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+---
+title: 相等比較
+slug: Web/JavaScript/Equality_comparisons_and_sameness
+translation_of: Web/JavaScript/Equality_comparisons_and_sameness
+---
+<div>{{jsSidebar("Intermediate")}}</div>
+
+<p>在 ES2015，有四個相等比較方法：</p>
+
+<ul>
+ <li>一般相等 (<code>==</code>)</li>
+ <li>嚴格相等 (<code style="font-weight: 700; font-style: normal;">===</code>)：被用於 <code style="font-weight: 700; font-style: normal;">Array.prototype.indexOf</code>、 <code style="font-weight: 700; font-style: normal;">Array.prototype.lastIndexOf </code>和 <code style="font-weight: 700; font-style: normal;">case</code>-matching </li>
+ <li>零值相等：被用於 <code>%TypedArray%</code> 和 <code>ArrayBuffer</code> 建構子，以及 <code>Map</code> 和 <code>Set</code> 運算子，還有將在 ES2016 新增的 <code>String.prototype.includes。</code></li>
+ <li>同值相等： 用在除上面提及的所有情況。</li>
+</ul>
+
+<p>JavaScript 提供三種不同的值比較運算操作：</p>
+
+<ul>
+ <li>嚴格相等 (或稱 "三等於"、"全等") 使用 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Identity">===</a></li>
+ <li>一般相等 ("雙等於") 使用 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Equality">==</a></li>
+ <li>還有 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> (ECMAScript 2015 新加入)</li>
+</ul>
+
+<p>要用哪個操作取決於你要哪種類型的比較。</p>
+
+<p>簡單來說，一般相等會將型別一致化後比較；嚴格相等則不會（也就是說若型別不同，就會回傳 fasle）；<code>Object.is</code> 會和嚴格相等做同樣的事，但會將 <code>NaN</code>、<code>-0<font face="Open Sans, Arial, sans-serif"> 和 </font></code><code>+0 </code>獨立處理，因此這三個不會相等，而 <code>Object.is(NaN, NaN)</code> 則會回傳 true 。（用一般相等或嚴格相等比較兩個 <code>NaN</code> 時會回傳 <code>false</code> ，因為 IEEE 754 如此規範。） 切記，這三種判斷必須考慮原型，因為他們在設計上不被考慮為相等。對於任何非原型物件 x、y，即使他們有著相同結構，但如果是不同物件，比較就會是 false。</p>
+
+<h2 id="嚴格相等（）">嚴格相等（<code>===</code>）</h2>
+
+<p>嚴格相等比較兩個值，而被比較的兩個值都不會轉換成其他型別。如果值是不同型別，就會被視為不相等。如果兩值型別相同但不是數字，若值相同，則為相等。此外，如果兩個值皆為數字，只要他們是 NaN 以外的同一值，或者 +0 和 -0，則為相等。</p>
+
+<pre class="brush: js">var num = 0;
+var obj = new String("0");
+var str = "0";
+
+console.log(num === num); // true
+console.log(obj === obj); // true
+console.log(str === str); // true
+
+console.log(num === obj); // false
+console.log(num === str); // false
+console.log(obj === str); // false
+console.log(null === undefined); // false
+console.log(obj === null); // false
+console.log(obj === undefined); // false
+</pre>
+
+<p>嚴格比較適合在絕大多數情況下使用。對於所有非數字的值，嚴格比較就如字面：一個值只相等於自己。而數字則使用稍微不同的方式：第一種情況是浮點數 0 同時為正和負，在解決某些數學問題時，<code>+0</code> 和 <code>-0 </code>是不同的，但在大部分情況下我們不需要考慮這種情境，因此嚴格比較將他們視為相同的。第二種情況是非數字，<code>NaN</code>，用來表示某些定義不明確的數學問題的解， 例如：負無窮加正無窮，嚴格比較認為 <code>NaN</code> 不等於任何值，包含他本身。（<code>(x !== x)</code>只有在 <code>x</code> 是 <code>NaN </code>時會是 <code>true</code>。）</p>
+
+<h2 id="一般相等（）">一般相等（==）</h2>
+
+<p>一般相等會<em>先將</em>比較值轉換成同型別後比較。轉換後（可能一個或兩個都被轉換），接著進行的幾乎和嚴格比較（<code>===</code>）一樣。 一般相等會<em>對稱</em>： <code>A == B</code> 等同 <code>B == A</code> ，無論 <code>A</code> 和 <code>B</code> 是什麼。（除了型別轉換的順序）</p>
+
+<p>不同型別的一般相等運作如下表：</p>
+
+<table class="standard-table">
+ <thead>
+  <tr>
+   <th scope="row"></th>
+   <th colspan="7" scope="col" style="text-align: center;">比較值 B</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <th scope="row"></th>
+   <td></td>
+   <td style="text-align: center;">Undefined</td>
+   <td style="text-align: center;">Null</td>
+   <td style="text-align: center;">Number</td>
+   <td style="text-align: center;">String</td>
+   <td style="text-align: center;">Boolean</td>
+   <td style="text-align: center;">Object</td>
+  </tr>
+  <tr>
+   <th colspan="1" rowspan="6" scope="row">比較值 A</th>
+   <td>Undefined</td>
+   <td style="text-align: center;"><code>true</code></td>
+   <td style="text-align: center;"><code>true</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td>Null</td>
+   <td style="text-align: center;"><code>true</code></td>
+   <td style="text-align: center;"><code>true</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td>Number</td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>A === B</code></td>
+   <td style="text-align: center;"><code>A === ToNumber(B)</code></td>
+   <td style="text-align: center;"><code>A === ToNumber(B)</code></td>
+   <td style="text-align: center;"><code>A == ToPrimitive(B)</code></td>
+  </tr>
+  <tr>
+   <td>String</td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>ToNumber(A) === B</code></td>
+   <td style="text-align: center;"><code>A === B</code></td>
+   <td style="text-align: center;"><code>ToNumber(A) === ToNumber(B)</code></td>
+   <td style="text-align: center;"><code>A == ToPrimitive(B)</code></td>
+  </tr>
+  <tr>
+   <td>Boolean</td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>ToNumber(A) === B</code></td>
