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<div id="content_views" class="markdown_views prism-atom-one-dark"> <p>数据类型：number、string、boolean、null、undefined、object</p> <h4><a id="1_number__string_1" rel="nofollow"></a>1. number ==> string</h4> <p>+‘’ 、String(number) 、number.toString()</p> <pre><code>var a=12; var b=13; console.log(a+''+b,typeof(a+''+b)) /* 1213 string */ console.log(a.toString()+b,typeof(a.toString()+b)); /* 1213 string */ console.log(String(a)+11,typeof (String(a)+11)); /* 1211 string */</code></pre> <h4><a id="2_string__number_11" rel="nofollow"></a>2. string ==> number</h4> <p>Number()、+string、-string、parseFloat(s) 保留全部<br /> ~~string、parseInt(s) 只保留整数</p> <pre><code>var s1='12.9'; console.log(parseInt(s1)+1); /* 13 */ console.log(parseFloat(s1)+1); /* 13.9 */ console.log(Number(s1)+1); /* 13.9 */ var s3='12.9'; console.log(+s3+1); /* 13.9 */ console.log(-s3+1); /* 11.9 */ console.log(~~s3); /* 12 */</code></pre> <p>Number() 和 parseFloat() 区别：</p> <pre><code>var s2='12.9abc'; console.log(parseFloat(s2)); /* 12.9 */ console.log(Number(s2)); /* NaN ： not a number 非数字*/</code></pre> <p><strong>ps：不过NaN为number的一种，即typeof NaN为number</strong><br /> 注意NaN的特殊：</p> <pre><code>console.log(Number==Number); /* true */ console.log(null==null); /* true */ console.log(undefined==undefined); /* true */ console.log(NaN==NaN); /* false */ console.log(typeof NaN); /* number */</code></pre> <h4><a id="3_numberstring__boolean_44" rel="nofollow"></a>3. number、string ==> boolean</h4> <p>Boolean()、!!number/string</p> <pre><code>var a=0,b=1,s1='',s2='hello',s3=null,s4=undefined; console.log(Boolean(a)); /* false */ console.log(Boolean(b)); /* true */ console.log(Boolean(s1)); /* false */ console.log(Boolean(s2)); /* true */ console.log(Boolean(s3)); /* false */ console.log(Boolean(s4)); /* false */ console.log(!!b); /* true */ console.log(!!s4); /* false */</code></pre> <h4><a id="4_boolean_number_59" rel="nofollow"></a>4. boolean==> number</h4> <pre><code>console.log(false+1); /* 1 */ console.log(true+1); /* 2 */</code></pre> <h4><a id="5__65" rel="nofollow"></a>5. 注意</h4> <p>【有些想测试的还没测。。。感兴趣的可以测一下】</p> <pre><code>console.log(undefined); /* undefined */ console.log(null); /* null */ console.log(undefined==null); /* true */ console.log(undefined==false); /* false */ console.log(!!undefined==false); /* true */ console.log(NaN==NaN); /* false */</code></pre> <blockquote> <p>【解释为什么console.log(undefined<mark>null); /* true */】（有的说undefined</mark>null是因为null为object，而undefined为类型，是null一特殊存在，不过好像最初是这样用，现在摒弃了。。。但不确定这样说对不对。。。）<br /><strong>null</strong> 表示 “没有对象”，即该处不应该有值（典型用法：（1）作为函数的参数，表示该函数的参数不是对象；（2）作为对象原型链的终点。（这里不是很懂QAQ。。。））<br /><strong>undefined</strong>表示“缺少值“，即此处该有个值，但是没有定义（典型用法：（1）变量被声明了，但没有赋值时，就等于undefined；（2）调用函数时，应该提供的参数没有提供，该参数等于undefined；（3）对象没有赋值的属性，该属性的值为undefined；（4）函数没有返回值时，默认返回undefined）</p> </blockquote> </div> <div class="clearfix"></div>
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