Hey, I'm Christopher Chedeau aka Vjeux! I started this blog to talk about the various projects I am working on and to reveal some of my programming tricks! I hope you will find some of my stuff fun if not useful :)

I'm a Facebook Software Engineer in the Photos Team. Before that, I went to EPITA, a 5-year Computer Science school and majored in its R&D lab LRDE. I also worked for Curse during the nights and week-ends.

I was reading about text layout algorithms when I found a striking parallel with Google Plus image layout algorithm. This blog article explains how to find the best line breaks for the image layout.

Line Breaking Algorithm

"Breaking Paragraphs into Lines" article written in 1980 by Donald Knuth and Michael Plass explain how to find the best line breaks in order to make justified text look pretty. The goal is to find where to put line breaks on this paragraph:

In olden times when wishing still helped one, there lived a king
whose daughters were all beautiful; and the youngest was so
beautiful that the sun itself, which has seen so much, was aston-
ished whenever it shone in her face. Close by the king’s castle lay
a great dark forest, and under an old lime-tree in the forest was
a well, and when the day was very warm, the king’s child went
out into the forest and sat down by the side of the cool fountain;
and when she was bored she took a golden ball, and threw it up
on high and caught it; and this ball was her favorite plaything.

The first thing to notice is that you cannot break everywhere. In bold, are all the words that can be end of lines while maintaining the constraint of reasonably stretching white spaces in the line.

The first line only has two allowed words while the line before last line has seven. However, the seven are not allowed at the same time, each of them depend on the previous breaks configuration. You can draw a graph that shows all the possible breaks configurations.

graph-knuth

Each transition in this graph represents a line of text (between two breaks). If you can give a fitness value to each line, then you can apply a shortest path algorithm on the graph to find the best breaks configuration. The rest of the article studies a flexible fitness framework that covers all major text layout constraints, but that will not be of any help for our problem.

Parallel with Image Layout

Let's say we want to layout the following five images:

There are many ways to split the images into lines. Here is an exhaustive list of the lines we can have for our five images:

 1
 12      2
 123     23     3
 1234    234    34    4
 12345   2345   345   45

And, we need to link the lines such that they form 12345 at the end. For example 1-2-3-4-5, 12345, 123-4-5 ... I've drawn the full graph so you get an idea of what it looks like:

Like in the line breaking algorithm for text, not all the break positions are wanted. Having only one image would make it way too big, having more than three would make them too small. So we're going to remove them from the graph.

By disallowing some lines, some nodes are now unreachable, trimming down the graph as well. In our example, we are left with only two valid layouts: 123-45 and 12-345.

possible

Cost Metrics

I found it very hard to visually evaluate two layouts. Most of the time, you can easily spot extreme things you don't want such as a giant image or too many small ones. Then, the goal of the cost metric is to minimize the number of those occurrences.

Max height

The first approach tries to respect the constraints of the first-fit heuristic: as soon as the height goes below a threshold, we display the images. We therefore have a MAX_HEIGHT constant that no line can go above.

Because the graph is acyclic, we can use Dijkstra shortest path algorithm to find the longest path (Proof), we just have to return negative costs.

function cost(images, i, j) {
  var height = compute_height(images, i, j);
  if (height > MAX_HEIGHT) { return null; }
 
  // Maximize the number of breaks
  return -1;
  // Maximize the total height
  return -height;
  // Maximize the average image height
  return -(height * (j - i));
}

Target Height

The other approach is to have a target height per line and try to have all the lines as close as possible to the target. The function should return 0 when the line is exactly the size of the target line.

We can write several versions that are going to weight errors differently. You can have a greater variance with a more precise mean, or the opposite. You can prefer being a bit bigger than smaller ...

function cost(images, i, j) {
  var height = compute_height(images, i, j);
 
  // Minimize the total height difference from the target
  return Math.abs(TARGET_HEIGHT - height);
 
  // Minimize the difference to the target and
  // penalize more harshly big differences
  return Math.pow(TARGET_HEIGHT - height, 2);
 
  // Minimize the difference area from the target
  var diff = 0;
  for (; i < = j; ++i) {
    var ratio = images[i].width / images[i].height;
    var target_area = TARGET_HEIGHT * TARGET_HEIGHT * ratio;
    var area = height * height * ratio;
 
    diff += Math.abs(target_area - area);
  }
  return diff;
}

Conclusion

Check-out the first-fit demo versus best break demo (using cost = (target_height - height)^2)

Thanks to an unrelated reading, I've been able to draw a parallel between text display and image layout. Who would have thought that graph theory would be involved there too :)

We can now find the best layout at a cheap cost (nearly-linear with the number of images if you only allow a small number of possible breaks per line). The cost metric allow you to easily disallow patterns you find bad looking.

