refactor: Move handlebars template rendering to separate service

Logic was embedded in EmailService.  Extracting allows for easier reuse.

src/app/common/template.service.ts

@@ -0,0 +1,56 @@
+import fs from 'fs/promises';
+
+import config from 'config';
+import handlebars from 'handlebars';
+import _ from 'lodash';
+
+handlebars.registerHelper('not', (value: boolean) => !value);
+handlebars.registerHelper(
+	'isEqual',
+	(value1: unknown, value2: unknown) => value1 === value2
+);
+handlebars.registerHelper('lower', (str: string) => str?.toLowerCase());
+handlebars.registerHelper('upper', (str: string) => str?.toUpperCase());
+handlebars.registerHelper(
+	'head',
+	(str: string, count: number) => str?.substring(0, count)
+);
+handlebars.registerHelper(
+	'tail',
+	(str: string, count: number) => str?.substring(str.length - count)
+);
+
+class TemplateService {
+	async renderTemplate(
+		templatePath: string,
+		data: Record<string, unknown>,
+		options: RuntimeOptions = {}
+	) {
+		const template = await fs.readFile(templatePath, 'utf-8');
+		const templateData = _.merge(
+			{},
+			{
+				app: config.get('app')
+			},
+			data
+		);
+		return handlebars.compile(template)(templateData, options);
+	}
+
+	renderTemplateStr(
+		templateStr: string,
+		data: Record<string, unknown>,
+		options: RuntimeOptions = {}
+	) {
+		const templateData = _.merge(
+			{},
+			{
+				app: config.get('app')
+			},
+			data
+		);
+		return handlebars.compile(templateStr)(templateData, options);
+	}
+}
+
+export = new TemplateService();

src/app/core/email/email.service.ts

@@ -1,14 +1,11 @@
-import fs from 'fs';
 import path from 'path';
 
-import handlebars from 'handlebars';
 import _ from 'lodash';
 
 import { EmailProvider, MailOptions } from './providers/email.provider';
 import { config } from '../../../dependencies';
 import { logger } from '../../../lib/logger';
-
-handlebars.registerHelper('toLowerCase', (str) => str.toLowerCase());
+import templateService from '../../common/template.service';
 
 class EmailService {
 	provider: EmailProvider;
@@ -64,44 +61,6 @@ class EmailService {
 		logger.debug(`Sent email to: ${mailOptions.to}`);
 	}
 
-	async buildEmailContent(templatePath, user, overrides = {}) {
-		const templateString = await new Promise((resolve, reject) => {
-			fs.readFile(templatePath, 'utf-8', (err, source) => {
-				if (err) {
-					reject(err);
-				} else {
-					resolve(source);
-				}
-			});
-		});
-
-		// Set email header/footer
-		const data = _.merge(
-			{},
-			config.get('coreEmails.default'),
-			{
-				app: config.get('app'),
-				user: user
-			},
-			overrides
-		);
-
-		return handlebars.compile(templateString)(data);
-	}
-
-	buildEmailSubject(template, user, overrides = {}): string {
-		const data = _.merge(
-			{},
-			config.get('coreEmails.default'),
-			{
-				app: config.get('app'),
-				user: user
-			},
-			overrides
-		);
-		return handlebars.compile(template)(data);
-	}
-
 	async generateMailOptions(
 		user,
 		req,
@@ -143,6 +102,36 @@ class EmailService {
 		);
 	}
 
+	buildEmailContent(
+		templatePath: string,
+		user,
+		overrides = {}
+	): Promise<string> {
+		// Set email header/footer
+		const data = _.merge(
+			{},
+			config.get('coreEmails.default'),
+			{
+				user: user
+			},
+			overrides
+		);
+
+		return templateService.renderTemplate(templatePath, data);
+	}
+
+	buildEmailSubject(template: string, user, overrides = {}): string {
+		const data = _.merge(
+			{},
+			config.get('coreEmails.default'),
+			{
+				user: user
+			},
+			overrides
+		);
+		return templateService.renderTemplateStr(template, data);
+	}
+
 	/**
 	 * Initializes the provider only once. Use the getProvider() method
 	 * to create and/or retrieve this singleton