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diff --git a/files/zh-tw/web/javascript/reference/errors/bad_return_or_yield/index.html b/files/zh-tw/web/javascript/reference/errors/bad_return_or_yield/index.html
new file mode 100644
index 0000000000..7a28fb4be3
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/bad_return_or_yield/index.html
@@ -0,0 +1,51 @@
+---
+title: 'SyntaxError: return not in function'
+slug: Web/JavaScript/Reference/Errors/Bad_return_or_yield
+translation_of: Web/JavaScript/Reference/Errors/Bad_return_or_yield
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">SyntaxError: return not in function
+SyntaxError: yield not in function
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("SyntaxError")}}</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/return">return</a></code> 或 <code><a href="/zh-TW/docs/Web/JavaScript/Reference/Operators/yield">yield</a></code> 宣告在<a href="/zh-TW/docs/Web/JavaScript/Guide/Functions">函式</a>以外的地方被呼叫。也許少寫了一個大括號？<code>return</code> 與 <code>yield</code> 宣告必須要寫在函式裡面，因為它們結束（或暫停並恢復）函式的執行，並且回傳了特定值。</p>
+
+<h2 id="實例">實例</h2>
+
+<pre class="brush: js example-bad">var cheer = function(score) {
+  if (score === 147)
+    return "Maximum!";
+  };
+  if (score &gt; 100) {
+    return "Century!";
+  }
+}
+
+// SyntaxError: return not in function</pre>
+
+<p>乍看之下大括號寫對了，但其實在第一個 <code>if</code> 的後面，少了一個 <code>{</code>。正確的寫法應該是：</p>
+
+<pre class="brush: js example-good">var cheer = function(score) {
+  if (score === 147) {
+    return "Maximum!";
+  }
+  if (score &gt; 100) {
+    return "Century!";
+  }
+};</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/return">return</a></code></li>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Operators/yield">yield</a></code></li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/index.html b/files/zh-tw/web/javascript/reference/errors/index.html
new file mode 100644
index 0000000000..8f553faa75
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/index.html
@@ -0,0 +1,22 @@
+---
+title: JavaScript 錯誤參考資料
+slug: Web/JavaScript/Reference/Errors
+translation_of: Web/JavaScript/Reference/Errors
+---
+<p>{{jsSidebar("Errors")}}</p>
+
+<p>在這裡，你可以看到一些由 JavaScript 拋出的錯誤一覽。這些錯誤訊息對你的除錯很有幫助，但拋出的錯誤也不是每次都能講清楚。本頁回提供這些錯誤的詳細資訊。所有的錯誤都是由 {{jsxref("Error")}} 物件所建立的物件，有著 <code>name</code> 與 <code>message</code>。</p>
+
+<p>錯誤會出現在網路主控台、可能還連接到相應頁面，以幫助你儘速理解程式碼裡面的問題。</p>
+
+<h2 id="錯誤一覽表">錯誤一覽表</h2>
+
+<p>In this list, each page is listed by name (the type of error) and message (a more detailed human-readable error message). Together, these two properties provide a starting point toward understanding and resolving the error. For more information, follow the links below!</p>
+
+<p>{{ListSubPages("/zh-TW/docs/Web/JavaScript/Reference/Errors")}}</p>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><a href="/zh-TW/docs/Learn/JavaScript/First_steps/What_went_wrong">哪裡出錯了？JavaScript 除錯</a>：針對初學者的 JavaScript 除錯入門教程。</li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/invalid_array_length/index.html b/files/zh-tw/web/javascript/reference/errors/invalid_array_length/index.html
new file mode 100644
index 0000000000..ee2c5f08e4
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/invalid_array_length/index.html
@@ -0,0 +1,74 @@
+---
+title: 'RangeError: invalid array length'
+slug: Web/JavaScript/Reference/Errors/Invalid_array_length
+translation_of: Web/JavaScript/Reference/Errors/Invalid_array_length
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">RangeError: Array length must be a finite positive integer (Edge)
+RangeError: invalid array length (Firefox)