In this talk I give an overview of the big categories of image layout algorithms with examples for each of them and present criterias to look at when evaluating them.

I'm implementing a layout algorithm in C and want to let the user specify a callback to compute the height based on the width.

Using function pointers, we can provide the callback:

typedef struct {
  float (*measure)(float width);
} layout_node_t;
 
void layout(layout_node_t *node) {
  float width = 10;
  float height = node->measure(width);
}

It works well if we have a function measure that only uses global variables:

float measure(float width) {
  return width * 2;
}
 
int main() {
  layout_node_t node;
  node.measure = measure;
  layout(&node);
}

However, I would like my measure function to take some dynamic input. For example in order to measure an image, you need to take its aspect ratio into account. In JavaScript, I would write the following:

var aspect_ratio = 1.5;
node.measure = function mesure_image(width) {
  return width * aspect_ratio;
}

Unfortunately, C doesn't support closures. I haven't been able to find a way to get a function pointer alone somehow hold some state. The best trade-off I found was to have a void * metadata in the struct and pass it along with the function call. (Thanks Scott and Felix for the help!)

typedef struct {
  float (*measure)(void *context, float width);
  void *measure_context;
} layout_node_t;
 
void layout(layout_node_t *node) {
  float width = 10;
  float height = node->measure(node->measure_context, width);
}

The void * value lets us put anything we want in it. So, with some casting we are able to simulate a closure and write our measure_image function :)

float measure_image(void *context, float width) {
  float aspect_ratio = *(float *)context;
  return width / aspect_ratio;
}
 
int main() {
  layout_node_t node;
  node.measure = measure_image;
  float aspect_ratio = 1.5;
  node.measure_context = (void *)&aspect_ratio;
  layout(&node);
}

To compute the height of the image we use a float, but in order to handle text, we can pass a const char * instead. It works as well!

float measure_text(void *content, float width) {
  const char *text = (const char *)content;
  float line_height = 11;
  return ceil(strlen(text) / width) * line_height;
}
 
int main() {
  layout_node_t node;
  node.measure = measure_text;
  node.measure_context = (void *)"this is some super long text";
  layout(&node);
}

This solves the use case pretty well, which is remarkable since C doesn't support closure. The downside is that we are losing all the type information, have to do a lot of type casting and renaming.

PHP and JavaScript are both renowned to be languages with a lot of quirks. However two major initiatives on both sides, Hack for PHP and ES6 for JavaScript made the languages much better and modern. In this article I'm going to show all the ES6 features that are also in Hack.

Arrow Function

Both languages adopted the same shorter way to write functions. On JavaScript side, the main advantage is the automatic binding of this and for PHP it removes the need to declare all the variables you want to use from outside. ES6, Hack.

// JavaScript
var odds = evens.map(v => v + 1);
var nums = evens.map((v, i) => v + i);
nums.filter(v => {
  if (v % 5 === 0) {
    console.log(v);
    return true;
  }
  return false;
});
// Hack
$odds = array_map($v ==> $v + 1, $evens);
$nums = array_map(($v, $i) ==> $v + $i, $evens);
array_filter($nums, $v ==> {
  if ($v % 5 === 0) {
    echo $v;
    return true;
  }
  return false;
});

Class

JavaScript finally gets a class abstraction with ES6. It is however the bare minimal one to be useful, you cannot define constants, protected/private methods, traits ... PHP on this side is much better, without any Hack addition. ES6, PHP5.

