refactor: Allow default template in template engines.

.componentsignore

@@ -4,5 +4,6 @@
   "Error",
   "EventEmitter",
   "HttpErrorOptions",
+  "Template",
   "ValuePreferencesArg"
 ]

.eslintrc.js

@@ -86,6 +86,10 @@ module.exports = {
     'unicorn/prefer-ternary': 'off',
     'yield-star-spacing': [ 'error', 'after' ],
 
+    // Need to use the typescript version of this rule to support overloading
+    "no-dupe-class-members": "off",
+    "@typescript-eslint/no-dupe-class-members": ["error"],
+
     // Naming conventions
     '@typescript-eslint/naming-convention': [
       'error',

config/identity/pod/resource-generators/templated.json

@@ -9,7 +9,7 @@
       "factory": {
         "@type": "ExtensionBasedMapperFactory"
       },
-      "engine": {
+      "templateEngine": {
         "@type": "HandlebarsTemplateEngine"
       }
     }

config/util/representation-conversion/default.json

@@ -26,17 +26,24 @@
             { "@id": "urn:solid-server:default:QuadToRdfConverter" },
             { "@type": "ErrorToQuadConverter" },
             {
+              "comment": "Converts an error into a Markdown description of its details.",
               "@type": "ErrorToTemplateConverter",
-              "engine": { "@type": "HandlebarsTemplateEngine" },
-              "templatePath": "$PACKAGE_ROOT/templates/error/main.md",
-              "descriptions": "$PACKAGE_ROOT/templates/error/descriptions/",
-              "contentType": "text/markdown",
-              "extension": ".md"
+              "templateEngine": {
+                "@type": "HandlebarsTemplateEngine",
+                "template": { "templateFile": "$PACKAGE_ROOT/templates/error/main.md" }
+              },
+              "templatePath": "$PACKAGE_ROOT/templates/error/descriptions/",
+              "extension": ".md",
+              "contentType": "text/markdown"
             },
             {
+              "comment": "Renders Markdown snippets into the main page template.",
               "@type": "MarkdownToHtmlConverter",
-              "engine": { "@type": "HandlebarsTemplateEngine" },
-              "templatePath": "$PACKAGE_ROOT/templates/main.html"
+              "templateEngine": {
+                "@id": "urn:solid-server:default:MainTemplateEngine",
+                "@type": "HandlebarsTemplateEngine",
+                "template": { "templateFile": "$PACKAGE_ROOT/templates/main.html" }
+              }
             }
           ]
         }

src/index.ts

@@ -174,14 +174,12 @@ export * from './pods/generate/variables/VariableSetter';
 export * from './pods/generate/BaseComponentsJsFactory';
 export * from './pods/generate/ComponentsJsFactory';
 export * from './pods/generate/GenerateUtil';
-export * from './pods/generate/HandlebarsTemplateEngine';
 export * from './pods/generate/IdentifierGenerator';
 export * from './pods/generate/PodGenerator';
 export * from './pods/generate/ResourcesGenerator';
 export * from './pods/generate/SubdomainIdentifierGenerator';
 export * from './pods/generate/SuffixIdentifierGenerator';
 export * from './pods/generate/TemplatedPodGenerator';
-export * from './pods/generate/TemplateEngine';
 export * from './pods/generate/TemplatedResourcesGenerator';
 
 // Pods/Settings
@@ -223,8 +221,8 @@ export * from './storage/conversion/ChainedConverter';
 export * from './storage/conversion/ConstantConverter';
 export * from './storage/conversion/ContentTypeReplacer';
 export * from './storage/conversion/ConversionUtil';
-export * from './storage/conversion/ErrorToTemplateConverter';
 export * from './storage/conversion/ErrorToQuadConverter';
+export * from './storage/conversion/ErrorToTemplateConverter';
 export * from './storage/conversion/IfNeededConverter';
 export * from './storage/conversion/MarkdownToHtmlConverter';
 export * from './storage/conversion/PassthroughConverter';
@@ -311,6 +309,10 @@ export * from './util/locking/ResourceLocker';
 export * from './util/locking/SingleThreadedResourceLocker';
 export * from './util/locking/WrappedExpiringReadWriteLocker';
 
