blob: 62a09df0156c5c69cef4b17daaeda717bfcfaefb (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
|
---
title: 區塊
slug: Web/JavaScript/Reference/Statements/block
translation_of: Web/JavaScript/Reference/Statements/block
---
<p>{{jsSidebar("Statements")}}</p>
<h2 id="總覽">總覽</h2>
<p>區塊陳述用來組合零個或多個陳述。我們使用一對大括號 { } 以界定區塊。</p>
<table class="standard-table">
<tbody>
<tr>
<td class="header" colspan="2">陳述句</td>
</tr>
<tr>
<td>Implemented in</td>
<td>JavaScript 1.0</td>
</tr>
<tr>
<td>ECMAScript edition</td>
<td>ECMA-262 1st edition</td>
</tr>
</tbody>
</table>
<h2 id="語法">語法</h2>
<pre class="syntaxbox">{
<var>陳述_1</var>
<var>陳述_2</var>
...
<var>陳述_n</var>
}
</pre>
<h3 id="參數">參數</h3>
<dl>
<dt><code>陳述_1</code>, <code>陳述_2</code>, <code>陳述_n</code></dt>
<dd>區塊陳述中的陳述句群。</dd>
</dl>
<h2 id="說明">說明</h2>
<p>區塊陳述通常配合流程控制陳述（如 <code>if</code>、<code>for</code>、<code>while</code>）一併使用。</p>
<h4 id="var"><code>var</code></h4>
<p>使用<code>var</code>區塊中定義的變數，其存取範圍是整個整個函式或是腳本，即為Execution Context的範圍中。</p>
<pre class="brush: js">var x = 1;
{
var x = 2;
}
alert(x); // outputs 2
</pre>
<p>輸出結果是 2。因為var是宣告於整個腳本範圍中。</p>
<h4 id="let_和_const"><code>let </code>和 <code>const</code></h4>
<p>當使用<code>let</code>或是<code>const</code>進行宣告時，其存取範圍是只有本身定義的區塊中。</p>
<pre class="brush: js line-numbers language-js"><code class="language-js"><span class="keyword token">let</span> x <span class="operator token">=</span> <span class="number token">1</span><span class="punctuation token">;</span>
<span class="punctuation token">{</span>
<span class="keyword token">let</span> x <span class="operator token">=</span> <span class="number token">2</span><span class="punctuation token">;</span>
<span class="punctuation token">}</span>
console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span>x<span class="punctuation token">)</span><span class="punctuation token">;</span> <span class="comment token">// logs 1</span></code></pre>
<h4 id="function"><code>function</code></h4>
<p>當function被呼叫時，會建立此function的Execution Context，因此在function區塊使用<code>var</code>整個function區塊中都可對其進行存取。</p>
<pre class="brush: js">function foo() {
{
var a = 'var';
{
let a = 'let';
console.log(a); // let
}
}
console.log(a); // var
}
foo();</pre>
|
