Jessica: The JS ES6 Template Engine

"Jessica," drawn from JESS as an anagram of "JS ES6," is a simple, super fast, and extendable Template Engine for Node and Express applications using pure ES6 JavaScript syntax, as a JavaScript module. It works by scanning files in a working directory, then reading the contents of the files and converting them from plain strings to ES6 template literals. Once conversion is complete, it is then compiled to plain text by the V8 engine. Being less than 1kb, Jessica offloads a lot of the processing directly to the V8 interpreter, which compiles the code and runs as fast as the rest of the Express App. In fact, Jessica will add very little, if any, overhead to your project at all! It should also allow you to implement any functionality you'd like within the bounds of JavaScript.

Template literals, enclosed by back ticks, feature String Interpolation, Embedded Expressions, Multiline strings and String Tagging for safe HTML escaping, localisation, and more.

Minimum requirements Node.js v13.0.0, for native ES6 modules support.

Installation

$ npm i @nonsensecodes/jessica --save

Features

No Dependencies
Fully Configurable
Compiled and interpreted by V8 (Super Fast)
Learning new syntax is not required
Partials Support
Conditional Support
Iterators Support
Native Javascript Support

Usage

Prerequisites

List of used html files in the views folder:

index.html

<!DOCTYPE html>
<html>
<body>
    <h1>${title}</h1>
</body>
</html>

template.html

<!DOCTYPE html>
<html>
<body>
    <h1>${title}</h1>
    <main>${partial}</main>
</body>
</html>

partial.html

<p>The fastest javascript template string engine!</p>

partial-conditional.html

Jessica is ${maintainedBy ? `a template engine maintained by ${maintainedBy}` : 'not maintained anymore'}.

partial-iteration.html

<dl>
  ${features.map(f => `
    <dt>${f.dt}</dt>
    <dd>${f.dd}</dd>
  `).join('')}
</dl>

Setup with Express

The basics required to integrate Jessica in your app are pretty simple and easy to implement:

import express from 'express';
import jessica from 'jessica';
  
const app = express();
  
app.engine('html', jessica);
app.set('views', 'views');
app.set('view engine', 'html');

app.get('/', function(req, res) {
  res.render('index', {locals: {title: 'Welcome!'}});
});

app.listen(3000);

Before Express can render template files, the following application settings must be set:

views, the directory where the template files are located. Eg: app.set('views', './views')
view engine, the template engine to use. Eg: app.set('view engine', 'html')

HTML template file named index.html in the views directory is needed (the content of the file can be found in the prerequisites section). Route to render the html file is expected. If the view engine property is not set, we must specify the extension of the view file. Otherwise, it can be safely omitted.

app.render('index', {locals: {title: 'jessica'}});

Express-compliant template engines such as Jessica export a function named __express(filePath, options, callback), which is called by the res.render() function to render the template code. When a request is made to the home page, the index.html file will be rendered as HTML.

Setup without Express

To get up and running without having to worry about managing extra libraries one only needs the following:

import jessica from 'jessica';
jessica(
    __dirname + '/views/index.html',
    { locals: { title:  'jessica' } },
    (err, content) => err || content
);

The content below will be rendered on the client side as a response from both setups:

<!DOCTYPE html>
<html>
<body>
    <h1>JESSica</h1>
</body>
</html>

Rendering a template

Within your app route callback, call res.render, passing any partials and local variables required by your template. For example:

app.get('/', function(req, res) {
  res.render('template', {
      locals: {
        title:  'jessica'
      },
      partials: {
        partial: __dirname + '/views/partial'
      }
  });
});

Partial with a file name partial.html (see the content of the file in the prerequisites section above) will be injected into template.html:

<!DOCTYPE html>
<html>
<body>
    <h1>Jessica</h1>
    <main><p>The fastest javascript template string engine!</p></main>
</body>
</html>

All templates files paths are defined as absolute to the root directory of the project.

Compiling a string

Jessica's rendering functionality has separate scanning, parsing, string generation and response sending phases. Compilation is pretty much the same but without the response sending phase. This feature can be useful for pre-processing templates on the server. Compiling has the following syntax:

const titleTpl = '${engineName} - The fastest javascript template string engine!';
const cb = (err, content) => err || content;

// sync - second parameter is a string representation of an array of variable names.
// The returned function is called with a string representation of an array of variable values.
const compiled = jessica(titleTpl, 'engineName')('jessica');
// async - second parameter is an object and third parameter is a callback function
jessica(titleTpl,{ template: true, locals:{ engineName: 'jessica' } }, cb);

Both methods will result in the following output:

jessica - The fastest javascript template string engine!

The template engine allows both synchronous and asynchronous method invocations. If string is rendered as in the examples provided above a 'template' option needs to be set to true. The preceding synchronous invocation returns an output immediately in response to the function execution. Alternatively, you can specify partials and omit template parameter to force file lookup and content reading and invoke the function asynchronously.

