equal(42).pizza;
     }, 'Invalid Chai property: pizza');
+
+    // .then is excluded from property validation for promise support
+    expect(function () {
+      expect(42).then;
+    }).to.not.throw();
   });
 
   it('no-op chains', function() {

test/should.js

@@ -25,6 +25,11 @@ describe('should', function() {
     err(function () {
       (42).should.equal(42).pizza;
     }, 'Invalid Chai property: pizza');
+
+    // .then is excluded from property validation for promise support
+    (function () {
+      (42).should.then;
+    }).should.not.throw();
   });
 
   it('no-op chains', function() {

test/utilities.js

@@ -869,5 +869,14 @@ describe('utilities', function () {
         pizza.mushrooms;
       }).to.throw('Invalid Chai property: mushrooms');
     });
+
+    // .then is excluded from property validation for promise support
+    it('doesn\'t throw error if non-existent `then` is read', function () {
+      var pizza = proxify({});
+
+      expect(function () {
+        pizza.then;
+      }).to.not.throw();
+    });
   });
 });