+   <td style="text-align: center;"><code>ToNumber(A) === ToNumber(B)</code></td>
+   <td style="text-align: center;"><code>A === B</code></td>
+   <td style="text-align: center;"><code>ToNumber(A) == ToPrimitive(B)</code></td>
+  </tr>
+  <tr>
+   <td>Object</td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>false</code></td>
+   <td style="text-align: center;"><code>ToPrimitive(A) == B</code></td>
+   <td style="text-align: center;"><code>ToPrimitive(A) == B</code></td>
+   <td style="text-align: center;"><code>ToPrimitive(A) == ToNumber(B)</code></td>
+   <td style="text-align: center;"><code>A === B</code></td>
+  </tr>
+ </tbody>
+</table>
+
+<p>根據上表， <code>ToNumber(A)</code> 嘗試在比較前轉換成一個數字。 這等同 <code>+A</code> （單 + 運算子）。<code>ToPrimitive(A)</code> 嘗試從物件轉換成原生值，透過嘗試對 A 使用 <code>A.toString</code> 和 <code>A.valueOf</code> 方法。</p>
+
+<p>一般來說，根據 ECMAScript 規範，所有物件應該不等於 <code>undefined</code> 和 <code>null</code>。但大多數瀏覽器允許很小部分的物件（尤其是所有頁面的 <code>document.all</code> 物件）在某些情況下<em>當成</em> <code>undefined</code>。一般相等是其中一種：當 A 是個被<em>模擬</em> 成 <code>undefined </code>的物件<code><font face="Open Sans, Arial, sans-serif">，</font></code><code>null == A</code> 和 <code>undefined == A</code> 會是 true。而在其他情況下物件不會等同於 <code>undefined</code> 或 <code>null。</code></p>
+
+<pre class="brush: js">var num = 0;
+var obj = new String("0");
+var str = "0";
+
+console.log(num == num); // true
+console.log(obj == obj); // true
+console.log(str == str); // true
+
+console.log(num == obj); // true
+console.log(num == str); // true
+console.log(obj == str); // true
+console.log(null == undefined); // true
+
+// 除了少數情況，這兩個應該是 false。
+console.log(obj == null);
+console.log(obj == undefined);
+</pre>
+
+<p>部分開發者認為最好別用一般相等。嚴格比較更容易預測，且因為不必轉型，因此效率更好。</p>
+
+<h2 id="同值相等">同值相等</h2>
+
+<p>同值相等解決了最後一個情況：比較兩個值是否<em>功能相同</em> 。（這裡用了 <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" title="http://en.wikipedia.org/wiki/Liskov_substitution_principle">Liskov 替換原則（英）</a> 為例）當試圖修改一個不可變的屬性：</p>
+
+<pre class="brush: js">// 新增一個不可變 NEGATIVE_ZERO 屬性到 Number 原型。
+Object.defineProperty(Number, "NEGATIVE_ZERO",
+                      { value: -0, writable: false, configurable: false, enumerable: false });
+
+function attemptMutation(v)
+{
+  Object.defineProperty(Number, "NEGATIVE_ZERO", { value: v });
+}
+</pre>
+
+<p>當修改一個不可變屬性時， <code>Object.defineProperty</code> 會出現例外，但若沒有真的要求修改，就沒事。如果 <code>v</code> 是 <code>-0</code>，就不會有修改，也就不會有錯誤出現。但若 <code>v</code> 是 <code>+0</code>，<code>Number.NEGATIVE_ZERO</code> 不再擁有自己的不可變屬性。在內部，當一個不可變屬性被重新定義，新的值會用同值相等和原值比較。</p>
+
+<p><a href="/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> 方法提供同值相等比較。</p>
+
+<h2 id="零值相等">零值相等</h2>
+
+<p>和同值相等一樣，但將 <code>+0</code> 和 <code>-0 </code>視為相同。</p>
+
+<h2 id="一般相等、嚴格相等和同值相等的規範">一般相等、嚴格相等和同值相等的規範</h2>
+
+<p>在 ES5，一般相等 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators"><code>==</code></a> 在 <a href="http://ecma-international.org/ecma-262/5.1/#sec-11.9.3" title="http://ecma-international.org/ecma-262/5.1/#sec-11.9.3">Section 11.9.3, The Abstract Equality Algorithm</a> 中規範。嚴格相等 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators"><code>===</code></a> 在 <a href="http://ecma-international.org/ecma-262/5.1/#sec-11.9.6" title="http://ecma-international.org/ecma-262/5.1/#sec-11.9.6">11.9.6, The Strict Equality Algorithm</a>。（可以看看，這很簡短且可讀。註：先讀嚴格相等。）ES5 也在 <a href="http://ecma-international.org/ecma-262/5.1/#sec-9.12" title="http://ecma-international.org/ecma-262/5.1/#sec-9.12">Section 9.12, The SameValue Algorithm</a> 規範 JS 引擎的行為。他幾乎和嚴格相等一樣，除了 11.9.6.4 和 9.12.4 在處理 <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number">Number</a></code> 時的不同。ES2015 簡短的提出了 <code><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is">Object.is</a>。</code></p>
+
+<p>我們可以發現在 11.9.6.1 中，除了 11.9.6.1 規範型別檢查，嚴格相等規範是從屬於一般相等規範，因為 11.9.6.2–7 和 11.9.3.1.a–f相應。</p>
+
+<h2 id="理解相等比較模型">理解相等比較模型</h2>
+
+<p>ES2015 以後，你或許會將雙等於和三等於解讀成是彼此的「加強版」。比如，有人或許會說雙等於是三等於的延伸版本，因為前者做的事情和後者事情一模一樣，只差在運算元的型別轉換。舉例來說，<code>6 == "6"</code> （又或者說，有人可能會講說雙等於是基底，而三等於是加強版，因為它要求兩個運算元是同型別，所以它多了一個限制。至於哪個是較好的理解模型，取決於你的觀點。</p>
+
+<p>儘管如此，這個思考內建相同運算子的方法，並非是延伸 ES2015 中的 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> 方法。 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> 不是單純地比雙等號「更寬鬆」或比三等號「更嚴謹」，也不適合將其放在兩者之間（即，比雙等號嚴謹，但較三等號寬鬆）。我們可以從下方的比較表格看到，一切是起源於 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> 可以處理 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a> 的比較運算。要注意的是，如果 <code>Object.is(NaN, NaN)</code> 的運算結果是  <code>false</code> ，我們就可以因為它區分 <code>-0</code> 和 <code>+0</code> 的結果，使用寬鬆和嚴謹的範疇來界定它是比三等號更嚴謹的那一區段。然而，區別 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a> 的方式並不確實。Unfortunately, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> simply has to be thought of in terms of its specific characteristics, rather than its looseness or strictness with regard to the equality operators.</p>