+// Util/Templates
+export * from './util/templates/HandlebarsTemplateEngine';
+export * from './util/templates/TemplateEngine';
+
 // Util
 export * from './util/ContentTypes';
 export * from './util/GuardedStream';

src/pods/generate/HandlebarsTemplateEngine.ts

@@ -1,12 +0,0 @@
-import { compile } from 'handlebars';
-import type { TemplateEngine } from './TemplateEngine';
-
-/**
- * Fills in Handlebars templates.
- */
-export class HandlebarsTemplateEngine implements TemplateEngine {
-  public apply(template: string, options: NodeJS.Dict<string>): string {
-    const compiled = compile(template);
-    return compiled(options);
-  }
-}

src/pods/generate/TemplateEngine.ts

@@ -1,8 +0,0 @@
-import Dict = NodeJS.Dict;
-
-/**
- * A template engine takes as input a template and applies the given options to it.
- */
-export interface TemplateEngine {
-  apply: (template: string, options: Dict<string>) => string;
-}

src/pods/generate/TemplatedResourcesGenerator.ts

@@ -13,8 +13,8 @@ import { guardStream } from '../../util/GuardedStream';
 import type { Guarded } from '../../util/GuardedStream';
 import { joinFilePath, isContainerIdentifier, resolveAssetPath } from '../../util/PathUtil';
 import { guardedStreamFrom, readableToString } from '../../util/StreamUtil';
+import type { TemplateEngine } from '../../util/templates/TemplateEngine';
 import type { Resource, ResourcesGenerator } from './ResourcesGenerator';
-import type { TemplateEngine } from './TemplateEngine';
 import Dict = NodeJS.Dict;
 
 interface TemplateResourceLink extends ResourceLink {
@@ -33,7 +33,7 @@ interface TemplateResourceLink extends ResourceLink {
 export class TemplatedResourcesGenerator implements ResourcesGenerator {
   private readonly templateFolder: string;
   private readonly factory: FileIdentifierMapperFactory;
-  private readonly engine: TemplateEngine;
+  private readonly templateEngine: TemplateEngine;
   private readonly templateExtension: string;
 
   /**
@@ -41,28 +41,28 @@ export class TemplatedResourcesGenerator implements ResourcesGenerator {
    *
    * @param templateFolder - Folder where the templates are located.
    * @param factory - Factory used to generate mapper relative to the base identifier.
-   * @param engine - Template engine for generating the resources.
+   * @param templateEngine - Template engine for generating the resources.
    * @param templateExtension - The extension of files that need to be interpreted as templates.
    *                            Will be removed to generate the identifier.
    */
-  public constructor(templateFolder: string, factory: FileIdentifierMapperFactory, engine: TemplateEngine,
+  public constructor(templateFolder: string, factory: FileIdentifierMapperFactory, templateEngine: TemplateEngine,
     templateExtension = '.hbs') {
     this.templateFolder = resolveAssetPath(templateFolder);
     this.factory = factory;
-    this.engine = engine;
+    this.templateEngine = templateEngine;
     this.templateExtension = templateExtension;
   }
 
   public async* generate(location: ResourceIdentifier, options: Dict<string>): AsyncIterable<Resource> {
     const mapper = await this.factory.create(location.path, this.templateFolder);
     const folderLink = await this.toTemplateLink(this.templateFolder, mapper);
-    yield* this.parseFolder(folderLink, mapper, options);
+    yield* this.processFolder(folderLink, mapper, options);
   }
 
   /**
    * Generates results for all entries in the given folder, including the folder itself.
    */
-  private async* parseFolder(folderLink: TemplateResourceLink, mapper: FileIdentifierMapper, options: Dict<string>):
+  private async* processFolder(folderLink: TemplateResourceLink, mapper: FileIdentifierMapper, options: Dict<string>):
   AsyncIterable<Resource> {
     // Group resource links with their corresponding metadata links
     const links = await this.groupLinks(folderLink.filePath, mapper);
@@ -76,7 +76,7 @@ export class TemplatedResourcesGenerator implements ResourcesGenerator {
 
     for (const { link, meta } of Object.values(links)) {
       if (isContainerIdentifier(link.identifier)) {
-        yield* this.parseFolder(link, mapper, options);
+        yield* this.processFolder(link, mapper, options);
       } else {
         yield this.generateResource(link, options, meta);
       }
@@ -131,7 +131,7 @@ export class TemplatedResourcesGenerator implements ResourcesGenerator {
 
