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+---
+title: 加法赋值 (+=)
+slug: Web/JavaScript/Reference/Operators/Addition_assignment
+tags:
+  - +=
+translation_of: Web/JavaScript/Reference/Operators/Addition_assignment
+---
+<div>{{jsSidebar("Operators")}}</div>
+
+<p>加法赋值操作符 (<code>+=</code>) 将右操作数的值添加到变量，并将结果分配给该变量。两个操作数的类型确定加法赋值运算符的行为。加法或串联是可能的。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/expressions-addition-assignment.html")}}</div>
+
+<div></div>
+
+
+
+<h2 id="Syntax">Syntax</h2>
+
+<pre class="syntaxbox notranslate"><strong>Operator:</strong> x += y
+<strong>Meaning:</strong>  x  = x + y</pre>
+
+<h2 id="Examples">Examples</h2>
+
+<h3 id="Using_addition_assignment">Using addition assignment</h3>
+
+<pre class="brush: js notranslate">// Assuming the following variables
+//  foo = 'foo'
+//  bar = 5
+//  baz = true
+
+// Number + Number -&gt; addition
+bar += 2 // 7
+
+// Boolean + Number -&gt; addition
+baz += 1 // 2
+
+// Boolean + Boolean -&gt; addition
+baz += false // 1
+
+// Number + String -&gt; concatenation
+bar += 'foo' // "5foo"
+
+// String + Boolean -&gt; concatenation
+foo += false // "foofalse"
+
+// String + String -&gt; concatenation
+foo += 'bar' // "foobar"</pre>
+
+<h2 id="Specifications">Specifications</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">Specification</th>
+  </tr>
+  <tr>
+   <td>{{SpecName('ESDraft', '#sec-assignment-operators', 'Assignment operators')}}</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="Browser_compatibility">Browser compatibility</h2>
+
+
+
+<p>{{Compat("javascript.operators.addition_assignment")}}</p>
+
+<h2 id="See_also">See also</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Assignment">Assignment operators in the JS guide</a></li>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Addition">Addition operator</a></li>
+</ul>