+RangeError: Invalid array length (Chrome)
+RangeError: Invalid array buffer length (Chrome)
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("RangeError")}}</p>
+
+<h2 id="哪裡錯了">哪裡錯了?</h2>
+
+<p>一個無效的陣列長度可能發生於以下幾種情形:</p>
+
+<ul>
+ <li>建立了一個長度為負或大於等於2<sup>32</sup>的 {{jsxref("Array")}} 或 {{jsxref("ArrayBuffer")}} </li>
+ <li>將 {{jsxref("Array.length")}} 屬性設為負值或大於等於 2<sup>32</sup></li>
+</ul>
+
+<p>為什麼 <code>Array</code>  和 <code>ArrayBuffer</code> 的長度有限?   <code>Array</code> 和 <code>ArrayBuffer</code> 的屬性以一個32位元的非負整數表使，因此僅能儲存 0 到 2<sup>32</sup>-1 的數值。</p>
+
+<p>If you are creating an <code>Array</code>, using the constructor, you probably want to use the literal notation instead, as the first argument is interpreted as the length of the <code>Array</code>.</p>
+
+<p>Otherwise, you might want to clamp the length before setting the length property, or using it as argument of the constructor.</p>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="無效的案例">無效的案例</h3>
+
+<pre class="brush: js example-bad">new Array(Math.pow(2, 40))
+new Array(-1)
+new ArrayBuffer(Math.pow(2, 32))
+new ArrayBuffer(-1)
+
+let a = [];
+a.length = a.length - 1;         // set -1 to the length property
+
+let b = new Array(Math.pow(2, 32) - 1);
+b.length = b.length + 1;         // set 2^32 to the length property
+</pre>
+
+<h3 id="有效的案例">有效的案例</h3>
+
+<pre class="brush: js example-good">[ Math.pow(2, 40) ]                     // [ 1099511627776 ]
+[ -1 ]                                  // [ -1 ]
+new ArrayBuffer(Math.pow(2, 32) - 1)
+new ArrayBuffer(0)
+
+let a = [];
+a.length = Math.max(0, a.length - 1);
+
+let b = new Array(Math.pow(2, 32) - 1);
+b.length = Math.min(0xffffffff, b.length + 1);
+
+// 0xffffffff 是 2^32 - 1 的十六進位表示
+// 也可以寫成 (-1 &gt;&gt;&gt; 0)
+</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li>{{jsxref("Array")}}</li>
+ <li>{{jsxref("Array.length")}}</li>
+ <li>{{jsxref("ArrayBuffer")}}</li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/missing_curly_after_property_list/index.html b/files/zh-tw/web/javascript/reference/errors/missing_curly_after_property_list/index.html
new file mode 100644
index 0000000000..7e3728fc49
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/missing_curly_after_property_list/index.html
@@ -0,0 +1,47 @@
+---
+title: 'SyntaxError: missing } after property list'
+slug: Web/JavaScript/Reference/Errors/Missing_curly_after_property_list
+translation_of: Web/JavaScript/Reference/Errors/Missing_curly_after_property_list
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">SyntaxError: missing } after property list
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("SyntaxError")}}</p>
+
+<h2 id="何處出錯">何處出錯?</h2>
+
+<p>在物件初始化時，語法錯誤。  實際上可能遺漏一個大括號或是逗號。 例如， 同時檢查大括弧以及逗號是否以正確的順序關閉。 縮排或是有規則的排序代碼是有幫助您找出複雜的代碼錯誤。</p>
+
+<h2 id="範例">範例</h2>
+
+<h3 id="忘記逗號">忘記逗號</h3>
+
+<p>有時候，在初始化物件時，缺少一個逗號：</p>
+
+<pre class="brush: js example-bad">var obj = {
+  a: 1,
+  b: { myProp: 2 }
+  c: 3
+};
+</pre>
+
+<p>Correct would be:</p>
+
+<pre class="brush: js example-good">var obj = {
+  a: 1,
+  b: { myProp: 2 },
+  c: 3
+};
+</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer">Object initializer</a></li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/no_properties/index.html b/files/zh-tw/web/javascript/reference/errors/no_properties/index.html
new file mode 100644
index 0000000000..b355d15ea3
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/no_properties/index.html
@@ -0,0 +1,36 @@
+---
+title: 'TypeError: "x" has no properties'
+slug: Web/JavaScript/Reference/Errors/No_properties