     // Read file if it is not a container
     if (!isContainerIdentifier(link.identifier)) {
-      data = await this.parseFile(link, options);
+      data = await this.processFile(link, options);
       metadata.contentType = link.contentType;
     }
 
@@ -154,7 +154,7 @@ export class TemplatedResourcesGenerator implements ResourcesGenerator {
   Promise<RepresentationMetadata> {
     const metadata = new RepresentationMetadata(metaLink.identifier);
 
-    const data = await this.parseFile(metaLink, options);
+    const data = await this.processFile(metaLink, options);
     const parser = new Parser({ format: metaLink.contentType, baseIRI: metaLink.identifier.path });
     const quads = parser.parse(await readableToString(data));
     metadata.addQuads(quads);
@@ -165,11 +165,10 @@ export class TemplatedResourcesGenerator implements ResourcesGenerator {
   /**
    * Creates a read stream from the file and applies the template if necessary.
    */
-  private async parseFile(link: TemplateResourceLink, options: Dict<string>): Promise<Guarded<Readable>> {
+  private async processFile(link: TemplateResourceLink, options: Dict<string>): Promise<Guarded<Readable>> {
     if (link.isTemplate) {
-      const raw = await fsPromises.readFile(link.filePath, 'utf8');
-      const result = this.engine.apply(raw, options);
-      return guardedStreamFrom(result);
+      const rendered = await this.templateEngine.render(options, { templateFile: link.filePath });
+      return guardedStreamFrom(rendered);
     }
     return guardStream(createReadStream(link.filePath));
   }

src/storage/conversion/ErrorToTemplateConverter.ts

@@ -1,13 +1,11 @@
 import assert from 'assert';
-import { promises as fsPromises } from 'fs';
 import arrayifyStream from 'arrayify-stream';
 import { BasicRepresentation } from '../../ldp/representation/BasicRepresentation';
 import type { Representation } from '../../ldp/representation/Representation';
-import type { TemplateEngine } from '../../pods/generate/TemplateEngine';
 import { INTERNAL_ERROR } from '../../util/ContentTypes';
 import { HttpError } from '../../util/errors/HttpError';
 import { InternalServerError } from '../../util/errors/InternalServerError';
-import { joinFilePath, resolveAssetPath } from '../../util/PathUtil';
+import type { TemplateEngine } from '../../util/templates/TemplateEngine';
 import type { RepresentationConverterArgs } from './RepresentationConverter';
 import { TypedRepresentationConverter } from './TypedRepresentationConverter';
 
@@ -23,52 +21,45 @@ import { TypedRepresentationConverter } from './TypedRepresentationConverter';
  * using the variable `codeMessage`.
  */
 export class ErrorToTemplateConverter extends TypedRepresentationConverter {
-  private readonly engine: TemplateEngine;
+  private readonly templateEngine: TemplateEngine;
   private readonly templatePath: string;
-  private readonly descriptions: string;
-  private readonly contentType: string;
   private readonly extension: string;
+  private readonly contentType: string;
 
-  public constructor(engine: TemplateEngine, templatePath: string, descriptions: string, contentType: string,
-    extension: string) {
+  public constructor(templateEngine: TemplateEngine, templatePath: string, extension: string, contentType: string) {
     super(INTERNAL_ERROR, contentType);
-    this.engine = engine;
-    this.templatePath = resolveAssetPath(templatePath);
-    this.descriptions = resolveAssetPath(descriptions);
-    this.contentType = contentType;
+    this.templateEngine = templateEngine;
+    this.templatePath = templatePath;
     this.extension = extension;
+    this.contentType = contentType;
   }
 
   public async handle({ representation }: RepresentationConverterArgs): Promise<Representation> {
+    // Obtain the error from the representation stream
     const errors = await arrayifyStream(representation.data);
     if (errors.length !== 1) {
       throw new InternalServerError('Only single errors are supported.');
     }
     const error = errors[0] as Error;
 
