duktape/website/guide/es6features.html

<h1 id="es6features">Ecmascript E6 features</h1>

<p>This section describes the small set of features Duktape borrows from
<a href="http://www.ecma-international.org/ecma-262/6.0/index.html">Ecmascript E6</a>.
These features are not fully compliant; the intent is to minimize custom features
and to align with the ES6 specification when possible.</p>

<h2 id="es6-regexp-syntax">ES6 Annex B RegExp syntax</h2>

<p>Duktape supports some of the additional RegExp syntax defined in
<a href="http://www.ecma-international.org/ecma-262/6.0/#sec-regular-expressions-patterns">ES6 Annex B.1.4</a>.
These features can be enabled/disabled using the <code>DUK_USE_ES6_REGEXP_SYNTAX</code>
config option (enabled by default).</p>

<h2 id="es6-const">Const variables</h2>

<p>Duktape has minimal support for
<a href="http://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations">const</a>
declarations to allow existing code using <code>const</code> to run.  However,
<code>const</code> is mostly just an alias for <code>var</code> and currently has
the following non-standard semantics:</p>
<ul>
<li>Unlike <code>var</code> declarations, <code>const</code> declarations are
    required to have an initializer.</li>
<li>Const variables are writable.  In ES6 <code>const</code> variables are
    not writable.</li>
<li>Const variables are function scoped and "hoisted" to the top of the
    function like <code>var</code> declarations.  In ES6 <code>const</code>
    has block scoping like <code>let</code>.</li>
</ul>

<h2 id="es6-computed-property-names">Computed property names</h2>

<p>Computed property names are supported and allow syntax like
<code>{ [1 + 2]: "three" }</code>.</p>

<h2 id="es6-proto">Object.setPrototypeOf and Object.prototype.__proto__</h2>

<p><a href="http://www.ecma-international.org/ecma-262/6.0/index.html#sec-object.setprototypeof">Object.setPrototypeOf</a>
allows user to set the internal prototype of an object which is not supported in
Ecmascript E5.  Ecmascript E6 also provides
<a href="http://www.ecma-international.org/ecma-262/6.0/index.html#sec-object.prototype.__proto__">Object.prototype.__proto__</a>,
an accessor property (setter/getter) which provides the same functionality
but is compatible with existing code base which has relied on a non-standard
<code>__proto__</code> property for a while.  Duktape does not support the
<a href="http://www.ecma-international.org/ecma-262/6.0/index.html#sec-__proto__-property-names-in-object-initializers">__proto__ property name in an object initializer</a>.</p>

<p>These custom features can be disabled by disabling the config options
<code>DUK_USE_ES6_OBJECT_SETPROTOTYPEOF</code> and
<code>DUK_USE_ES6_OBJECT_PROTO_PROPERTY</code>.</p>

<h2 id="es6-proxy">Proxy object (subset)</h2>

<p>The Ecmascript E6 <code>Proxy</code> object allows property virtualization
and fine-grained access control for accessing an underlying plain object.
Duktape implements a strict subset of the <code>Proxy</code> object from
ES6.  The following traps are implemented:</p>

<table>
<thead>
<tr><th>Trap</th><th>Implemented</th><th>Notes</th></tr>
</thead>
<tbody>
<tr><td class="propname">getPrototypeOf</td><td>no</td><td></td></tr>
<tr><td class="propname">setPrototypeOf</td><td>no</td><td></td></tr>
<tr><td class="propname">isExtensible</td><td>no</td><td></td></tr>
<tr><td class="propname">preventExtension</td><td>no</td><td></td></tr>
<tr><td class="propname">getOwnPropertyDescriptor</td><td>no</td><td></td></tr>
<tr><td class="propname">defineProperty</td><td>no</td><td></td></tr>
<tr><td class="propname">has</td><td>yes</td><td><code>Object.hasOwnProperty()</code> does not invoke the trap at the moment, <code>key in obj</code> does</td></tr>
<tr><td class="propname">get</td><td>yes</td><td></td></tr>
<tr><td class="propname">set</td><td>yes</td><td></td></tr>
<tr><td class="propname">deleteProperty</td><td>yes</td><td></td></tr>
<tr><td class="propname">enumerate</td><td>yes</td><td></td></tr>
<tr><td class="propname">ownKeys</td><td>yes</td><td><code>Object.keys()</code> enumerability check limitation, trap result validation (non-configurable properties, non-extensible target) not done</td></tr>
<tr><td class="propname">apply</td><td>no</td><td></td></tr>
<tr><td class="propname">construct</td><td>no</td><td></td></tr>
</tbody>
</table>

<p>Limitations include:</p>
<ul>
<li>Only about half of the ES6 traps have been implemented.  This causes odd behavior
    if you e.g. call <code>Object.defineProperty()</code> on a proxy object.</li>
<li><code>Object.keys()</code> invokes the <code>ownKeys</code> trap, cleans up the trap
    result to a gap-free string list, but does not check that the property names returned
    by the trap are enumerable.  <code>Object.keys()</code> and <code>Object.getOwnPropertyNames()</code>
    thus currently return the same value for a proxy object implementing the <code>ownKeys</code>
    trap.</li>
<li>Proxy trap results are not validated, e.g. <code>ownKeys</code> trap result validation
    steps described in <a href="http://www.ecma-international.org/ecma-262/6.0/#sec-proxy-object-internal-methods-and-internal-slots-ownpropertykeys">[[OwnPropertyKeys]] ()</a>
    for non-configurable target properties and/or non-extensible target object are not
    yet implemented.</li>
<li>Proxy revocation feature of ES6 is not supported.</li>
<li>The target and handler objects given to <code>new Proxy()</code> cannot
    be proxy objects themselves.  ES6 poses no such limitation, but Duktape
    enforces it to simplify the internal implementation.</li>
</ul>

<p>This custom feature can be disabled by disabling the config option
<code>DUK_USE_ES6_PROXY</code>.</p>

<h2 id="es6-typedarray">Typed arrays</h2>

<p>Duktape implements Khronos typed array support which is a subset of ES6
typed arrays.  Support for typed arrays and Node.js Buffer can be disabled
by disabling the config option <code>DUK_USE_BUFFEROBJECT_SUPPORT</code>.</p>