// JavaScript
class SkinnedMesh extends THREE.Mesh {
  constructor(geometry, materials) {
    super(geometry, materials);
    this.idMatrix = SkinnedMesh.defaultMatrix();
    this.bones = [];
  }
  update(camera) {
    super.update();
  }
  static defaultMatrix() {
    return new THREE.Matrix4();
  }
}
// Hack
class SkinnedMesh extends THREE\Mesh {
  public function constructor($geometry, $materials) {
    parent::__construct($geometry, $materials);
    $this->idMatrix = SkinnedMesh::defaultMatrix();
    $this->bones = array();
  }
  public function update($camera) {
    parent::update();
  }
  static private function defaultMatrix() {
    return new THREE\Matrix4();
  }
}

Enhanced Object Literal

One long standing issue with object literals in JavaScript is the inability to use an expression as a key. This is fixed with the bracket notation in ES6. PHP 5.4 introduced a short notation for arrays as well. ES6, PHP.

// JavaScript
var obj = { [Math.random()]: true };
// Hack
$obj = [rand() => true];

Template Strings

Multiline strings and variable interpolations are something that have always been possible in PHP, yet they only start to work in ES6! ES6, PHP.

// JavaScript
var multiline = `In JavaScript this is
 not legal.`
var name = 'Bob',
    time = 'today';
`Hello ${name}, how are you ${time}?`
// Hack
$multiline = 'In PHP this is
 legal.';
$name = 'Bob';
$time = 'today';
"Hello $name, how are you $time?";

Default Arguments

It was possible to write default arguments in JavaScript but ES6 adds proper support for it right in the function declaration. Guess what, PHP had support for it all along. ES6, PHP.

// JavaScript
function f(x, y=12) {
  return x + y;
}
f(3) === 15;
f(2, 10) === 12;
// Hack
function f($x, $y=12) {
  return $x + $y;
}
f(3) === 15;
f(2, 10) === 12;

Iterator + for of

JavaScript has two ways to iterate on collections, either

for (var i = 0; i < array.length; ++i) { var element = array[i]; /* ... */ }
for (var key in object) { var element = object[key]; /* ... */ }

ES6 is now introducing a unified way to do iteration, that PHP always had, as well as a way to write custom collections via the iterator pattern, introduced in PHP5. ES6, PHP, PHP5.

// JavaScript
var fibonacci = {
  [Symbol.iterator]: function() {
    var previous = 0;
    var current = 1;
    return {
      next: function() {
        var new_previous = current; 
        current += previous; 
        previous = new_previous; 
 
 
        return {
          value: current,
 
 
          done: false
        }
      }
    }
  }
}
 
 
 
 
 
for (var n of fibonacci) {
  if (n > 1000) break;
  console.log(n);
}
// Hack
class Fibonacci implements Iterator<int> { 
  private $key = 0;    
  private $previous = 1;
  private $current = 0;
 
  public function next() { 
      $new_previous = $this->current; 
      $this->current += $this->previous; 
      $this->previous = $new_previous; 
      $this->key++; 
  } 
  public function current() { 
      return $this->current; 
  } 
  public function valid() { 
      return true; 
  } 
  public function key() { 
      return $this->key; 
  } 
  public function rewind() { 
      $this->previous = 1; 
      $this->current = 0; 
      $this->key = 0; 
  } 
}
foreach (new Fibonacci() as $n) { 
  if ($n > 1000) break; 
  echo $n; 
}

Generators

Python pioneered generators as another tool to manage control flow. It has originally been designed and promoted as an easier way to write iterators, but really shined as a better way to write asynchronous operations than callbacks. ES6, PHP5.

// JavaScript
var fibonacci = {
  [Symbol.iterator]: function*() {
    var previous = 1;
    var current = 0;
    for (;;) {
      var new_previous = current; 
      current += previous; 
      previous = new_previous; 
      yield current;
    }
  }
}
for (var n of fibonacci) {
  if (n > 1000) break;
  console.log(n);
}
// Hack
 
function fibonacci() {
  $previous = 1;
  $current = 0;
  for (;;) {
    $new_previous = $current; 
    $current += $previous; 
    $previous = $new_previous; 
    yield $current;
  }
}
 
foreach (fibonacci() as $n) { 
  if ($n > 1000) break; 
  echo $n; 
}

ES7 Async Await

C# introduced the concept of async/await combination to deal with asynchronous programming. The underlying implementation is very similar to generators but has proper syntax support. It is an addition of Hack on-top of PHP. ES7, Hack.

