Add a helper function for writing parameterized JS tests

Mocha lacks built-in support [1] for writing parameterized tests and the
suggested solution [2] involves a bunch of boilerplate* which has IMO
resulted in different styles of parameterized tests in our codebase and
not having parameterized tests when they would be useful to attain more
complete coverage.

This adds a helper inspired by [3] for writing parameterized tests and
switches several existing places in our code to use it.

* Though less with ES2015 syntax.

[1] https://github.com/mochajs/mocha/issues/1454
[2] https://mochajs.org/#dynamically-generating-tests
[3] https://github.com/lawrencec/Unroll

h/static/scripts/directive/test/search-status-bar-test.js

@@ -3,6 +3,7 @@
 var angular = require('angular');
 
 var util = require('./util');
+var unroll = require('../../test/util').unroll;
 
 describe('searchStatusBar', function () {
   before(function () {
@@ -25,21 +26,19 @@ describe('searchStatusBar', function () {
   });
 
   context('when there is a selection', function () {
-    var cases = [
+    var FIXTURES = [
       {count: 0, message: 'Show all annotations'},
       {count: 1, message: 'Show all annotations'},
       {count: 10, message: 'Show all 10 annotations'},
     ];
 
-    cases.forEach(function (testCase) {
-      it('should display the "Show all annotations" message', function () {
+    unroll('should display the "Show all annotations" message when there are #count annotations', function (testCase) {
       var elem = util.createDirective(document, 'searchStatusBar', {
         selectionCount: 1,
         totalCount: testCase.count
       });
       var clearBtn = elem[0].querySelector('button');
       assert.include(clearBtn.textContent, testCase.message);
-      });
-    });
+    }, FIXTURES);
   });
 });

h/static/scripts/test/markdown-commands-test.js

 'use strict';
 
 var commands = require('../markdown-commands');
+var unroll = require('./util').unroll;
 
 /**
  * Convert a string containing '<sel>' and '</sel>' markers
@@ -72,33 +73,30 @@ describe('markdown commands', function () {
   });
 
   describe('block formatting', function () {
-    var CASES = {
-      'adds formatting to blocks': {
+    var FIXTURES = [{
+      tag: 'adds formatting to blocks',
       input: 'one\n<sel>two\nthree</sel>\nfour',
       output: 'one\n> <sel>two\n> three</sel>\nfour',
-      },
-      'removes formatting from blocks': {
+    },{
+      tag: 'removes formatting from blocks',
       input: 'one \n<sel>> two\n> three</sel>\nfour',
       output: 'one \n<sel>two\nthree</sel>\nfour',
-      },
-      'preserves the selection': {
+    },{
+      tag: 'preserves the selection',
       input: 'one <sel>two\nthree </sel>four',
       output: '> one <sel>two\n> three </sel>four',
-      },
-      'inserts the block prefix before an empty selection': {
+    },{
+      tag: 'inserts the block prefix before an empty selection',
       input: '<sel></sel>',
       output: '> <sel></sel>',
-      }
-    };
+    }];
 
-    Object.keys(CASES).forEach(function (case_) {
-      it(case_, function () {
+    unroll('#tag', function (fixture) {
       var output = commands.toggleBlockStyle(
-          parseState(CASES[case_].input), '> '
+        parseState(fixture.input), '> '
       );
-        assert.equal(formatState(output), CASES[case_].output);
-      });
-    });
+      assert.equal(formatState(output), fixture.output);
+    }, FIXTURES);
   });
 
   describe('link formatting', function () {

h/static/scripts/test/media-embedder-test.js

@@ -34,7 +34,7 @@ describe('media-embedder', function () {
     var urls = [
       'https://youtu.be/QCkm0lL-6lc',
       'https://youtu.be/QCkm0lL-6lc/',
-    ]
+    ];
     urls.forEach(function (url) {
       var element = domElement('<a href="' + url + '">' + url + '</a>');
 
@@ -55,7 +55,7 @@ describe('media-embedder', function () {
       'https://vimeo.com/149000090/#fragment',
       'https://vimeo.com/149000090?foo=bar&a=b',
       'https://vimeo.com/149000090/?foo=bar&a=b',
-    ]
+    ];
     urls.forEach(function (url) {
       var element = domElement('<a href="' + url + '">' + url + '</a>');
 

h/static/scripts/test/util.js

@@ -22,6 +22,61 @@ function noCallThru(stubs) {
   return Object.assign(stubs, {'@noCallThru':true});
 }
 
+/**
+ * Helper for writing parameterized tests.
+ *
+ * This is a wrapper around the `it()` function for creating a Mocha test case
+ * which takes an array of fixture objects and calls it() once for each fixture,
+ * passing in the fixture object as an argument to the test function.
+ *
+ * Usage:
+ *   unroll('should return #output with #input', function (fixture) {
+ *     assert.equal(functionUnderTest(fixture.input), fixture.output);
+ *   },[
+ *    {input: 'foo', output: 'bar'}
+ *   ]);
+ *
+ * Based on https://github.com/lawrencec/Unroll with the following changes:
+ *
+ *  1. Support for test functions that return promises
+ *  2. Mocha's `it()` is the only supported test function
+ *  3. Fixtures are objects rather than arrays
+ *
+ * @param {string} description - Description with optional '#key' placeholders
+ *        which are replaced by the values of the corresponding key from each
+ *        fixture object.
+ * @param {Function} testFn - Test function which can accept either `fixture`
+ *        or `done, fixture` as arguments, where `done` is the callback for
+ *        reporting completion of an async test and `fixture` is an object
+ *        from the `fixtures` array.
+ * @param {Array<T>} fixtures - Array of fixture objects.
+ */
+function unroll(description, testFn, fixtures) {
+  fixtures.forEach(function (fixture) {
+    var caseDescription = Object.keys(fixture).reduce(function (desc, key) {
+      return desc.replace('#' + key, String(fixture[key]));
+    }, description);
+    it(caseDescription, function (done) {
+      if (testFn.length === 1) {
+        // Test case does not accept a 'done' callback argument, so we either
+        // call done() immediately if it returns a non-Promiselike object
+        // or when the Promise resolves otherwise
+        var result = testFn(fixture);
+        if (typeof result === 'object' && result.then) {
+          result.then(function () { done(); }, done);
+        } else {
+          done();
+        }
+      } else {
+        // Test case accepts a 'done' callback argument and takes responsibility
+        // for calling it when the test completes.
+        testFn(done, fixture);
+      }
+    });
+  });
+}
+
 module.exports = {
   noCallThru: noCallThru,
+  unroll: unroll,
 };