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---
title: with
slug: Web/JavaScript/Reference/Statements/with
translation_of: Web/JavaScript/Reference/Statements/with
original_slug: Web/JavaScript/Referencia/Sentencias/with
---
<div class="warning">El uso de la declaración no es recomendado, ya que puede ser el origen de los errores de confusión y problemas de compatibilidad. See the "Ambiguity Con" paragraph in the "Description" section below for details.</div>
<div>{{jsSidebar("Statements")}}</div>
<p>La <strong>sentencia with</strong> extiende el alcance de una cadena con la declaración.</p>
<h2 id="Sintaxis">Sintaxis</h2>
<pre class="syntaxbox">with (expresión) {
<em>declaración</em>
}
</pre>
<dl>
<dt><font face="Consolas, Liberation Mono, Courier, monospace">expresión</font></dt>
<dd>Añade la expresión dada a la declaración. Los parentesis alrededor de la expresión son necesarios.</dd>
<dt><code>declaración</code></dt>
<dd>Se puede ejecutar cualquier declaración. Para ejecutar varias declaraciónes, utilizar una declaración de bloque ({ ... }) para agrupar esas declaraciónes.</dd>
</dl>
<h2 id="Descripción">Descripción</h2>
<p>JavaScript looks up an unqualified name by searching a scope chain associated with the execution context of the script or function containing that unqualified name. The 'with' statement adds the given object to the head of this scope chain during the evaluation of its statement body. If an unqualified name used in the body matches a property in the scope chain, then the name is bound to the property and the object containing the property. Otherwise a {{jsxref("ReferenceError")}} is thrown.</p>
<div class="note">Using <code>with</code> is not recommended, and is forbidden in ECMAScript 5 <a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode" title="JavaScript/Strict mode">strict mode</a>. The recommended alternative is to assign the object whose properties you want to access to a temporary variable.</div>
<h3 id="Performance_pro_contra">Performance pro & contra</h3>
<p><strong>Pro:</strong> The <code>with</code> statement can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. The scope chain change required by 'with' is not computationally expensive. Use of 'with' will relieve the interpreter of parsing repeated object references. Note, however, that in many cases this benefit can be achieved by using a temporary variable to store a reference to the desired object.</p>
<p><strong>Contra:</strong> The <code>with</code> statement forces the specified object to be searched first for all name lookups. Therefore all identifiers that aren't members of the specified object will be found more slowly in a 'with' block. Where performance is important, 'with' should only be used to encompass code blocks that access members of the specified object.</p>
<h3 id="Ambiguity_contra">Ambiguity contra</h3>
<p><strong>Contra:</strong> The <code>with</code> statement makes it hard for a human reader or JavaScript compiler to decide whether an unqualified name will be found along the scope chain, and if so, in which object. So given this example:</p>
<pre class="brush: js">function f(x, o) {
with (o)
print(x);
}</pre>
<p>Only when <code>f</code> is called is <code>x</code> either found or not, and if found, either in <code>o</code> or (if no such property exists) in <code>f</code>'s activation object, where <code>x</code> names the first formal argument. If you forget to define <code>x</code> in the object you pass as the second argument, or if there's some similar bug or confusion, you won't get an error -- just unexpected results.</p>
<p><strong>Contra: </strong>Code using <code>with</code> may not be forward compatible, especially when used with something else than a plain object. Consider this example:</p>
<div>
<pre class="brush:js">function f(foo, values) {
with (foo) {
console.log(values)
}
}
</pre>
<p>If you call <code>f([1,2,3], obj)</code> in an ECMAScript 5 environment, then the <code>values</code> reference inside the <code>with</code> statement will resolve to <code>obj</code>. However, ECMAScript 6 introduces a <code>values</code> property on <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/prototype">Array.prototype</a></code> (so that it will be available on every array). So, in a JavaScript environment that supports ECMAScript 6, the <code>values</code> reference inside the <code>with</code> statement will resolve to <code>[1,2,3].values</code>.</p>
</div>
<h2 id="Examples">Examples</h2>
<h3 id="Using_with">Using <code>with</code></h3>
<p>The following <code>with</code> statement specifies that the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math" title="JavaScript/Reference/Global_Objects/Math"><code>Math</code></a> object is the default object. The statements following the <code>with</code> statement refer to the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/PI" title="JavaScript/Reference/Global_Objects/Math/PI"><code>PI</code></a> property and the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/cos" title="JavaScript/Reference/Global_Objects/Math/cos"><code>cos</code></a> and <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sin" title="JavaScript/Reference/Global_Objects/Math/sin"><code>sin</code></a> methods, without specifying an object. JavaScript assumes the <code>Math</code> object for these references.</p>
<pre class="brush:js">var a, x, y;
var r = 10;
with (Math) {
a = PI * r * r;
x = r * cos(PI);
y = r * sin(PI / 2);
}</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-with-statement', 'with statement')}}</td>
<td>{{Spec2('ES6')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-12.10', 'with statement')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td>Now forbidden in strict mode.</td>
</tr>
<tr>
<td>{{SpecName('ES3', '#sec-12.10', 'with statement')}}</td>
<td>{{Spec2('ES3')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES1', '#sec-12.10', 'with statement')}}</td>
<td>{{Spec2('ES1')}}</td>
<td>Initial definition</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{CompatibilityTable}}</p>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Statements/block", "block")}}</li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode">Strict mode</a></li>
</ul>
|