// JavaScript
async function chainAnimationsAsync(element, animations) {
  var result = null;
  try {
    for (var animation in animations) {
      result = await animation(element);
    }
  } catch (e) { /* ignore and keep going */ }
  return result;
}
// Hack
async function chainAnimationsAsync($element, $animations) {
  $result = null;
  try {
    foreach ($animations as $animation) {
      $result = await animation($element);
    }
  } catch (Exception $e) { /* ignore and keep going */ }
  return $result;
}

Map + Set

Both JavaScript and PHP are notorious for attempting to fit all the collection use cases into a single general purpose type. Both ES6 and Hack bring to the table proper support for Map and Set. ES6, Hack

// JavaScript
var s = new Set();
s.add('hello').add('goodbye').add('hello');
s.size === 2;
s.has('hello') === true;
 
var m = new Map();
m.set('hello', 42);
m.get('hello') === 42;
// Hack
$s = new Set();
$s->add('hello')->add('goodbye')->add('hello');
$s->count() === 2;
$s->contains('hello') === true;
 
$m = new Map();
$m->set('hello', 42);
$m->get('hello') === 42;

TypeScript

Last but not least, both languages are getting gradual typing. TypeScript, Hack.

// JavaScript
class Greeter<T> {
  greeting: T;
  constructor(message: T) {
    this.greeting = message;
  }
  greet() {
    return this.greeting;
  }
}
 
var greeter = new Greeter<string>("Hello, world");
console.log(greeter->greet());
// Hack
class Greeter<T> {
 
  public function __construct(private T $greeting) {}
 
 
  public function greet() {
    return $this->greeting;
  }
}
 
$greeter = new Greeter("Hello, world");
echo $greeter->greet();

Conclusion

With ES6 and Hack efforts, JavaScript and PHP are becoming languages with modern features. If you tried them 5 years ago, you should take another look, they are not as crappy as they once were :)

E4X (ECMAScript for XML) is a Javascript syntax extension and a runtime to manipulate XML. It was promoted by Mozilla but failed to become mainstream and is now deprecated. JSX was inspired by E4X. In this article, I'm going to go over all the features of E4X and explain the design decisions behind JSX.

Historical Context

E4X has been created in 2002 by John Schneider. This was the golden age of XML where it was being used for everything: data, configuration files, code, interfaces (DOM) ... E4X was first implemented inside of Rhino, a Javascript implementation from Mozilla written in Java.

At the time, a very common operation was to transform XML documents into other XML documents, especially in the Java world. The two major ways to do that were either XSLT or the DOM API. Both those technologies suffer from very bad reputation as they are very tedious to work with.

Since then, the Javascript landscape evolved and the assumptions E4X was developed under do not hold true anymore. JSON has now largely replaced XML to represent data and JSON can be manipulated natively within Javascript. Libraries like jQuery made DOM searching, filtering and basic manipulation a lot easier thank to CSS selectors.

Creating a DOM structure is the only major problem that is still not properly solved. Current solutions involve creating a different "templating" language language (Mustache, Jade), creating a poor man's DSL (MagicDOM) or to use HTML modified with special attributes (Angular). This is the problem JSX is trying to solve.

The Good Parts

XML syntax is particularly good at expressing interfaces. Many people (including myself) have tried to create pure Javascript libraries that have a syntax similar to XML but none of them look really good.

With E4X, you can write XML within Javascript like this:

var header =
  <div>
    <h1><a href="/">Vjeux</a></h1>
    <h2>French Web Developer</h2>
  </div>;

The real power of E4X comes from the interpolation mechanism. You can write Javascript expressions within {}. This lets you write dynamic HTML.

var links = [
  {name: 'Talks & Written Reports', url: '/reports'},
  {name: 'Contact', url: '/contact/'}
];
var body =
  <body>
    {header}
    <div class="left_nav">
      <input type="text" name="search" />
      <h2>About Me</h2>
      <ul>{section.elements.map(function(element) {
        var isActive = element.url == window.location;
        return <li class={isActive ? 'active' : ''}>
          {isActive ?
            element.name :
            <a href={element.url}>{element.name}</a>
          }
        </li>;
      })}</ul>
    </div>
  </body>

This example shows that this simple construct coupled with Javascript can solve what templates have been designed to.

  • partials: header has been defined somewhere else and the resulting DOM node is just being included. In this case header was just a variable but I could as easily have used a function call to create it.
  • lists: The result of the interpolation can be a Javascript array. In order to create it, you can use the default map, filter, or any Javascript library that manipulate arrays (like Underscore for example).
  • conditions: Again, I don't need a special syntax here, Javascript already has the ternary operator. For more complex conditions you can call a function that will contain if/then/else statements.
  • nesting: Within an interpolated block, you can write XML in which you can use another interpolated block, and so on ... With templates you can only do that one level, if you have a problem that requires you to have more, then you have to go back to string concatenation.

