tests: add waitForPromise method and test for it

tests/frontend/helper.js

@@ -197,6 +197,17 @@ var helper = {};
     return deferred;
   };
 
+  /**
+   * Same as `waitFor` but using Promises
+   *
+   */
+  helper.waitForPromise = async function(...args) {
+    // Note: waitFor() has a strange API: On timeout it rejects, but it also throws an uncatchable
+    // exception unless .fail() has been called. That uncatchable exception is disabled here by
+    // passing a no-op function to .fail().
+    return await this.waitFor(...args).fail(() => {});
+  };
+
   helper.selectLines = function($startLine, $endLine, startOffset, endOffset){
     // if no offset is provided, use beginning of start line and end of end line
     startOffset = startOffset || 0;

tests/frontend/specs/helper.js

@@ -215,6 +215,53 @@ describe("the test helper", function(){
     });
   });
 
+  describe('the waitForPromise method', function() {
+    it('takes a timeout and waits long enough', async function() {
+      this.timeout(2000);
+      var startTime = Date.now();
+      await helper.waitForPromise(function() {
+        return false;
+      }, 1500).catch(function() {
+        var duration = Date.now() - startTime;
+        expect(duration).to.be.greaterThan(1490);
+      });
+    });
+
+    it('takes an interval and checks on every interval', async function() {
+      this.timeout(4000);
+      var checks = 0;
+      await helper.waitForPromise(function() {
+        checks++;
+        return false;
+      }, 2000, 100).catch(function() {
+        expect(checks).to.be.greaterThan(15);
+        expect(checks).to.be.lessThan(21);
+      });
+    });
+
+    describe('returns a Promise', function() {
+      it('calls then after success', async function() {
+        let called = false;
+        await helper.waitForPromise(function() {
+          return true;
+        }).then(function() {
+          called = true;
+        });
+        expect(called).to.be(true);
+      });
+
+      it('calls catch on failure', async function() {
+        let called = false;
+        await helper.waitForPromise(function() {
+          return false;
+        },0).catch(function() {
+          called = true;
+        });
+        expect(called).to.be(true);
+      });
+    });
+  });
+
   describe("the selectLines method", function(){
     // function to support tests, use a single way to represent whitespaces
     var cleanText = function(text){