-    // Render the template
+    // Render the error description using an error-specific template
+    let description: string | undefined;
+    if (HttpError.isInstance(error)) {
+      try {
+        const templateFile = `${error.errorCode}${this.extension}`;
+        assert(/^[\w.-]+$/u.test(templateFile), 'Invalid error template name');
+        description = await this.templateEngine.render(error.details ?? {},
+          { templateFile, templatePath: this.templatePath });
+      } catch {
+        // In case no template is found, or rendering errors, we still want to convert
+      }
+    }
+
+    // Render the main template, embedding the rendered error description
     const { name, message, stack } = error;
-    const description = await this.getErrorCodeMessage(error);
     const variables = { name, message, stack, description };
-    const template = await fsPromises.readFile(this.templatePath, 'utf8');
-    const rendered = this.engine.apply(template, variables);
+    const rendered = await this.templateEngine.render(variables);
 
     return new BasicRepresentation(rendered, representation.metadata, this.contentType);
   }
-
-  private async getErrorCodeMessage(error: Error): Promise<string | undefined> {
-    if (HttpError.isInstance(error)) {
-      let template: string;
-      try {
-        const fileName = `${error.errorCode}${this.extension}`;
-        assert(/^[\w.-]+$/u.test(fileName), 'Invalid error template name');
-        template = await fsPromises.readFile(joinFilePath(this.descriptions, fileName), 'utf8');
-      } catch {
-        // In case no template is found we still want to convert
-        return;
-      }
-
-      return this.engine.apply(template, (error.details ?? {}) as NodeJS.Dict<string>);
-    }
-  }
 }

src/storage/conversion/MarkdownToHtmlConverter.ts

@@ -1,41 +1,34 @@
-import { promises as fsPromises } from 'fs';
 import marked from 'marked';
 import { BasicRepresentation } from '../../ldp/representation/BasicRepresentation';
 import type { Representation } from '../../ldp/representation/Representation';
-import type { TemplateEngine } from '../../pods/generate/TemplateEngine';
 import { TEXT_HTML, TEXT_MARKDOWN } from '../../util/ContentTypes';
-import { resolveAssetPath } from '../../util/PathUtil';
 import { readableToString } from '../../util/StreamUtil';
+import type { TemplateEngine } from '../../util/templates/TemplateEngine';
 import type { RepresentationConverterArgs } from './RepresentationConverter';
 import { TypedRepresentationConverter } from './TypedRepresentationConverter';
 
 /**
- * Converts markdown data to HTML.
+ * Converts Markdown data to HTML.
  * The generated HTML will be injected into the given template using the parameter `htmlBody`.
- * A standard markdown string will be converted to a <p> tag, so html and body tags should be part of the template.
- * In case the markdown body starts with a header (#), that value will also be used as `title` parameter.
+ * A standard Markdown string will be converted to a <p> tag, so html and body tags should be part of the template.
+ * In case the Markdown body starts with a header (#), that value will also be used as `title` parameter.
  */
 export class MarkdownToHtmlConverter extends TypedRepresentationConverter {
-  private readonly engine: TemplateEngine;
-  private readonly templatePath: string;
+  private readonly templateEngine: TemplateEngine;
 
-  public constructor(engine: TemplateEngine, templatePath: string) {
+  public constructor(templateEngine: TemplateEngine) {
     super(TEXT_MARKDOWN, TEXT_HTML);
-    this.engine = engine;
-    this.templatePath = resolveAssetPath(templatePath);
+    this.templateEngine = templateEngine;
   }
 
   public async handle({ representation }: RepresentationConverterArgs): Promise<Representation> {
     const markdown = await readableToString(representation.data);
+    // Try to extract the main title for use in the <title> tag
+    const title = /^#+\s*([^\n]+)\n/u.exec(markdown)?.[1];
 