The XML notation and the extremely powerful interpolation mechanism have been re-used as is in JSX. Now let's see the other parts of E4X that didn't work so well and what JSX does to address them.

XML Objects

While the XML object looks and behaves in a similar way to a regular JavaScript object, the two are not the same thing. E4X introduces new syntax that only works with E4X XML objects. The syntax is designed to be familiar to JavaScript programmers, but E4X does not provide a direct mapping from XML to native JavaScript objects; just the illusion of one.

Downsides

The major use case of XML within Javascript is to write HTML tags. Unfortunately, what E4X generates is not a DOM node. In order to use it to generate DOM nodes, you've got to do a conversion phase not provided by default.

The second use case of XML is to represent data. In Javascript world, this use case is already being fulfilled by JSON. E4X only supports strings as a data type where Javascript objects can contain numbers, booleans, functions ...

All the code is not going to be converted to E4X right away. There's going to be a transition phase where E4X and non E4X code will have to co-exist. The fact that the objects E4X generates are not accessible from non E4X code means that none of the libraries ever written can work with E4X structures.

JSX

JSX, contrary to E4X, does not contain a runtime for XML, it is only a syntactic sugar. It translates XML syntax into function calls:

<Node attr1="first" attr2="second">
  <Child />
</Node>

is converted into the following standard Javascript:

Node({attr1: 'first', attr2: 'second'},
  Child({})
)

The syntax is no longer tied to a specific implementation of the node representation. JSX is pure syntactic sugar. There are cases where you cannot use JSX, for example if you are writing your application in CoffeeScript, but it doesn't prevent you from using the underlying implementation of the XML nodes.

Because it is only a syntactic sugar, there is no need to provide a way to express all the edge cases. Computed attributes, for example, are tricky because they introduce a lot of implementation specific questions when the same attribute is specified more than once. Instead of dealing with them, it has been decided not to support it in JSX and let the users do it in regular Javascript.

var attributes = {a: 1, b: 2, c: 3};
<Node *{attributes} /> // Not valid JSX
Node(attributes) // Regular Javascript equivalent

Namespaces

Each node is identified by a name. In order to prevent conflict in the meaning of the nodes, each node also contains a namespace, encoded as an URI.

default xml namespace = "http://www.w3.org/1999/xhtml";
<div />.name();
// { localName: 'div', uri: 'http://www.w3.org/1999/xhtml' }
 
<svg xmlns="http://www.w3.org/2000/svg" />.name();
// { localName: 'svg', uri: 'http://www.w3.org/1999/svg' }
 
<svg xmlns="http://www.w3.org/2000/svg"><circle /></svg>..circle.name();
// { localName: 'circle', uri: 'http://www.w3.org/1999/svg' }

Every single element in E4X contains a namespace. Most of them use the default namespace that you can override using a special JS syntax default xml namespace =. You can also set a namespace on a node using the xmlns attribute and it's going to be propagated to all the sub-tree.

The namespaces solve the problem of name conflict. E4X implements it making an identifier (URI namespace, String name) unique. This is indeed working but have downsides.

Downsides

This is very weird to have to specify a URL because this URL is just a unique identifier, it is not going to get fetched or influence the way the program run.

Maintaining a unique URL and making sure it is going to stay valid is not a trivial task. There are also many questions that are raised such as what happens if your project is private. How do you deal with versioning.

URLs are usually very long and they are a distraction when you look at the code. Everyone just copy and pasted the doctype value pre-HTML5.

In the end, the XML namespaces didn't get really popular in the front-end community.

