Extracted email preview service

refs https://github.com/TryGhost/Team/issues/694

- The code complexity in the email preview's read controller method was breaking the complexity rule in ESLint. To reduce the complexity extracted common parts into mega service

core/server/api/canary/email-preview.js

@@ -4,6 +4,11 @@ const errors = require('@tryghost/errors');
 const mega = require('../../services/mega');
 const labs = require('../../../shared/labs');
 
+const emailPreview = new mega.EmailPreview({
+    apiVersion: 'canary',
+    isSet: labs.isSet.bind(labs)
+});
+
 module.exports = {
     docName: 'email_preview',
 
@@ -34,23 +39,7 @@ module.exports = {
                 });
             }
 
-            let emailContent = await mega.postEmailSerializer.serialize(model, {
-                isBrowserPreview: true,
-                apiVersion: 'canary'
-            });
-            if (labs.isSet('emailCardSegments') && frame.options.memberSegment) {
-                emailContent = mega.postEmailSerializer.renderEmailForSegment(emailContent, frame.options.memberSegment);
-            }
-            const replacements = mega.postEmailSerializer.parseReplacements(emailContent);
-
-            replacements.forEach((replacement) => {
-                emailContent[replacement.format] = emailContent[replacement.format].replace(
-                    replacement.match,
-                    replacement.fallback || ''
-                );
-            });
-
-            return emailContent;
+            return emailPreview.generateEmailContent(model, frame.options.memberSegment);
         }
     },
     sendTestEmail: {

core/server/api/v3/email-preview.js

@@ -3,6 +3,10 @@ const i18n = require('../../../shared/i18n');
 const errors = require('@tryghost/errors');
 const mega = require('../../services/mega');
 
+const emailPreview = new mega.EmailPreview({
+    apiVersion: 'v3'
+});
+
 module.exports = {
     docName: 'email_preview',
 
@@ -32,20 +36,7 @@ module.exports = {
                 });
             }
 
-            let emailContent = await mega.postEmailSerializer.serialize(model, {
-                isBrowserPreview: true,
-                apiVersion: 'v3'
-            });
-            const replacements = mega.postEmailSerializer.parseReplacements(emailContent);
-
-            replacements.forEach((replacement) => {
-                emailContent[replacement.format] = emailContent[replacement.format].replace(
-                    replacement.match,
-                    replacement.fallback || ''
-                );
-            });
-
-            return emailContent;
+            return emailPreview.generateEmailContent(model, frame.options.memberSegment);
         }
     },
     sendTestEmail: {

core/server/services/mega/email-preview.js

@@ -0,0 +1,43 @@
+const postEmailSerializer = require('./post-email-serializer');
+
+class EmailPreview {
+    /**
+     * @constructor
+     * @param {Object} options
+     * @param {String} options.apiVersion
+     * @param {Function} [options.isSet]
+     */
+    constructor({apiVersion, isSet}) {
+        this.apiVersion = apiVersion;
+        this.isSet = isSet;
+    }
+
+    /**
+     * @param {Object} post - Post model object instance
+     * @param {String} memberSegment - member segment filter
+     * @returns {Promise<Object>}
+     */
+    async generateEmailContent(post, memberSegment) {
+        let emailContent = await postEmailSerializer.serialize(post, {
+            isBrowserPreview: true,
+            apiVersion: this.apiVersion
+        });
+
+        if (this.isSet && this.isSet('emailCardSegments') && memberSegment) {
+            emailContent = postEmailSerializer.renderEmailForSegment(emailContent, memberSegment);
+        }
+
+        const replacements = postEmailSerializer.parseReplacements(emailContent);
+
+        replacements.forEach((replacement) => {
+            emailContent[replacement.format] = emailContent[replacement.format].replace(
+                replacement.match,
+                replacement.fallback || ''
+            );
+        });
+
+        return emailContent;
+    }
+}
+
+module.exports = EmailPreview;

core/server/services/mega/index.js

@@ -5,6 +5,10 @@ module.exports = {
 
     get postEmailSerializer() {
         return require('./post-email-serializer');
+    },
+
+    get EmailPreview() {
+        return require('./email-preview');
     }
 };