-    // See if there is a title we can use
-    const match = /^\s*#+\s*([^\n]+)\n/u.exec(markdown);
-    const title = match?.[1];
-
+    // Place the rendered Markdown into the HTML template
     const htmlBody = marked(markdown);
-
-    const template = await fsPromises.readFile(this.templatePath, 'utf8');
-    const html = this.engine.apply(template, { htmlBody, title });
+    const html = await this.templateEngine.render({ htmlBody, title });
 
     return new BasicRepresentation(html, representation.metadata, TEXT_HTML);
   }

src/util/templates/HandlebarsTemplateEngine.ts

@@ -0,0 +1,29 @@
+/* eslint-disable tsdoc/syntax */
+// tsdoc/syntax can't handle {json} parameter
+import type { TemplateDelegate } from 'handlebars';
+import { compile } from 'handlebars';
+import type { TemplateEngine, Template } from './TemplateEngine';
+import { readTemplate } from './TemplateEngine';
+import Dict = NodeJS.Dict;
+
+/**
+ * Fills in Handlebars templates.
+ */
+export class HandlebarsTemplateEngine<T extends Dict<any> = Dict<any>> implements TemplateEngine<T> {
+  private readonly applyTemplate: Promise<TemplateDelegate>;
+
+  /**
+   * @param template - The default template @range {json}
+   */
+  public constructor(template?: Template) {
+    this.applyTemplate = readTemplate(template)
+      .then((templateString: string): TemplateDelegate => compile(templateString));
+  }
+
+  public async render(contents: T): Promise<string>;
+  public async render<TCustom = T>(contents: TCustom, template: Template): Promise<string>;
+  public async render<TCustom = T>(contents: TCustom, template?: Template): Promise<string> {
+    const applyTemplate = template ? compile(await readTemplate(template)) : await this.applyTemplate;
+    return applyTemplate(contents);
+  }
+}

src/util/templates/TemplateEngine.ts

@@ -0,0 +1,49 @@
+import { promises as fsPromises } from 'fs';
+import { joinFilePath, resolveAssetPath } from '../PathUtil';
+import Dict = NodeJS.Dict;
+
+export type Template = TemplateString | TemplatePath;
+
+export interface TemplateString {
+  // String contents of the template
+  templateString: string;
+}
+
+export interface TemplatePath {
+  // Name of the template file
+  templateFile: string;
+  // Path of the template file
+  templatePath?: string;
+}
+
+/* eslint-disable @typescript-eslint/method-signature-style */
+/**
+ * A template engine renders content into a template.
+ */
+export interface TemplateEngine<T extends Dict<any> = Dict<any>> {
+  /**
+   * Renders the given contents into the template.
+   *
+   * @param contents - The contents to render.
+   * @param template - The template to use for rendering;
+   *                   if omitted, a default template is used.
+   * @returns The rendered contents.
+   */
+  render(contents: T): Promise<string>;
+  render<TCustom = T>(contents: TCustom, template: Template): Promise<string>;
+}
+/* eslint-enable @typescript-eslint/method-signature-style */
+
+/**
+ * Reads the template and returns it as a string.
+ */
+export async function readTemplate(template: Template = { templateString: '' }): Promise<string> {
+  // The template has already been given as a string
+  if ('templateString' in template) {
+    return template.templateString;
+  }
+  // The template needs to be read from disk
+  const { templateFile, templatePath } = template;
+  const fullTemplatePath = templatePath ? joinFilePath(templatePath, templateFile) : templateFile;
+  return fsPromises.readFile(resolveAssetPath(fullTemplatePath), 'utf8');
+}

test/unit/pods/generate/HandlebarsTemplateEngine.test.ts

@@ -1,11 +0,0 @@
-import { HandlebarsTemplateEngine } from '../../../../src/pods/generate/HandlebarsTemplateEngine';
-
-describe('A HandlebarsTemplateEngine', (): void => {
-  const engine = new HandlebarsTemplateEngine();
-
-  it('fills in Handlebars templates.', async(): Promise<void> => {
-    const template = '<{{webId}}> a <http://xmlns.com/foaf/0.1/Person>.';
-    const options = { webId: 'http://alice/#profile' };
-    expect(engine.apply(template, options)).toBe('<http://alice/#profile> a <http://xmlns.com/foaf/0.1/Person>.');
-  });
-});

