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JS基础语法
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<p>JS基础语法</p>
<a id="more"></a>
<h2 id="一-JS注释">一.JS注释</h2>
<ul>
<li>和其他高级语言差不多, 分为:</li>
<li>多行注释: /**/</li>
<li>单行注释: //</li>
<li>注释中的代码不会被执行, 只能在源码中看见注释的内容</li>
</ul>
<h2 id="二-JS的变量与字面量">二.JS的变量与字面量</h2>
<p>变量的作用是给某一个值或对象标注名称, 方便复用, 变量也用来保存字面量</p>
<p>字面量: 可以直接使用, 不能更改其值, 像: 123、“Hello”、1.23等</p>
<ul>
<li>变量的声明: 使用<font style='color:red;'>var关键字声明一个变量</font>, 例: var a;</li>
<li>变量的赋值: 使用=为变量赋值, 例: a=123;</li>
<li>声明和赋值同时进行, 例, var a = 123;</li>
</ul>
<h2 id="三-JS标识符">三.JS标识符</h2>
<p>所谓标识符, 就是指变量、函数、属性的名字, 或函数的参数.要求:</p>
<ul>
<li>第一个字符必须是一个字母、下划线_或一个美元符号$.</li>
<li>不能以数字开头, 不能是关键字或保留字</li>
<li>其他字符可以是字母、下划线、美元符号或数字.</li>
<li>标识符一般采用驼峰命名法(规范): 首字母小写, 每个单词首字母大写其他字母小写, 例: xxxYyyZzz</li>
</ul>
<p>JS底层保存标识符是用Unicode编码的</p>
<p>命名时, 尽量见名知意, 遵守它的规则和行业规范.</p>
<h2 id="四-数据类型">四.数据类型</h2>
<ul>
<li>String(字符串型), 例: "Hello World"或’Hello World’</li>
<li>Number(数值型), 例: 123、1.23</li>
<li>Boolean(布尔型), 只有 true false两个取值</li>
<li>Null(null型), 只有一个取值: null, 用来表示一个对象为空; 用 typeof null;会返回object</li>
<li>Undefined(undefined型), 未定义, 当声明了一个变量而没赋值时, 变量的值就是undefined</li>
<li>上述五种基本类型之外的类型: Object</li>
</ul>
<p>String: 使用单或双引号括起来的整体, 但是单双引号不能嵌套. 了解一些常见的转义字符</p>
<p>Number: 正无穷(Infinity), 负无穷(-Infinity); Nut a Number(NaN)</p>
<p><font style='color:red;font-weight:bold;'>尽量避免用JS做一些高精度的计算问题</font></p>
<p>用typeof运算符可以检查变量的类型, 例:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">123</span>;</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> a);<span class="comment">//输出结果为:number</span></span><br><span class="line"><span class="keyword">var</span> b = <span class="built_in">Number</span>.MAX_VALUE + <span class="number">1</span>;</span><br><span class="line"><span class="built_in">console</span>.log(b);<span class="comment">//输出结果为:Infinity</span></span><br><span class="line"><span class="keyword">var</span> c = <span class="built_in">Number</span>.MIN_VALUE;<span class="comment">//JS能表示的大于零的最小值</span></span><br><span class="line"><span class="comment">//Number有它能表示的最大值, 为Number.MAX_VALUE;</span></span><br><span class="line"><span class="comment">//当计算结果超过它时, 输出结果为:Infinity(number型)即正无穷</span></span><br><span class="line"><span class="keyword">var</span> str = <span class="string">"123"</span> * <span class="string">"123"</span>;</span><br><span class="line"><span class="comment">//当计算结果非数字时, 会返回一个NaN(number型)</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> str);<span class="comment">//结果为:NaN</span></span><br></pre></td></tr></table></figure>
<h2 id="五-强制类型转换">五.强制类型转换</h2>
<ul>
<li>其他类型变量转换为String, 调用变量的toString()方法, 或调用String()函数</li>
<li>其他类型变量转换为Number, 调用Number()函数, 对于字符串还可调用parseInt()或parseFloat()函数</li>
<li>其他类型变量转换为Boolean, 调用Boolean()函数</li>
</ul>
<details>
<summary>
One More Thing 👇
</summary>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//其他类型变量转换为String</span></span><br><span class="line"><span class="keyword">var</span> a = <span class="number">123</span>;<span class="comment">//注意:当a = null;或a = undefined;时, 不能调用a.toString()方法</span></span><br><span class="line">a = a.toString();<span class="comment">//toString()方法, 无参数, 将a的内容变为string后返回</span></span><br><span class="line"><span class="keyword">var</span> b = <span class="literal">null</span>;</span><br><span class="line">b = <span class="built_in">String</span>(b);<span class="comment">//String()函数, 将要转换的变量作为参数传递给函数, 后函数将结果返回</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"a = "</span> + a);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> a);<span class="comment">//输出:string</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"b = "</span> + b);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> b);</span><br><span class="line"></span><br><span class="line"><span class="comment">//其他类型变量转换为Number</span></span><br><span class="line"><span class="keyword">var</span> str = <span class="string">"123"</span>;<span class="comment">//若字符串内容有非数字和空格, Number()函数会将内容转换为NaN</span></span><br><span class="line">str = <span class="built_in">Number</span>(str);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"str = "</span> + str);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> str);</span><br><span class="line"><span class="keyword">var</span> bool = <span class="literal">true</span>;<span class="comment">//true==>1 false==>0 null==>0 undefined==>NaN</span></span><br><span class="line">bool = <span class="built_in">Number</span>(bool);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"bool = "</span> + bool);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> bool);</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> px = <span class="string">"123px"</span>;</span><br><span class="line">px = <span class="built_in">parseInt</span>(px);<span class="comment">//从第一个字符开始转换, 直到遇到非空和非数字.parseFolat()函数类似</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"px = "</span> + px);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> px);</span><br><span class="line"><span class="comment">//JS中数字的进制, 0x开头表示16进制, 0开头表示8进制</span></span><br><span class="line">px = <span class="string">"070"</span>;</span><br><span class="line">px = <span class="built_in">parseInt</span>(px,<span class="number">10</span>);<span class="comment">//第二个参数, 表示以几进制进行解析</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"px = "</span> + px);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> px);</span><br><span class="line"></span><br><span class="line"><span class="comment">//其他类型变量转换为Boolean</span></span><br><span class="line"><span class="keyword">var</span> num = <span class="number">123</span>;<span class="comment">//数字只有0和NaN转换为false, 字符串只有空串转换为false</span></span><br><span class="line">num = <span class="built_in">Boolean</span>(num);<span class="comment">//null和undefined转换为false, 对象都转换为true</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"num = "</span> + num);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> num);</span><br><span class="line"></span><br><span class="line"><span class="comment">//这里说一个isNaN()函数, 判断一个数字是不是NaN, 若是返回true, 不是返回false</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="built_in">isNaN</span>(num));</span><br></pre></td></tr></table></figure>
</details>
<h2 id="六-运算符">六.运算符</h2>
<ul>
<li>算术运算符 + - * / ++(前置和后置) --(前置和后置) +(符号不变) -(符合取反) %(取模)</li>
<li>逻辑运算符 && || ! & |</li>
<li>关系运算符 > < >= <= == != === !==</li>
<li>赋值运算符 = += -= *= /= %=</li>
<li>条件运算符 ?:, 格式: 条件表达式?表达式1:表达式2; 若条件表达式为true, 整个表达式值为表达式1的值, 否则为表达式2的值.</li>
<li>位运算符 ^ & | ! (用于数值类型的操作)</li>
<li>typeof运算符 判断类型并以字符串形式返回</li>
</ul>
<h3 id="A-算术运算符">A.算术运算符:</h3>
<p>+的特殊性, 可以用来拼接字符串, 例:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">1</span> + <span class="number">2</span> + <span class="string">"3"</span>;<span class="comment">//33</span></span><br><span class="line"><span class="keyword">var</span> b = <span class="string">"1"</span> + <span class="number">2</span> + <span class="number">3</span>;<span class="comment">//123</span></span><br></pre></td></tr></table></figure>