+
+<table class="standard-table">
+ <caption>Sameness Comparisons</caption>
+ <thead>
+  <tr>
+   <th scope="col" style="text-align: center;">x</th>
+   <th scope="col" style="text-align: center;">y</th>
+   <th scope="col" style="width: 10em; text-align: center;"><code>==</code></th>
+   <th scope="col" style="width: 10em; text-align: center;"><code>===</code></th>
+   <th scope="col" style="width: 10em; text-align: center;"><code>Object.is</code></th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <td><code>undefined</code></td>
+   <td><code>undefined</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>null</code></td>
+   <td><code>null</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>true</code></td>
+   <td><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>false</code></td>
+   <td><code>false</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>"foo"</code></td>
+   <td><code>"foo"</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>{ foo: "bar" }</code></td>
+   <td><code>x</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>0</code></td>
+   <td><code>0</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+  <tr>
+   <td><code>+0</code></td>
+   <td><code>-0</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>0</code></td>
+   <td><code>false</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>""</code></td>
+   <td><code>false</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>""</code></td>
+   <td><code>0</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>"0"</code></td>
+   <td><code>0</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>"17"</code></td>
+   <td><code>17</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>[1,2]</code></td>
+   <td><code>"1,2"</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>new String("foo")</code></td>
+   <td><code>"foo"</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>null</code></td>
+   <td><code>undefined</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>null</code></td>
+   <td><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>undefined</code></td>
+   <td><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>{ foo: "bar" }</code></td>
+   <td><code>{ foo: "bar" }</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>new String("foo")</code></td>
+   <td><code>new String("foo")</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>0</code></td>
+   <td><code>null</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>0</code></td>
+   <td><code>NaN</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>"foo"</code></td>
+   <td><code>NaN</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+  </tr>
+  <tr>
+   <td><code>NaN</code></td>
+   <td><code>NaN</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(255, 144, 144); text-align: center;"><code>false</code></td>
+   <td style="background-color: rgb(144, 255, 144); text-align: center;"><code>true</code></td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="When_to_use_Object.is_versus_triple_equals">When to use <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> versus triple equals</h2>
+
+<p>Aside from the way it treats <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a>, generally, the only time <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a>'s special behavior towards zeros is likely to be of interest is in the pursuit of certain meta-programming schemes, especially regarding property descriptors when it is desirable for your work to mirror some of the characteristics of <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty"><code>Object.defineProperty</code></a>. If your use case does not require this, it is suggested to avoid <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> and use <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators"><code>===</code></a> instead. Even if your requirements involve having comparisons between two <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a> values evaluate to <code>true</code>, generally it is easier to special-case the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a> checks (using the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN"><code>isNaN</code></a> method available from previous versions of ECMAScript) than it is to work out how surrounding computations might affect the sign of any zeros you encounter in your comparison.</p>