test/unit/pods/generate/TemplatedResourcesGenerator.test.ts

@@ -1,5 +1,4 @@
 import type { ResourceIdentifier } from '../../../../src/ldp/representation/ResourceIdentifier';
-import { HandlebarsTemplateEngine } from '../../../../src/pods/generate/HandlebarsTemplateEngine';
 import { TemplatedResourcesGenerator } from '../../../../src/pods/generate/TemplatedResourcesGenerator';
 import type {
   FileIdentifierMapper,
@@ -8,6 +7,7 @@ import type {
 } from '../../../../src/storage/mapping/FileIdentifierMapper';
 import { ensureTrailingSlash, trimTrailingSlashes } from '../../../../src/util/PathUtil';
 import { readableToString } from '../../../../src/util/StreamUtil';
+import { HandlebarsTemplateEngine } from '../../../../src/util/templates/HandlebarsTemplateEngine';
 import { mockFs } from '../../../util/Util';
 
 jest.mock('fs');

test/unit/storage/conversion/ErrorToTemplateConverter.test.ts

@@ -1,32 +1,23 @@
 import { BasicRepresentation } from '../../../../src/ldp/representation/BasicRepresentation';
-import type { TemplateEngine } from '../../../../src/pods/generate/TemplateEngine';
 import { ErrorToTemplateConverter } from '../../../../src/storage/conversion/ErrorToTemplateConverter';
 import { BadRequestHttpError } from '../../../../src/util/errors/BadRequestHttpError';
 import { InternalServerError } from '../../../../src/util/errors/InternalServerError';
 import { readableToString } from '../../../../src/util/StreamUtil';
-import { mockFs } from '../../../util/Util';
-
-jest.mock('fs');
+import type { TemplateEngine } from '../../../../src/util/templates/TemplateEngine';
 
 describe('An ErrorToTemplateConverter', (): void => {
-  let cache: { data: any };
   const identifier = { path: 'http://test.com/error' };
-  const templatePath = '/templates/error.template';
-  const descriptions = '/templates/codes';
+  const templatePath = '/templates/codes';
   const errorCode = 'E0001';
-  let engine: TemplateEngine;
+  let templateEngine: jest.Mocked<TemplateEngine>;
   let converter: ErrorToTemplateConverter;
   const preferences = {};
 
   beforeEach(async(): Promise<void> => {
-    cache = mockFs('/templates');
-    cache.data['error.template'] = '{{ template }}';
-    cache.data.codes = { [`${errorCode}.html`]: '{{{ errorText }}}' };
-    engine = {
-      apply: jest.fn().mockReturnValue('<html>'),
+    templateEngine = {
+      render: jest.fn().mockReturnValue(Promise.resolve('<html>')),
     };
-
-    converter = new ErrorToTemplateConverter(engine, templatePath, descriptions, 'text/html', '.html');
+    converter = new ErrorToTemplateConverter(templateEngine, templatePath, '.html', 'text/html');
   });
 
   it('supports going from errors to the given content type.', async(): Promise<void> => {
@@ -46,13 +37,14 @@ describe('An ErrorToTemplateConverter', (): void => {
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
     await expect(prom).resolves.toBeDefined();
+
     const result = await prom;
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}', { name: 'Error', message: 'error text', stack: error.stack },
+    expect(templateEngine.render).toHaveBeenCalledTimes(1);
+    expect(templateEngine.render).toHaveBeenLastCalledWith(
+      { name: 'Error', message: 'error text', stack: error.stack },
     );
   });
 
@@ -60,15 +52,19 @@ describe('An ErrorToTemplateConverter', (): void => {
     const error = new BadRequestHttpError('error text');
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
+    templateEngine.render.mockRejectedValueOnce(new Error('error-specific template not found'));
     await expect(prom).resolves.toBeDefined();
+
     const result = await prom;
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}', { name: 'BadRequestHttpError', message: 'error text', stack: error.stack },
-    );
+    expect(templateEngine.render).toHaveBeenCalledTimes(2);
+    expect(templateEngine.render).toHaveBeenNthCalledWith(1,
+      {},
+      { templatePath: '/templates/codes', templateFile: 'H400.html' });
+    expect(templateEngine.render).toHaveBeenNthCalledWith(2,
+      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack });
   });
 
   it('only adds stack if it is defined.', async(): Promise<void> => {
@@ -76,18 +72,22 @@ describe('An ErrorToTemplateConverter', (): void => {
     delete error.stack;
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
+    templateEngine.render.mockRejectedValueOnce(new Error('error-specific template not found'));
     await expect(prom).resolves.toBeDefined();
+
     const result = await prom;
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}', { name: 'BadRequestHttpError', message: 'error text' },
-    );
+    expect(templateEngine.render).toHaveBeenCalledTimes(2);
+    expect(templateEngine.render).toHaveBeenNthCalledWith(1,
+      {},
+      { templatePath: '/templates/codes', templateFile: 'H400.html' });
+    expect(templateEngine.render).toHaveBeenNthCalledWith(2,
+      { name: 'BadRequestHttpError', message: 'error text' });
   });
 