<p>其他算数运算符在进行运算时, 会将操作数先转换为Number类型, 再进行运算, 并将结果以数值类型返回</p>
<p>前++: 先对原变量加1, 在使用该变量的值;后++: 先使用给变量的值, 再给变量加1;–类似</p>
<h3 id="B-逻辑运算符">B.逻辑运算符</h3>
<p>与 或 非, 运算时将其他类型转换为Boolean型, 再运算;</p>
<p>注意短路与(&&)和短路或(||):</p>
<p>短路与: 若第一个值为true, 则返回第二个值; 若第一个值为false, 则返回第一个值;</p>
<p>短路或: 若第一个值为true, 则返回第一个值; 若第一个值为false, 则返回第二个值;</p>
<h3 id="C-关系元算符">C.关系元算符</h3>
<p>比较两个字符串时, 比较的是字符串的字符编码(按位比较); 其他类型转换为数值类型比较; 比较结果为true或false, 以字符串形式返回比较的结果.</p>
<p>注意==判等时会做隐式类型转换, ===表示全等; != 与 !==一样</p>
<h3 id="D-赋值运算符">D.赋值运算符</h3>
<p>将运算符左边的表达式的值做一个操作, 然后将操作后的值赋值给右边的变量.</p>
<h2 id="七-Unicode编码表">七.Unicode编码表</h2>
<p>在JS的字符串中, 使用转义字符\u+四位16进制的编码表示Unicode编码</p>
<p>在HTML中, 使用 “&#10进制编码;” 表示Unicode编码.</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta"><!DOCTYPE html></span></span><br><span class="line"><span class="tag"><<span class="name">html</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">head</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">meta</span> <span class="attr">charset</span>=<span class="string">"utf-8"</span> /></span></span><br><span class="line"> <span class="tag"><<span class="name">title</span>></span>JS基础<span class="tag"></<span class="name">title</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">script</span> <span class="attr">type</span>=<span class="string">"text/javascript"</span>></span><span class="undefined"></span></span><br><span class="line"><span class="actionscript"> <span class="keyword">var</span> str = <span class="string">"\u2620"</span>;</span></span><br><span class="line"><span class="javascript"> <span class="built_in">console</span>.log(str);</span></span><br><span class="line"><span class="undefined"> </span><span class="tag"></<span class="name">script</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">head</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">h1</span>></span>&#9760;<span class="tag"></<span class="name">h1</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">body</span>></span></span><br><span class="line"><span class="tag"></<span class="name">html</span>></span></span><br></pre></td></tr></table></figure>
<p>注意:</p>
<ul>
<li>JS中严格区分大小写(HTML中不区分)</li>
<li>JS中一个分号代表一条命令的结束</li>
<li>JS会忽略多个空格与换行符, 因此我们可以用来格式化自己的代码</li>
<li>JS代码是一条条以;结尾的语句, 可用{}将语句分组, 一个{}也叫一个代码块</li>
</ul>
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<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#一-JS注释"><span class="toc-text">一.JS注释</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#二-JS的变量与字面量"><span class="toc-text">二.JS的变量与字面量</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#三-JS标识符"><span class="toc-text">三.JS标识符</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#四-数据类型"><span class="toc-text">四.数据类型</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#五-强制类型转换"><span class="toc-text">五.强制类型转换</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#六-运算符"><span class="toc-text">六.运算符</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#A-算术运算符"><span class="toc-text">A.算术运算符:</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#B-逻辑运算符"><span class="toc-text">B.逻辑运算符</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#C-关系元算符"><span class="toc-text">C.关系元算符</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#D-赋值运算符"><span class="toc-text">D.赋值运算符</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#七-Unicode编码表"><span class="toc-text">七.Unicode编码表</span></a></li></ol>
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