xjst

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Extensible JavaScript Transformations

XJST is a DSL for universal data transformations with compiler written on top of the Node.js1 and Ometa/JS2 and output code working in any browser or on server-side.

Data transformations?

Yes, traverse any data in specific flow using matching against conditions set to generate any output (see binary tree prefixer6).
For example, XJST can be used as:

• HTTP request router
• template engine
• AST transformator
• parser

Extensible

XJST makes possible to extend your previous transformation by overriding or specializing some of it's parts (example below is extending this.url === '/login' condition with redirection for logged in users).
XJST is a superset of JavaScript so you can use any popular libraries (that is jquery or underscore) within your transformation and write condition bodies in JavaScript.
Creating your own DSL based on XJST is also possible, because it's syntax parser is powered by ometajs2.

Basic example

Input:

template(this.url === '/')(function() {
  return render('home page')
});

template(this.url === '/login')(function() {
  return render('login form')
});

template(this.url === '/login', this.cookie.is_logined)(function() {
  return redirect('user page')
});

Output (simplified):

switch (this.url) {
  case '/login':
    switch (this.cookie.is_logined) {
      case true:
        return redirect('user page')
      default:
        return render('login form')
    }
  case '/':
    return render('home page')
}

More examples5

Installation

npm install xjst

Public API

var xjst = require('xjst'),

    fn = xjst.compile('template string', 'filename.xjst', options);

fn({ your: 'data' });

Syntax

XJST extends JavaScript syntax with a following keywords: template, local, apply, applyNext.

Template

template(expression1 === value1, ... , expressionN === valueN)(function() {
  // will be run if condition above equals to true
})

Multiple template statements will be grouped to construct optimal conditions graph. Order of the template statements matters, the priority decreases from the bottom to the top.
There're few restrictions for templates:

• Expressions in template predicate should have no side-effects

(that is should not change transformation context).

• It's preferred to use function calls or equality comparisons joined by

logical `&&` operator for expressions, as it can be better optimized at
compilation time.

Local

var obj = { x: 1 };

console.log(local(obj)({ x: 2 })(obj.x)); // 2
console.log(obj.x); // 1

local allows you to make temporary changes to a visible variables scope. Every assignment put inside parens will be reverted immediately after the expression execution.
You can make multiple assignments in the one statement:

local({ x: 2, y: 3 })(/* your code */)

Or use local with a block:

local({ ... })(function() { var a = 1; return a * 2; });

Or as an expression:

var newX = local(this)({ x: 2 })(this.x);

Apply

template(true)(function() {
  return apply({ type: 'first' });
});

template(this.type === 'first')(function() {
  return apply({ type: 'second' });
});

template(this.type === 'second')(function() {
  return 'here am I';
});

XJST is intended to be applied recursively to the same data, while making small temporary changes to it (all changes will be reverted back after operation). apply keyword works exactly like a local (applying changes in the parens and reverting them after the execution), but with small distinction - apply doesn't have a body, so it's just doing some changes to the data and applying template recursively (the context will be preserved).

Apply next

template(this.page === 'home' && this.action === 'login')(function() {
  // match here
});

template(this.page === 'home')(function() {
  applyNext();
});

applyNext() call will reapply all templates, except one where it was called, to the inputs data.

CLI interface

$ bin/xjst --help

Usage:
  xjst [OPTIONS] [ARGS]


Options:
  -h, --help : Help
  -i INPUT, --input=INPUT : Input file (default: stdin)
  -o OUTPUT, --output=OUTPUT : Output file (default: stdout)

$ bin/xjst -i template.xjst

.... some code ...

Optimizations

!Optimized graph7
XJST takes all the template statements and produces a tree with comparisons in nodes and template bodies in leafs. apply are handled and replaced by direct calls to the tree nodes (some of comparisons can be skipped, using known context state).
Input:

template(this.type === 'a')(function() {
  // body 1
});
template(this.type === 'b')(function() {
  // body 2
});

Output (simplified):

switch (this.type) {
  case 'a':
    // body 1
    break;
  case 'b':
    // body 2
    break;
}

Documentation

Here is the documented source3.
Some technical details (in Russian) can be found in doc/tech.ru.md4.

Contributors

• Sergey Berezhnoy
• Andrey Mischenko
• Fedor Indutny