-  it('adds additional information if an error code is found.', async(): Promise<void> => {
+  it('adds additional information if an error code description is found.', async(): Promise<void> => {
     const error = new BadRequestHttpError('error text', { errorCode, details: { key: 'val' }});
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
@@ -96,14 +96,12 @@ describe('An ErrorToTemplateConverter', (): void => {
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(2);
-    expect(engine.apply).toHaveBeenCalledWith(
-      '{{{ errorText }}}', { key: 'val' },
-    );
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}',
-      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack, description: '<html>' },
-    );
+    expect(templateEngine.render).toHaveBeenCalledTimes(2);
+    expect(templateEngine.render).toHaveBeenNthCalledWith(1,
+      { key: 'val' },
+      { templatePath: '/templates/codes', templateFile: 'E0001.html' });
+    expect(templateEngine.render).toHaveBeenNthCalledWith(2,
+      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack, description: '<html>' });
   });
 
   it('sends an empty object for additional error code parameters if none are defined.', async(): Promise<void> => {
@@ -111,33 +109,35 @@ describe('An ErrorToTemplateConverter', (): void => {
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
     await expect(prom).resolves.toBeDefined();
+
     const result = await prom;
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(2);
-    expect(engine.apply).toHaveBeenCalledWith(
-      '{{{ errorText }}}', { },
-    );
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}',
-      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack, description: '<html>' },
-    );
+    expect(templateEngine.render).toHaveBeenCalledTimes(2);
+    expect(templateEngine.render).toHaveBeenNthCalledWith(1,
+      {},
+      { templatePath: '/templates/codes', templateFile: 'E0001.html' });
+    expect(templateEngine.render).toHaveBeenNthCalledWith(2,
+      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack, description: '<html>' });
   });
 
   it('converts errors with a code as usual if no corresponding template is found.', async(): Promise<void> => {
     const error = new BadRequestHttpError('error text', { errorCode: 'invalid' });
     const representation = new BasicRepresentation([ error ], 'internal/error', false);
     const prom = converter.handle({ identifier, representation, preferences });
+    templateEngine.render.mockRejectedValueOnce(new Error('error-specific template not found'));
     await expect(prom).resolves.toBeDefined();
+
     const result = await prom;
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}',
-      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack },
-    );
+    expect(templateEngine.render).toHaveBeenCalledTimes(2);
+    expect(templateEngine.render).toHaveBeenNthCalledWith(1,
+      {},
+      { templatePath: '/templates/codes', templateFile: 'invalid.html' });
+    expect(templateEngine.render).toHaveBeenNthCalledWith(2,
+      { name: 'BadRequestHttpError', message: 'error text', stack: error.stack });
   });
 });

test/unit/storage/conversion/MarkdownToHtmlConverter.test.ts

@@ -1,27 +1,19 @@
 import { BasicRepresentation } from '../../../../src/ldp/representation/BasicRepresentation';
-import type { TemplateEngine } from '../../../../src/pods/generate/TemplateEngine';
 import { MarkdownToHtmlConverter } from '../../../../src/storage/conversion/MarkdownToHtmlConverter';
 import { readableToString } from '../../../../src/util/StreamUtil';
-import { mockFs } from '../../../util/Util';
-
-jest.mock('fs');
+import type { TemplateEngine } from '../../../../src/util/templates/TemplateEngine';
 
 describe('A MarkdownToHtmlConverter', (): void => {
-  let cache: { data: any };
   const identifier = { path: 'http://test.com/text' };
-  const templatePath = '/templates/error.template';
   const preferences = {};
-  let engine: TemplateEngine;
+  let templateEngine: TemplateEngine;
   let converter: MarkdownToHtmlConverter;
 
