Extract and refactor dropdown item rendering

Basic Intent: Allow all branch names without accidentally creating
layout or backstage DOM. e.g. a branch named `separator` should never
create a separator `li` element.  

Ideally, there should never be a string that could cause this kind of 
conflict.  

Implementation: All of `GitLabDropdown.renderItem` is extracted to a 
standalone module.  

To render a divider or separator, consumers must now pass in an object 
like `{ "type": "divider" }` or `{ "type": "separator" }`   

Notable choices:  
- All of the functions have a cyclomatic complexity of 3 or less
    - See: https://en.wikipedia.org/wiki/Cyclomatic_complexity
    - Note the "Correlation to number of defects" section
    - While software complexity may not have a directly causal
      relationship with defects, less complex software is generally
      easier to reason about, and **may** reduce defects.
      I personally try to maintain complexity of no higher than 3.
- Everything other than the branch names fix matches past behavior 
exactly
    - Including:
        - Unused variables
        - Class list order

app/assets/javascripts/gl_dropdown.js

@@ -7,6 +7,7 @@ import fuzzaldrinPlus from 'fuzzaldrin-plus';
 import axios from './lib/utils/axios_utils';
 import { visitUrl } from './lib/utils/url_utility';
 import { isObject } from './lib/utils/type_utility';
+import renderItem from './gl_dropdown/render';
 
 var GitLabDropdown, GitLabDropdownFilter, GitLabDropdownRemote, GitLabDropdownInput;
 
@@ -521,8 +522,8 @@ GitLabDropdown = (function() {
           html.push(
             this.renderItem(
               {
-                header: name,
-                // Add header for each group
+                content: name,
+                type: 'header',
               },
               name,
             ),
@@ -542,16 +543,7 @@ GitLabDropdown = (function() {
   };
 
   GitLabDropdown.prototype.renderData = function(data, group) {
-    if (group == null) {
-      group = false;
-    }
-    return data.map(
-      (function(_this) {
-        return function(obj, index) {
-          return _this.renderItem(obj, group, index);
-        };
-      })(this),
-    );
+    return data.map((obj, index) => this.renderItem(obj, group || false, index));
   };
 
   GitLabDropdown.prototype.shouldPropagate = function(e) {
@@ -688,104 +680,25 @@ GitLabDropdown = (function() {
   };
 
   GitLabDropdown.prototype.renderItem = function(data, group, index) {
-    var field, html, selected, text, url, value, rowHidden;
-
-    if (!this.options.renderRow) {
-      value = this.options.id ? this.options.id(data) : data.id;
-
-      if (value) {
-        value = value.toString().replace(/'/g, "\\'");
-      }
-    }
-
-    // Hide element
-    if (this.options.hideRow && this.options.hideRow(value)) {
-      rowHidden = true;
-    }
-    if (group == null) {
-      group = false;
-    }
-    if (index == null) {
-      // Render the row
-      index = false;
-    }
-    html = document.createElement('li');
-
-    if (rowHidden) {
-      html.style.display = 'none';
-    }
-
-    if (data === 'divider' || data === 'separator') {
-      html.className = data;
-      return html;
-    }
-    // Header
-    if (data.header != null) {
-      html.className = 'dropdown-header';
-      html.innerHTML = data.header;
-      return html;
-    }
-    if (this.options.renderRow) {
-      // Call the render function
-      html = this.options.renderRow.call(this.options, data, this);
-    } else {
-      if (!selected) {
-        const { fieldName } = this.options;
-
-        if (value) {
-          field = this.dropdown.parent().find(`input[name='${fieldName}'][value='${value}']`);
-          if (field.length) {
-            selected = true;
-          }
-        } else {
-          field = this.dropdown.parent().find(`input[name='${fieldName}']`);
-          selected = !field.length;
-        }
-      }
-      // Set URL
-      if (this.options.url != null) {
-        url = this.options.url(data);
-      } else {
-        url = data.url != null ? data.url : '#';
-      }
-      // Set Text
-      if (this.options.text != null) {
-        text = this.options.text(data);
-      } else {
-        text = data.text != null ? data.text : '';
-      }
-      if (this.highlight) {
-        text = data.template
-          ? this.highlightTemplate(text, data.template)
-          : this.highlightTextMatches(text, this.filterInput.val());
-      }
-      // Create the list item & the link
-      var link = document.createElement('a');
-
-      link.href = url;
-
-      if (this.icon) {
-        text = `<span>${text}</span>`;
-        link.classList.add('d-flex', 'align-items-center');
-        link.innerHTML = data.icon ? data.icon + text : text;
-      } else if (this.highlight) {
-        link.innerHTML = text;
-      } else {
-        link.textContent = text;
-      }
-
-      if (selected) {
-        link.classList.add('is-active');
-      }
-
-      if (group) {
-        link.dataset.group = group;
-        link.dataset.index = index;
-      }
-
-      html.appendChild(link);
-    }
-    return html;
+    let parent;
+
+    if (this.dropdown && this.dropdown[0]) {
+      parent = this.dropdown[0].parentNode;
+    }
+
+    return renderItem({
+      instance: this,
+      options: Object.assign({}, this.options, {
+        icon: this.icon,
+        highlight: this.highlight,
+        highlightText: text => this.highlightTextMatches(text, this.filterInput.val()),
+        highlightTemplate: this.highlightTemplate.bind(this),
+        parent,
+      }),
+      data,
+      group,
+      index,
+    });
   };
 