JSX

JSX uses Javascript to deal with the namespace problem. In E4X, each node type is not represented by a unique string across your program. In JSX a node type is represented by a Javascript variable. You can use all the Javascript features (eg variable hoisting, capturing via closure, function arguments ...) in order to find the proper node type you want to use.

var div = ReactDOM.div;
<div />
 
var div = JSXDOM.div;
<div />
 
var div = SomeJSFunctionThatReturnsADiv();
<div />

JSX is also totally optional. The actual representation of a node type is behind the scenes just a Javascript function. If you are not using JSX, you can create the node by calling it the following way:

// Using JSX:
<div attr="str"><br /></div>
 
// Without JSX:
ReactDOM.div({attr: 'str}, ReactDOM.br());

Because native DOM elements are used so often, JSX also have a way to declare a default namespace. You can add a special comment at the top of the file @jsx ReactDOM and JSX will assume that all the native elements are attributes of the object you mentioned. This is not the cleanest API but it works.

/** @jsx React.DOM */
<div /> // ReactDOM.div

Query Language

E4X is not only about creating XML fragments, it also extends Javascript syntax in order to find elements in an XML document. The integration is well done and expressive, it is worth looking at it.

var people = (
  <people>
    <person><name>Bob</name><age>32</age></person>
    <person><name>Joe</name><age>46</age></person>
  </people>
);
 
people.person.(name == "Joe").age // 46
 
// Filter expressions can even use JavaScript functions:
function over40(i) {
  return i > 40;
}
people.person.(over40(parseInt(age))).name // Joe
 
elem.@attr // attribute

Downsides

In my opinion, this is one of the main reason that prevented E4X from being more widely adopted.
In order to get a new language extension to be adopted by all the browsers, the bar is extremely high and this feature doesn't seem to cut it.

  • It isn't really an issue in the community. jQuery became pretty successful thanks to its way to query the DOM. The overall feeling is that the current solution is satisfying and here hasn't been a strong push for help at the syntax level.
  • The query language extends the surface area. Creating XML and querying XML are two orthogonal concepts that do not need to be addressed together. Bundling them together highly reduces the chance of them being accepted.
  • A query language is highly controversial. There isn't a consensus on what's the best way to query a XML document and will probably never be. It is even harder to sell as it isn't even using an already existing standard such as XPath or CSS selectors but comes up with a completely new one.

JSX

It is not part of JSX.

Interpolation

Since E4X is not manipulating a plain string, it is able to differentiate between the parts that are nodes and the parts that are attributes and children. This has the extremely strong property from a security point of view that it can prevent code injections by automatically escaping attributes.

var userInput = '"<script>alert("Pwn3d!");</script>';
<div class={userInput} />.toXMLString()
// <div class="&quot;&lt;script&gt;alert(&quot;Pwn3d!&quot;)&lt;/script&gt;"></div>

Downsides

XML can only manipulate strings and nodes. This means that all the attributes are first converted to strings.

JSX

Since we are now living in a Javascript world, we don't need to restrict ourself to strings. For example, it is possible to use a Javascript object to represent the style property:

<div style={{borderRadius: 10, borderColor: 'red'}} />
// <div style="-webkit-border-radius: 10px; border-color: red;" />

Callbacks don't need to be passed as string that is going to be evaluated, it is possible to use normal Javascript functions.

<div onClick={function() { console.log('Clicked!'); }} />

More is Less

When studying E4X, I stumbled across many small things that I either found weird or not really needed. This section is a compilation of them.

Attributes values are not Strings

The attributes values are not Javascript strings, they are special objects with the toString method implemented.

<div class="something">.@class === 'something'
// false
<div class="something">.@class.toString() === 'something'
// true

Prototype syntax

They introduced a special syntax function:: to add new methods on XML objects:

XML.prototype.function::fooCount = function fooCount() {
  return this..foo.length();
};
 
<foobar><foo/><foo/><foo/></foobar>.fooCount() // returns 3

Processing Instructions

E4X supports an obscure variant of XML tags: processing instructions of the form . There is a special flag to enable them in the output, but they are always properly parsed.

<foo><?process x="true"?></foo>.toXMLString()
// <foo />;
 
XML.ignoreProcessingInstructions = false;
<foo><?process x="true"?></foo>.toXMLString()
// <foo><?process x="true"?></foo>;

Operator overload +=

The += operator can be used to append new elements to an XMLList.

var xmlList = <></>;
xmlList += <node />;

Operator overload for attributes

You can get and set attributes using @attr.

var element = <foo bar="1" />;
console.log(element.@bar); // "1"
element.@bar = 2;

Conclusion

E4X had a lot of potential but unfortunately did not take off. JSX attempts to keep only the good parts of E4X. Since it is much smaller in features and only a Javascript transform, it is more likely to be adopted.