+
+<p>Here's an in-exhaustive list of built-in methods and operators that might cause a distinction between <code>-0</code> and <code>+0</code> to manifest itself in your code:</p>
+
+<dl>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#-_.28Unary_Negation.29" title="/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators"><code>- (unary negation)</code></a></dt>
+</dl>
+
+<dl>
+ <dd>
+ <p>It's obvious that negating <code>0</code> produces <code>-0</code>. But the abstraction of an expression can cause <code>-0</code> to creep in when you don't realize it. For example, consider:</p>
+
+ <pre class="brush:js">let stoppingForce = obj.mass * -obj.velocity</pre>
+
+ <p>If <code>obj.velocity</code> is <code>0</code> (or computes to <code>0</code>), a <code>-0</code> is introduced at that place and propogates out into <code>stoppingForce</code>.</p>
+ </dd>
+</dl>
+
+<dl>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/atan2" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/atan2"><code>Math.atan2</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/ceil" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/ceil"><code>Math.ceil</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/pow" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/pow"><code>Math.pow</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/round" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/round"><code>Math.round</code></a></dt>
+</dl>
+
+<dl>
+ <dd>It's possible for a <code>-0</code> to be introduced into an expression as a return value of these methods in some cases, even when no <code>-0</code> exists as one of the parameters. E.g., using <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/pow" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/pow"><code>Math.pow</code></a> to raise <code>-<a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Infinity" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Infinity">Infinity</a></code> to the power of any negative, odd exponent evaluates to <code>-0</code>. Refer to the documentation for the individual methods.</dd>
+</dl>
+
+<dl>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/floor" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/floor"><code>Math.floor</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/max" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/max"><code>Math.max</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/min" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/min"><code>Math.min</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sin" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sin"><code>Math.sin</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sqrt" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sqrt"><code>Math.sqrt</code></a></dt>
+ <dt><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/tan" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/tan"><code>Math.tan</code></a></dt>
+</dl>
+
+<dl>
+ <dd>It's possible to get a <code>-0</code> return value out of these methods in some cases where a <code>-0</code> exists as one of the parameters. E.g., <code>Math.min(-0, +0)</code> evalutes to <code>-0</code>. Refer to the documentation for the individual methods.</dd>
+</dl>
+
+<dl>
+ <dt><code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators">~</a></code></dt>
+ <dt><code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators">&lt;&lt;</a></code></dt>
+ <dt><code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators" title="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators">&gt;&gt;</a></code></dt>
+ <dd>Each of these operators uses the ToInt32 algorithm internally. Since there is only one representation for 0 in the internal 32-bit integer type, <code>-0</code> will not survive a round trip after an inverse operation. E.g., both <code>Object.is(~~(-0), -0)</code> and <code>Object.is(-0 &lt;&lt; 2 &gt;&gt; 2, -0)</code> evaluate to <code>false</code>.</dd>
+</dl>
+
+<p>Relying on <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" title="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is</code></a> when the signedness of zeros is not taken into account can be hazardous. Of course, when the intent is to distinguish between <code>-0</code> and <code>+0</code>, it does exactly what's desired.</p>
+
+<h2 id="See_also">See also</h2>
+
+<ul>
+ <li><a href="http://dorey.github.io/JavaScript-Equality-Table/">JS Comparison Table</a></li>
+</ul>