   GitLabDropdown.prototype.highlightTemplate = function(text, template) {
@@ -809,7 +722,6 @@ GitLabDropdown = (function() {
   };
 
   GitLabDropdown.prototype.noResults = function() {
-    var html;
     return '<li class="dropdown-menu-empty-item"><a>No matching results</a></li>';
   };
 

app/assets/javascripts/gl_dropdown/render.js

+const renderersByType = {
+  divider(element) {
+    element.classList.add('divider');
+
+    return element;
+  },
+  separator(element) {
+    element.classList.add('separator');
+
+    return element;
+  },
+  header(element, data) {
+    element.classList.add('dropdown-header');
+    element.innerHTML = data.content;
+
+    return element;
+  },
+};
+
+function getPropertyWithDefault(data, options, property, defaultValue = '') {
+  let result;
+
+  if (options[property] != null) {
+    result = options[property](data);
+  } else {
+    result = data[property] != null ? data[property] : defaultValue;
+  }
+
+  return result;
+}
+
+function getHighlightTextBuilder(text, data, options) {
+  if (options.highlight) {
+    return data.template
+      ? options.highlightTemplate(text, data.template)
+      : options.highlightText(text);
+  }
+
+  return text;
+}
+
+function getIconTextBuilder(text, data, options) {
+  if (options.icon) {
+    const wrappedText = `<span>${text}</span>`;
+    return data.icon ? `${data.icon}${wrappedText}` : wrappedText;
+  }
+
+  return text;
+}
+
+function getLinkText(data, options) {
+  const text = getPropertyWithDefault(data, options, 'text');
+
+  return [getHighlightTextBuilder, getIconTextBuilder].reduce(
+    (acc, fn) => fn(acc, data, options),
+    text,
+  );
+}
+
+function escape(text) {
+  return text ? String(text).replace(/'/g, "\\'") : text;
+}
+
+function getOptionValue(data, options) {
+  if (options.renderRow) {
+    return undefined;
+  }
+
+  return escape(options.id ? options.id(data) : data.id);
+}
+
+function shouldHide(data, { options }) {
+  const value = getOptionValue(data, options);
+
+  return options.hideRow && options.hideRow(value);
+}
+
+function hideElement(element) {
+  element.style.display = 'none';
+
+  return element;
+}
+
+function checkSelected(data, options) {
+  const value = getOptionValue(data, options);
+
+  if (!options.parent) {
+    return !data.id;
+  } else if (value) {
+    return (
+      options.parent.querySelector(`input[name='${options.fieldName}'][value='${value}']`) != null
+    );
+  }
+
+  return options.parent.querySelector(`input[name='${options.fieldName}']`) == null;
+}
+
+function createLink(url, selected, options) {
+  const link = document.createElement('a');
+
+  link.href = url;
+
+  if (options.icon) {
+    link.classList.add('d-flex', 'align-items-center');
+  }
+
+  link.classList.toggle('is-active', selected);
+
+  return link;
+}
+
+function assignTextToLink(el, data, options) {
+  const text = getLinkText(data, options);
+
+  if (options.icon || options.highlight) {
+    el.innerHTML = text;
+  } else {
+    el.textContent = text;
+  }
+
+  return el;
+}
+
+function renderLink(row, data, { options, group, index }) {
+  const selected = checkSelected(data, options);
+  const url = getPropertyWithDefault(data, options, 'url', '#');
+  const link = createLink(url, selected, options);
+
+  assignTextToLink(link, data, options);
+
+  if (group) {
+    link.dataset.group = group;
+    link.dataset.index = index;
+  }
+
+  row.appendChild(link);
+
+  return row;
+}
+
+function getOptionRenderer({ options, instance }) {
+  return options.renderRow && ((li, data) => options.renderRow(data, instance));
+}
+
+function getRenderer(data, params) {
+  return renderersByType[data.type] || getOptionRenderer(params) || renderLink;
+}
+
+export default function item({ data, ...params }) {
+  const renderer = getRenderer(data, params);
+  const li = document.createElement('li');
+
+  if (shouldHide(data, params)) {
+    hideElement(li);
+  }
+
+  return renderer(li, data, params);
+}

ee/app/assets/javascripts/projects/settings/access_dropdown.js

@@ -369,7 +369,7 @@ export default class AccessDropdown {
     });
 
     if (roles.length) {
-      consolidatedData = consolidatedData.concat([{ header: s__('AccessDropdown|Roles') }], roles);
+      consolidatedData = consolidatedData.concat([{ type: 'header', content: s__('AccessDropdown|Roles') }], roles);
     }
 
     if (groups.length) {
@@ -378,15 +378,15 @@ export default class AccessDropdown {
       }
 
       consolidatedData = consolidatedData.concat(
-        [{ header: s__('AccessDropdown|Groups') }],
+        [{ type: 'header', content: s__('AccessDropdown|Groups') }],
         groups,
       );
     }
 
     if (users.length) {
       consolidatedData = consolidatedData.concat(
-        ['divider'],
-        [{ header: s__('AccessDropdown|Users') }],
+        [{ type: 'divider' }],
+        [{ type: 'header', content: s__('AccessDropdown|Users') }],
         users,
       );
     }