Compiling a template

The two functions app.render and jessica are almost identical, but they require slightly different parameters to be passed. While app.render uses an absolute path, or a path relative to the views setting, jessica expects a path relative to root folder. They both return the rendered content of a view via the callback function. The callback function which is provided as a third parameter is called once the asynchronous activity is completed. The output in the two examples provided below is the same:

app.render('template', {
  locals: {
    title:  'jessica'
  },
  partials: {
    template: __dirname + '/views/partial'
  }
}, (err, content) => err || content);

jessica(__dirname + '/views/template.html', {
  locals: {
    title:  'jessica'
  },
  partials: {
    template: __dirname + '/views/partial.html'
  }
}, (err, content) => err || content);

On average jessica yields slightly better performance than app.render. Async function invocation of jessica also returns a promise, which enables us to chain method calls:

const compile = jessica(__dirname + '/views/template.html', {
  locals: {
    title:  'jessica'
  },
  partials: {
    template: __dirname + '/views/partial.html'
  }
}, (err, content) => err || content);
compile.then(output => res.send(output))

Compiling a nested template

Template nesting is currently not supported by the engine. A simple workaround to this issue would be to perform multiple template compilations:

const renderPage = (err, content) => res.render('template', {
  locals: {
    partial: content
  }
});
jessica(__dirname + '/views/partial-conditional.html', {
  locals: {
    maintainedBy:  'Good Samaritans'
  }
}, renderPage);

Precompiling

Jessica allows us bypassing Express view rendering for speed and modularity. Compiling a template is much slower than rendering it, so when it comes to speed, we should precompile our templates as part of the optimisation process. The result of precompilation can be stored to an object:

const text = '${engineName} - The fastest javascript template string engine in the whole ${place}!';
const precompiled = jessica(text, 'engineName, place');

and then invoked whenever needed:

console.log(precompiled('jessica', 'multiverse'));

To make use of this precompilation, templates should be compiled with names that the compiler would expect and the result function called with an argument list that consists of values relative to the names. If no property name is defined a default one is created with a value of '$':

const text = '${$.engineName} - The fastest javascript template string engine in the whole ${$.place}!';
console.log(jessica(text)({ engineName: 'jessica', place: 'multiverse' });

This allows you to create an application that is more flexible, independent from a framework, easier to understand and better performing.

Conditional statements

Jessica dynamically evaluates code in JavaScript. If the argument is an expression, jessica evaluates the expression. If the argument is one or more JavaScript statements, the engine executes the statements. A simplified example of using conditional statement is presented below.

A route path on the server side:

res.render('partial-conditional', {
    locals: {
      maintainedBy:  'Good Samaritans'
    }
  });

Will result in the following:

jessica is a template engine maintained by Good Samaritans.

Iterators

Iterating over arrays and objects is quite straight forward and intuitive (knowledge of basic javascript here is essential). An object literal is passed to an html template:

res.render('partial-iteration', {
    locals: {
      features:  [
      { 
        dt: 'Multi-line strings', 
        dd: 'Any new line characters inserted in the source are part of the template string.' 
      },
      { 
        dt: 'Expression interpolation', 
        dd: 'Template strings can contain placeholders. These are indicated by dollar sign and curly braces.' 
      },
    ]
    }
  });

The following is received by the client side:

<dl>
    <dt>Multi-line strings</dt>
    <dd>Any new line characters inserted in the source are part of the template string.</dd>
    <dt>Expression interpolation</dt>
    <dd>Template strings can contain placeholders. These are indicated by dollar sign and curly braces.</dd>
</dl>

Error Handling

Jessica catches and processes errors that occur both synchronously and asynchronously. Jessica comes with a default error handler so you don’t need to write your own to get started. Errors that occur in synchronous code require no extra work. If synchronous code throws an error, jessica will catch and process it. For example:

const text = '${engineName} - The fastest javascript template string engine in the whole ${place}!';
const err = jessica(text, 'engineName')('jessica', 'multiverse');
expect(err instanceof Error).to.equal(true);

For errors returned from asynchronous functions, you must pass them to the callback function, where jessica will catch and process them. A viable alternative will be to use the native promise catch or reject methods. The following example handles both cases simultaneously:

jessica(
    __dirname + "/index.html",
    { locals: { engineName: "jessica", footer: "MIT License" } },
    (err) => expect(err instanceof Error).to.equal(true);
).then((err) => expect(err instanceof Error).to.equal(true));

License

MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

@nonsensecodes/jessica

Jessica: The JS ES6 Template Engine

Installation

Features

Usage

Prerequisites

Setup with Express

Setup without Express

Rendering a template

Compiling a string

Compiling a template

Compiling a nested template

Precompiling

Conditional statements

Iterators

Error Handling

License

/@nonsensecodes/jessica/

Package Sidebar

Install

Repository

Homepage

Weekly Downloads

Version

License

Unpacked Size

Total Files

Last publish

Collaborators

@nonsensecodes/jessica

Jessica: The JS ES6 Template Engine

Installation

Features

Usage

Prerequisites

Setup with Express

Setup without Express

Rendering a template

Compiling a string

Compiling a template

Compiling a nested template

Precompiling

Conditional statements

Iterators

Error Handling

License

/@nonsensecodes/jessica/

Package Sidebar

Install

Repository

Homepage

DownloadsWeekly Downloads

Version

License

Unpacked Size

Total Files

Last publish

Collaborators

Weekly Downloads