Use the node v8 promisify API to remove unnecessary code

tests/fetch-template.js

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+'use strict';
+const fetchTemplate = require('../lib/fetch-template');
+const assert = require('assert');
+const sinon = require('sinon');
+const path = require('path');
+const fs = require('fs');
+
+describe('fetch-template', () => {
+    const mockParseTemplate = sinon.spy();
+    let mockRequest;
+    const templatePath = path.join(__dirname);
+    const testTemplatePath = `${templatePath}/test.html`;
+    const baseTemplatePath = `${templatePath}/base-template.html`;
+
+    beforeEach(() => {
+        fs.writeFileSync(testTemplatePath, '<div>test</div>');
+        fs.writeFileSync(baseTemplatePath, '<div>base-template</div>');
+    });
+
+    afterEach(() => {
+        if (fs.existsSync(testTemplatePath)) {
+            fs.unlinkSync(testTemplatePath);
+        }
+
+        if (fs.existsSync(baseTemplatePath)) {
+            fs.unlinkSync(baseTemplatePath);
+        }
+    });
+
+    it('should be able to fetch the static template with absolute path', () => {
+        mockRequest = {
+            url: 'http://localhost:8080/test'
+        };
+
+        return fetchTemplate(templatePath)(mockRequest, mockParseTemplate)
+            .then(() => {
+                assert(mockParseTemplate.calledOnce);
+                return assert(mockParseTemplate.calledWith('<div>test</div>'));
+            });
+    });
+
+    it('should be able to fetch template with relative path and baseTemplateFn', () => {
+        mockRequest = {
+            url: 'http://localhost:8080/test'
+        };
+        const baseTemplateFn = () => 'base-template';
+
+        return fetchTemplate(templatePath, baseTemplateFn)(mockRequest, mockParseTemplate)
+            .then(() => {
+                assert(mockParseTemplate.called);
+                return assert(mockParseTemplate.calledWith(
+                    '<div>base-template</div>',
+                    '<div>test</div>'
+                ));
+            });
+    });
+
+    it('should throw TEMPLATE_NOT_FOUND error for not present template', () => {
+        mockRequest = {
+            url: 'http://localhost:8080/test'
+        };
+
+        return fetchTemplate('templates')(mockRequest, mockParseTemplate)
+            .catch(err => {
+                assert(err.code, 1);
+                assert(err.presentable, 'template not found');
+            });
+    });
+});