+translation_of: Web/JavaScript/Reference/Errors/No_properties
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">TypeError: null has no properties
+TypeError: undefined has no properties
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("TypeError")}}.</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>{{jsxref("null")}} 與 {{jsxref("undefined")}} 並沒有可訪問的屬性。</p>
+
+<h2 id="示例">示例</h2>
+
+<pre class="brush: js example-bad">null.foo;
+// TypeError: null has no properties
+
+undefined.bar;
+// TypeError: undefined has no properties
+</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li>{{jsxref("null")}}</li>
+ <li>{{jsxref("undefined")}}</li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/not_a_function/index.html b/files/zh-tw/web/javascript/reference/errors/not_a_function/index.html
new file mode 100644
index 0000000000..24ce79a6e4
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/not_a_function/index.html
@@ -0,0 +1,80 @@
+---
+title: 'TypeError: "x" is not a function'
+slug: Web/JavaScript/Reference/Errors/Not_a_function
+translation_of: Web/JavaScript/Reference/Errors/Not_a_function
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">TypeError: "x" is not a function
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("TypeError")}}.</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>你想以函式呼叫一個數值，但該數值其實不是函式。程式碼期望你給出函式，但這份期望落空了。</p>
+
+<p>也許打錯了函式的名字？也許呼叫的物件並沒有這個函式？例如說 JavaScript 物件並沒有 <code>map</code> 函式，但 JavaScript Array（陣列）物件則有。</p>
+
+<p>許多內建函式都需要回呼（callback）的函式。為了讓下面的方法順利運作，你需要為它們提供函式：</p>
+
+<ul>
+ <li>如果是 {{jsxref("Array")}} 或 {{jsxref("TypedArray")}} 物件：
+  <ul>
+   <li>{{jsxref("Array.prototype.every()")}}、{{jsxref("Array.prototype.some()")}}、{{jsxref("Array.prototype.forEach()")}}、{{jsxref("Array.prototype.map()")}}、{{jsxref("Array.prototype.filter()")}}、{{jsxref("Array.prototype.reduce()")}}、{{jsxref("Array.prototype.reduceRight()")}}、{{jsxref("Array.prototype.find()")}}</li>
+  </ul>
+ </li>
+ <li>如果是 {{jsxref("Map")}} 與 {{jsxref("Set")}} 物件：
+  <ul>
+   <li>{{jsxref("Map.prototype.forEach()")}} 與 {{jsxref("Set.prototype.forEach()")}}</li>
+  </ul>
+ </li>
+</ul>
+
+<h2 id="實例">實例</h2>
+
+<h3 id="函式的名字打錯了">函式的名字打錯了</h3>
+
+<p>這種事太常發生了。下例就有個方法打錯：</p>
+
+<pre class="brush: js example-bad">var x = document.getElementByID("foo");
+// TypeError: document.getElementByID is not a function
+</pre>
+
+<p>該函式的正確名字為 <code>getElementByI<strong>d</strong></code>：</p>
+
+<pre class="brush: js example-good">var x = document.getElementById("foo");
+</pre>
+
+<h3 id="函式呼叫到錯誤的物件">函式呼叫到錯誤的物件</h3>
+
+<p>某些方法需要你提供回呼的函式，該函式只能作用於特定物件。以本例而言，我們使用的 {{jsxref("Array.prototype.map()")}} 就只能作用於 {{jsxref("Array")}} 物件。</p>
+
+<pre class="brush: js example-bad">var obj = { a: 13, b: 37, c: 42 };
+
+obj.map(function(num) {
+  return num * 2;
+});
+
+// TypeError: obj.map is not a function</pre>
+
+<p>請改用陣列：</p>
+
+<pre class="brush: js example-good">var numbers = [1, 4, 9];
+
+numbers.map(function(num) {
+  return num * 2;
+});
+
+// Array [ 2, 8, 18 ]
+</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><a href="/zh-TW/docs/Web/JavaScript/Reference/Functions">Functions</a></li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/not_defined/index.html b/files/zh-tw/web/javascript/reference/errors/not_defined/index.html
new file mode 100644
index 0000000000..fa79033977
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/not_defined/index.html
@@ -0,0 +1,67 @@
+---
+title: 'ReferenceError: "x" is not defined'
+slug: Web/JavaScript/Reference/Errors/Not_defined
+translation_of: Web/JavaScript/Reference/Errors/Not_defined
