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---
title: Logical operators
slug: Web/JavaScript/Reference/Operators/Logical_Operators
translation_of: Web/JavaScript/Reference/Operators
translation_of_original: Web/JavaScript/Reference/Operators/Logical_Operators
---
<div>{{jsSidebar("Operators")}}</div>
<p>邏輯運算子通常會搭配 {{jsxref("Boolean")}} (logical) 值。若是，則回傳布林值。然而， <code>&&</code> 和 <code>||</code> 運算子通常回傳其中一運算元的值, 因此若這些運算子搭配非布林值使用，他們會回傳非布林值。</p>
<div>{{EmbedInteractiveExample("pages/js/expressions-logicaloperator.html")}}</div>
<h2 id="說明">說明</h2>
<p>邏輯運算子的使用方式如下(<code><em>expr</em></code> 可能會是 <a href="/en-US/docs/Glossary/Data_structure">type</a>，不只是布林值)：</p>
<table class="fullwidth-table">
<tbody>
<tr>
<th>Operator</th>
<th>Syntax</th>
<th>Description</th>
</tr>
<tr>
<td>邏輯 AND (<code>&&</code>)</td>
<td><code><em>expr1</em> && <em>expr2</em></code></td>
<td>若 <code>expr<strong>1</strong></code> 可被轉換成 <code>true</code>, 回傳 <code>expr<strong>2</strong></code>; 否則 回傳 <code>expr<strong>1</strong></code>.</td>
</tr>
<tr>
<td>邏輯 OR (<code>||</code>)</td>
<td><code><em>expr1</em> || <em>expr2</em></code></td>
<td>若 <code>expr<strong>1</strong></code> 可被轉換成 <code>true</code>, 回傳 <code>expr<strong>1</strong></code>; 否則 回傳 <code>expr<strong>2</strong></code>.</td>
</tr>
<tr>
<td>邏輯 NOT (<code>!</code>)</td>
<td><code>!<em>expr</em></code></td>
<td>回傳 <code>false</code> 若它的單一運算元可被轉換成 <code>true</code>; 否則回傳 <code>true</code>.</td>
</tr>
</tbody>
</table>
<p>If a value can be converted to <code>true</code>, the value is so-called {{Glossary("truthy")}}. If a value can be converted to <code>false</code>, the value is so-called {{Glossary("falsy")}}.</p>
<p>Examples of expressions that can be converted to false are:</p>
<ul>
<li><code>null</code>;</li>
<li><code>NaN</code>;</li>
<li><code>0</code>;</li>
<li>empty string (<code>""</code> or <code>''</code> or <code>``</code>);</li>
<li><code>undefined</code>.</li>
</ul>
<p>Even though the <code>&&</code> and <code>||</code> operators can be used with operands that are not Boolean values, they can still be considered boolean operators since their return values can always be converted to <a href="/en-US/docs/Web/JavaScript/Data_structures#Boolean_type">boolean primitives</a>. To explicitly convert their return value (or any expression in general) to the corresponding boolean value, use a double <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators#Logical_NOT">NOT operator</a> or the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean">Boolean</a> constructor.</p>
<h3 id="Short-circuit_evaluation">Short-circuit evaluation</h3>
<p>As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:</p>
<ul>
<li><code>(some falsy expression) && <em>expr</em></code> is short-circuit evaluated to the falsy expression;</li>
<li><code>(some truthy expression) || <em>expr</em></code> is short-circuit evaluated to the truthy expression.</li>
</ul>
<p>Short circuit means that the <em>expr</em> parts above are <strong>not evaluated</strong>, hence any side effects of doing so do not take effect (e.g., if <em>expr</em> is a function call, the calling never takes place). This happens because the value of the operator is already determined after the evaluation of the first operand. See example:</p>
<pre class="brush: js">function A(){ console.log('called A'); return false; }
function B(){ console.log('called B'); return true; }
console.log( A() && B() );
// logs "called A" due to the function call,
// then logs false (which is the resulting value of the operator)
console.log( B() || A() );
// logs "called B" due to the function call,
// then logs true (which is the resulting value of the operator)
</pre>
<h3 id="Operator_precedence">Operator precedence</h3>
<p>The following expressions might seem equivalent, but they are not, because the <code>&&</code> operator is executed before the <code>||</code> operator (see <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence">operator precedence</a>).</p>
<pre class="brush: js">true || false && false // returns true, because && is executed first
(true || false) && false // returns false, because operator precedence cannot apply</pre>
<h3 id="Logical_AND_()"><a name="Logical_AND">Logical AND (<code>&&</code>)</a></h3>
<p>The following code shows examples of the <code>&&</code> (logical AND) operator.</p>
<pre class="brush: js">a1 = true && true // t && t returns true
a2 = true && false // t && f returns false
a3 = false && true // f && t returns false
a4 = false && (3 == 4) // f && f returns false
a5 = 'Cat' && 'Dog' // t && t returns "Dog"
a6 = false && 'Cat' // f && t returns false
a7 = 'Cat' && false // t && f returns false
a8 = '' && false // f && f returns ""
a9 = false && '' // f && f returns false
</pre>
<h3 id="Logical_OR_()"><a name="Logical_OR">Logical OR (<code>||</code>)</a></h3>
<p>The following code shows examples of the <code>||</code> (logical OR) operator.</p>