   beforeEach(async(): Promise<void> => {
-    cache = mockFs('/templates');
-    cache.data['error.template'] = '{{ template }}';
-    engine = {
-      apply: jest.fn().mockReturnValue('<html>'),
+    templateEngine = {
+      render: jest.fn().mockReturnValue(Promise.resolve('<html>')),
     };
-
-    converter = new MarkdownToHtmlConverter(engine, templatePath);
+    converter = new MarkdownToHtmlConverter(templateEngine);
   });
 
   it('supports going from markdown to html.', async(): Promise<void> => {
@@ -38,9 +30,9 @@ describe('A MarkdownToHtmlConverter', (): void => {
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}', { htmlBody: '<p>Text <code>code</code> more text.</p>\n' },
+    expect(templateEngine.render).toHaveBeenCalledTimes(1);
+    expect(templateEngine.render).toHaveBeenLastCalledWith(
+      { htmlBody: '<p>Text <code>code</code> more text.</p>\n' },
     );
   });
 
@@ -53,9 +45,9 @@ describe('A MarkdownToHtmlConverter', (): void => {
     expect(result.binary).toBe(true);
     expect(result.metadata.contentType).toBe('text/html');
     await expect(readableToString(result.data)).resolves.toBe('<html>');
-    expect(engine.apply).toHaveBeenCalledTimes(1);
-    expect(engine.apply).toHaveBeenLastCalledWith(
-      '{{ template }}', { htmlBody: '<h1 id="title-text">title text</h1>\n<p>more text</p>\n', title: 'title text' },
+    expect(templateEngine.render).toHaveBeenCalledTimes(1);
+    expect(templateEngine.render).toHaveBeenLastCalledWith(
+      { htmlBody: '<h1 id="title-text">title text</h1>\n<p>more text</p>\n', title: 'title text' },
     );
   });
 });

test/unit/util/templates/HandlebarsTemplateEngine.test.ts

@@ -0,0 +1,23 @@
+import { HandlebarsTemplateEngine } from '../../../../src/util/templates/HandlebarsTemplateEngine';
+
+jest.mock('../../../../src/util/templates/TemplateEngine', (): any => ({
+  readTemplate: jest.fn(async({ templateString }): Promise<string> => `${templateString}: {{detail}}`),
+}));
+
+describe('A HandlebarsTemplateEngine', (): void => {
+  const template = { templateString: 'xyz' };
+  const contents = { detail: 'a&b' };
+  let templateEngine: HandlebarsTemplateEngine;
+
+  beforeEach((): void => {
+    templateEngine = new HandlebarsTemplateEngine(template);
+  });
+
+  it('uses the default template when no template was passed.', async(): Promise<void> => {
+    await expect(templateEngine.render(contents)).resolves.toBe('xyz: a&amp;b');
+  });
+
+  it('uses the passed template.', async(): Promise<void> => {
+    await expect(templateEngine.render(contents, { templateString: 'my' })).resolves.toBe('my: a&amp;b');
+  });
+});

test/unit/util/templates/TemplateEngine.test.ts

@@ -0,0 +1,36 @@
+import { resolveAssetPath } from '../../../../src/util/PathUtil';
+import { readTemplate } from '../../../../src/util/templates/TemplateEngine';
+import { mockFs } from '../../../util/Util';
+
+jest.mock('fs');
+
+describe('readTemplate', (): void => {
+  const templateFile = 'template.xyz';
+  const templatePath = 'other';
+
+  beforeEach(async(): Promise<void> => {
+    const { data } = mockFs(resolveAssetPath(''));
+    Object.assign(data, {
+      'template.xyz': '{{template}}',
+      other: {
+        'template.xyz': '{{other}}',
+      },
+    });
+  });
+
+  it('returns the empty string when no template is provided.', async(): Promise<void> => {
+    await expect(readTemplate()).resolves.toBe('');
+  });
+
+  it('accepts string templates.', async(): Promise<void> => {
+    await expect(readTemplate({ templateString: 'abc' })).resolves.toBe('abc');
+  });
+
+  it('accepts a filename.', async(): Promise<void> => {
+    await expect(readTemplate({ templateFile })).resolves.toBe('{{template}}');
+  });
+
+  it('accepts a filename and path.', async(): Promise<void> => {
+    await expect(readTemplate({ templateFile, templatePath })).resolves.toBe('{{other}}');
+  });
+});