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">ReferenceError: "x" is not defined
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("ReferenceError")}}.</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>有個地方參照到不存在的變數了。這個變數需要宣告、或確定在目前腳本、或在 {{Glossary("scope")}} 裡可用。</p>
+
+<div class="note">
+<p><strong>注意：</strong>如果要使用函式庫（例如 jQuery）的話，請確定在你使用諸如 $ 這樣的函式庫變數前，就已載入完畢。把載入函式庫的 {{HTMLElement("script")}} 標籤，放在你使用的程式碼之前。
+</p>
+</div>
+
+<h2 id="實例">實例</h2>
+
+<h3 id="變數未宣告">變數未宣告</h3>
+
+<pre class="brush: js example-bad">foo.substring(1); // ReferenceError: foo is not defined
+</pre>
+
+<p>"foo" 變數在任何地方都沒被定義到。它需要字串使 {{jsxref("String.prototype.substring()")}} 得以運作。</p>
+
+<pre class="brush: js example-good">var foo = "bar";
+foo.substring(1); // "ar"</pre>
+
+<h3 id="作用域錯誤">作用域錯誤</h3>
+
+<p>A variable need to be available in the current context of execution. Variables defined inside a <a href="/en-US/docs/Web/JavaScript/Reference/Functions">function</a> cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function</p>
+
+<pre class="brush: js example-bad">function numbers () {
+  var num1 = 2,
+      num2 = 3;
+  return num1 + num2;
+}
+
+console.log(num1); // ReferenceError num1 is not defined.</pre>
+
+<p>However, a function can access all variables and functions defined inside the scope in which it is defined. In other words, a function defined in the global scope can access all variables defined in the global scope.</p>
+
+<pre class="brush: js example-good">var num1 = 2,
+    num2 = 3;
+
+function numbers () {
+  return num1 + num2;
+}
+
+console.log(num1); // 2</pre>
+
+<h2 id="參閱">參閱</h2>
+
+<ul>
+ <li>{{Glossary("Scope")}}</li>
+ <li><a href="/zh-TW/docs/Web/JavaScript/Guide/Grammar_and_types#Declaring_variables">Declaring variables in the JavaScript Guide</a></li>
+ <li><a href="/zh-TW/docs/Web/JavaScript/Guide/Functions#Function_scope/en-US/docs/">Function scope in the JavaScript Guide</a></li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/redeclared_parameter/index.html b/files/zh-tw/web/javascript/reference/errors/redeclared_parameter/index.html
new file mode 100644
index 0000000000..e9ba8cbbe0
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/redeclared_parameter/index.html
@@ -0,0 +1,57 @@
+---
+title: 'SyntaxError: redeclaration of formal parameter "x"'
+slug: Web/JavaScript/Reference/Errors/Redeclared_parameter
+translation_of: Web/JavaScript/Reference/Errors/Redeclared_parameter
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">SyntaxError: redeclaration of formal parameter "x" (Firefox)
+SyntaxError: Identifier "x" has already been declared (Chrome)
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("SyntaxError")}}</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>當相同的變數名作為函式的參數、接著又在函式體（function body）內用了 <code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/let">let</a></code> 重複宣告並指派時出現。在 JavaScript 裡面，不允許在相同的函式、或是作用域區塊（block scope）內重複宣告相同的 <code>let</code> 變數。</p>
+
+<h2 id="實例">實例</h2>
+
+<p>在這裡，「arg」變數的參數被重複宣告。</p>
+
+<pre class="brush: js example-bad">function f(arg) {
+  let arg = 'foo';
+}
+
+// SyntaxError: redeclaration of formal parameter "arg"
+</pre>
+
+<p>If you want to change the value of "arg" in the function body, you can do so, but you do not need to declare the same variable again. In other words: you can omit the <code>let</code> keyword. If you want to create a new variable, you need to rename it as conflicts with the function parameter already.</p>
+
+<pre class="brush: js example-good">function f(arg) {
+  arg = 'foo';
+}
+
+function f(arg) {
+  let bar = 'foo';
+}
+</pre>
+
+<h2 id="相容性註解">相容性註解</h2>