<pre class="brush: js">o1 = true || true // t || t returns true
o2 = false || true // f || t returns true
o3 = true || false // t || f returns true
o4 = false || (3 == 4) // f || f returns false
o5 = 'Cat' || 'Dog' // t || t returns "Cat"
o6 = false || 'Cat' // f || t returns "Cat"
o7 = 'Cat' || false // t || f returns "Cat"
o8 = '' || false // f || f returns false
o9 = false || '' // f || f returns ""
o10 = false || varObject // f || object returns varObject
</pre>
<h3 id="Logical_NOT_(!)"><a name="Logical_NOT">Logical NOT (<code>!</code>)</a></h3>
<p>The following code shows examples of the <code>!</code> (logical NOT) operator.</p>
<pre class="brush: js">n1 = !true // !t returns false
n2 = !false // !f returns true
n3 = !'' // !f returns true
n4 = !'Cat' // !t returns false
</pre>
<h4 id="Double_NOT_(!!)">Double NOT (<code>!!</code>)</h4>
<p>It is possible to use a couple of NOT operators in series to explicitly force the conversion of any value to the corresponding <a href="/en-US/docs/Web/JavaScript/Data_structures#Boolean_type">boolean primitive</a>. The conversion is based on the "truthyness" or "falsyness" of the value (see {{Glossary("truthy")}} and {{Glossary("falsy")}}).</p>
<p>The same conversion can be done through the {{jsxref("Boolean")}} function.</p>
<pre class="brush: js">n1 = !!true // !!truthy returns true
n2 = !!{} // !!truthy returns true: <strong>any</strong> object is truthy...
n3 = !!(new Boolean(false)) // ...even Boolean objects with a false <em>.valueOf()</em>!
n4 = !!false // !!falsy returns false
n5 = !!"" // !!falsy returns false
n6 = !!Boolean(false) // !!falsy returns false
</pre>
<h3 id="Conversion_rules_for_booleans">Conversion rules for booleans</h3>
<h4 id="Converting_AND_to_OR">Converting AND to OR</h4>
<p>The following operation involving <strong>booleans</strong>:</p>
<pre class="brush: js">bCondition1 && bCondition2</pre>
<p>is always equal to:</p>
<pre class="brush: js">!(!bCondition1 || !bCondition2)</pre>
<h4 id="Converting_OR_to_AND">Converting OR to AND</h4>
<p>The following operation involving <strong>booleans</strong>:</p>
<pre class="brush: js">bCondition1 || bCondition2</pre>
<p>is always equal to:</p>
<pre class="brush: js">!(!bCondition1 && !bCondition2)</pre>
<h4 id="Converting_between_NOTs">Converting between NOTs</h4>
<p>The following operation involving <strong>booleans</strong>:</p>
<pre class="brush: js">!!bCondition</pre>
<p>is always equal to:</p>
<pre class="brush: js">bCondition</pre>
<h3 id="Removing_nested_parentheses">Removing nested parentheses</h3>
<p>As logical expressions are evaluated left to right, it is always possible to remove parentheses from a complex expression following some rules.</p>
<h4 id="Removing_nested_AND">Removing nested AND</h4>
<p>The following composite operation involving <strong>booleans</strong>:</p>
<pre class="brush: js">bCondition1 || (bCondition2 && bCondition3)</pre>
<p>is always equal to:</p>
<pre class="brush: js">bCondition1 || bCondition2 && bCondition3</pre>
<h4 id="Removing_nested_OR">Removing nested OR</h4>
<p>The following composite operation involving <strong>booleans</strong>:</p>
<pre class="brush: js">bCondition1 && (bCondition2 || bCondition3)</pre>
<p>is always equal to:</p>
<pre class="brush: js">!(!bCondition1 || !bCondition2 && !bCondition3)</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('ES1')}}</td>
<td>{{Spec2('ES1')}}</td>
<td>Initial definition.</td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-11.11')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td>Defined in several sections of the specification: <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-11.4.9">Logical NOT Operator</a>, <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-11.11">Binary Logical Operators</a></td>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-binary-logical-operators')}}</td>
<td>{{Spec2('ES6')}}</td>
<td>Defined in several sections of the specification: <a href="http://www.ecma-international.org/ecma-262/6.0/#sec-logical-not-operator">Logical NOT Operator</a>, <a href="http://www.ecma-international.org/ecma-262/6.0/#sec-binary-logical-operators">Binary Logical Operators</a></td>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-binary-logical-operators')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td>Defined in several sections of the specification: <a href="http://tc39.github.io/ecma262/#sec-logical-not-operator">Logical NOT Operator</a>, <a href="http://tc39.github.io/ecma262/#sec-binary-logical-operators">Binary Logical Operators</a></td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{Compat("javascript.operators.logical")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators">Bitwise operators</a></li>
<li>{{jsxref("Boolean")}}</li>
<li><a href="/en-US/docs/Glossary/Truthy">Truthy</a></li>
<li><a href="/en-US/docs/Glossary/Falsy">Falsy</a></li>
</ul>
|
