---
title: 陳述式與宣告
slug: Web/JavaScript/Reference/Statements
tags:
  - JavaScript
  - Reference
  - statements
translation_of: Web/JavaScript/Reference/Statements
---
<div>{{jsSidebar("Statements")}}</div>

<p>JavaScript 應用程式由適當的陳述式組成。一個單一的陳述式可以跨用好幾行。 多個陳述式也可以藉由分號分隔來寫在同一行。 這不是一個關鍵字，而是一群關鍵字。</p>

<h2 id="陳述式與宣告分類">陳述式與宣告分類</h2>

<p>For an alphabetical listing see the sidebar on the left.</p>

<h3 id="流程控制">流程控制</h3>

<dl>
 <dt>{{jsxref("Statements/block", "Block")}}</dt>
 <dd>A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.</dd>
 <dt>{{jsxref("Statements/break", "break")}}</dt>
 <dd>中斷當下的迴圈、條件判斷（switch）或是標籤（label）陳述式，並將程式流程轉到被中斷陳述式後的陳述式。</dd>
 <dt>{{jsxref("Statements/continue", "continue")}}</dt>
 <dd>Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.</dd>
 <dt>{{jsxref("Statements/Empty", "Empty")}}</dt>
 <dd>An empty statement is used to provide no statement, although the JavaScript syntax would expect one.</dd>
 <dt>{{jsxref("Statements/if...else", "if...else")}}</dt>
 <dd>當特定的條件為真時執行一段陳述式，若為假則另一段陳述式就會被執行。</dd>
 <dt>{{jsxref("Statements/switch", "switch")}}</dt>
 <dd>Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.</dd>
 <dt>{{jsxref("Statements/throw", "throw")}}</dt>
 <dd>Throws a user-defined exception.</dd>
 <dt>{{jsxref("Statements/try...catch", "try...catch")}}</dt>
 <dd>Marks a block of statements to try, and specifies a response, should an exception be thrown.</dd>
</dl>

<h3 id="宣告">宣告</h3>

<dl>
 <dt>{{jsxref("Statements/var", "var")}}</dt>
 <dd>Declares a variable, optionally initializing it to a value.</dd>
 <dt>{{jsxref("Statements/let", "let")}}</dt>
 <dd>Declares a block scope local variable, optionally initializing it to a value.</dd>
 <dt>{{jsxref("Statements/const", "const")}}</dt>
 <dd>Declares a read-only named constant.</dd>
</dl>

<h3 id="函數與類別（Class）">函數與類別（Class）</h3>

<dl>
 <dt>{{jsxref("Statements/function", "function")}}</dt>
 <dd>用指定的變數宣告一個函數</dd>
 <dt>{{jsxref("Statements/function*", "function*")}}</dt>
 <dd>Generators functions enable writing <a href="/en-US/docs/Web/JavaScript/Guide/The_Iterator_protocol">iterators</a> more easily.</dd>
 <dt>{{jsxref("Statements/async_function", "async function")}}</dt>
 <dd>Declares an async function with the specified parameters.</dd>
 <dt>{{jsxref("Statements/return", "return")}}</dt>
 <dd>Specifies the value to be returned by a function.</dd>
 <dt>{{jsxref("Statements/class", "class")}}</dt>
 <dd>Declares a class.</dd>
</dl>

<h3 id="迭代（Iteration）">迭代（Iteration）</h3>

<dl>
 <dt>{{jsxref("Statements/do...while", "do...while")}}</dt>
 <dd>Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.</dd>
 <dt>{{jsxref("Statements/for", "for")}}</dt>
 <dd>Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.</dd>
 <dt>{{deprecated_inline}} {{non-standard_inline()}} {{jsxref("Statements/for_each...in", "for each...in")}}</dt>
 <dd>Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.</dd>
 <dt>{{jsxref("Statements/for...in", "for...in")}}</dt>
 <dd>Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.</dd>
 <dt>{{jsxref("Statements/for...of", "for...of")}}</dt>
 <dd>Iterates over iterable objects (including {{jsxref("Global_Objects/Array","arrays","","true")}}, array-like objects, <a href="/en-US/docs/JavaScript/Guide/Iterators_and_Generators">iterators and generators</a>), invoking a custom iteration hook with statements to be executed for the value of each distinct property.</dd>
 <dt>{{jsxref("Statements/while", "while")}}</dt>
 <dd>Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.</dd>
</dl>

<h3 id="其他">其他</h3>

<dl>
 <dt>{{jsxref("Statements/debugger", "debugger")}}</dt>
 <dd>Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.</dd>
 <dt>{{jsxref("Statements/export", "export")}}</dt>
 <dd>Used to export functions to make them available for imports in external modules, another scripts.</dd>
 <dt>{{jsxref("Statements/import", "import")}}</dt>
 <dd>Used to import functions exported from an external module, another script.</dd>
 <dt>{{jsxref("Statements/label", "label")}}</dt>
 <dd>Provides a statement with an identifier that you can refer to using a <code>break</code> or <code>continue</code> statement.</dd>
</dl>

<dl>
 <dt>{{deprecated_inline}} {{jsxref("Statements/with", "with")}}</dt>
 <dd>Extends the scope chain for a statement.</dd>
</dl>

<h2 id="規範">規範</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">技術規格</th>
   <th scope="col">狀態</th>
   <th scope="col">備註</th>
  </tr>
  <tr>
   <td>{{SpecName('ES1', '#sec-12', 'Statements')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition</td>
  </tr>
  <tr>
   <td>{{SpecName('ES3', '#sec-12', 'Statements')}}</td>
   <td>{{Spec2('ES3')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-12', 'Statements')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-ecmascript-language-statements-and-declarations', 'ECMAScript Language: Statements and Declarations')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>New: function*, let, for...of, yield, class</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-ecmascript-language-statements-and-declarations', 'ECMAScript Language: Statements and Declarations')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td></td>
  </tr>
 </tbody>
</table>

<h2 id="瀏覽器相容性">瀏覽器相容性</h2>



<p>{{Compat("javascript.statements")}}</p>

<h2 id="參見">參見</h2>

<ul>
 <li><a href="/docs/Web/JavaScript/Reference/Operators">運算式與運算子</a></li>
</ul>