+
+<ul>
+ <li>在 Firefox 49 {{geckoRelease(49)}} 之前，這個錯誤被歸為 {{jsxref("TypeError")}}。 ({{bug(1275240)}})</li>
+</ul>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/let">let</a></code></li>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/const">const</a></code></li>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/var">var</a></code></li>
+ <li>在 <a href="/zh-TW/docs/Web/JavaScript/Guide">JavaScript 教學</a>內<a href="/zh-TW/docs/Web/JavaScript/Guide/Grammar_and_Types#Declarations">宣告變數</a> </li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/too_much_recursion/index.html b/files/zh-tw/web/javascript/reference/errors/too_much_recursion/index.html
new file mode 100644
index 0000000000..1708683ffa
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/too_much_recursion/index.html
@@ -0,0 +1,50 @@
+---
+title: 'InternalError: too much recursion'
+slug: Web/JavaScript/Reference/Errors/Too_much_recursion
+translation_of: Web/JavaScript/Reference/Errors/Too_much_recursion
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">InternalError: too much recursion
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("InternalError")}}</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>一個呼叫自己的函式稱為<em>遞迴函式</em>（recursive function）。在某些方面，遞迴和迴圈很像。它們都需要在指定條件（以避免無窮迴圈，或是本例的無窮遞迴）下，重複執行數次相同的程式碼。如果遞迴執行太多次、或成為無窮遞迴的話，JavaScript 就會出現這個錯誤。</p>
+
+<h2 id="實例">實例</h2>
+
+<p>以下的遞迴函式，會根據終止條件，而運行十次。</p>
+
+<pre class="brush: js">function loop(x) {
+  if (x &gt;= 10) // "x &gt;= 10" 是終止條件
+    return;
+  // do stuff
+  loop(x + 1); // 遞迴呼叫
+}
+loop(0);</pre>
+
+<p>如果把終止條件的次數設得太高，函式就不會運作了：</p>
+
+<pre class="brush: js example-bad">function loop(x) {
+  if (x &gt;= 1000000000000)
+    return;
+  // do stuff
+  loop(x + 1);
+}
+loop(0);
+
+// InternalError: too much recursion</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li>{{Glossary("Recursion")}}</li>
+ <li><a href="/zh-TW/docs/Web/JavaScript/Guide/Functions#Recursion">遞迴函式</a></li>
+</ul>
diff --git a/files/zh-tw/web/javascript/reference/errors/unexpected_type/index.html b/files/zh-tw/web/javascript/reference/errors/unexpected_type/index.html
new file mode 100644
index 0000000000..d7399baf5d
--- /dev/null
+++ b/files/zh-tw/web/javascript/reference/errors/unexpected_type/index.html
@@ -0,0 +1,49 @@
+---
+title: 'TypeError: "x" is (not) "y"'
+slug: Web/JavaScript/Reference/Errors/Unexpected_type
+translation_of: Web/JavaScript/Reference/Errors/Unexpected_type
+---
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("TypeError")}}</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>有一個意想不到的類型。這與 {{jsxref("undefined")}} 或 {{jsxref("null")}} 值經常發生。</p>
+
+<p>另外，某些方法，如 {{jsxref("Object.create()")}} 或 {{jsxref("Symbol.keyFor()")}} 要求特定類型，即必須提供。</p>
+
+<h2 id="實例">實例</h2>
+
+<h3 id="無效的情況下">無效的情況下</h3>
+
+<pre class="brush: js example-bad">// undefined 和 null 的情況下在其上的子方法不起作用
+var foo = undefined;
+foo.substring(1); // TypeError: foo is undefined
+
+var foo = null;
+foo.substring(1); // TypeError: foo is null
+
+
+// 某些方法可能要求特定類型
+var foo = {}
+Symbol.keyFor(foo); // TypeError: foo is not a symbol
+
+var foo = "bar"
+Object.create(foo); // TypeError: "foo" is not an object or null
+</pre>
+
+<h3 id="修復問題">修復問題</h3>
+
+<p>為了解決空指針 <code>undefined</code> 或 <code>null</code> 值，可以使用 <a href="/zh-TW/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a> 運算符，例如。 operator, for example.</p>
+
+<pre class="brush: js">if (typeof foo !== 'undefined') {
+  // 現在我們知道foo被定義，我們可以繼續進行。
+}</pre>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li>{{jsxref("undefined")}}</li>
+ <li>{{jsxref("null")}}</li>
+</ul>