【javascript笔记】js基本数据类型<零>

不念不忘少年蓝@ 2022-08-09 09:55 9阅读 0赞

.记录一下以前没掌握的

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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> 变量</th> 
   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> 解释</th> 
   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> 示例</th> 
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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> <a href="https://developer.mozilla.org/en-US/docs/Glossary/String" title="String: In any computer programming language, a string is a sequence of characters used to represent text." style="margin:0px; padding:0px; border-width:0px 0px 1px; border-bottom-style:dashed; border-bottom-color:rgb(170,170,170); text-decoration:none" rel="nofollow">String</a></th> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)">字符串，一段文本。 要指示变量是字符串，你应该将它们用引号包裹起来。</td> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)"><code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">var myVariable = 'Bob';</code></td> 
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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> <a href="https://developer.mozilla.org/en-US/docs/Glossary/Number" title="Number: In JavaScript, Number is a numeric data type in the double-precision 64-bit floating point format (IEEE 754). In other programming languages different numeric types can exist, for examples: Integers, Floats, Doubles, or Bignums." style="margin:0px; padding:0px; border-width:0px 0px 1px; border-bottom-style:dashed; border-bottom-color:rgb(170,170,170); text-decoration:none" rel="nofollow">Number</a></th> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)">数字，一个数字。不用引号包围。是只有<span style="background-color:rgb(51,255,51)">一个单一的数据类型</span>，内部被表示为<span style="background-color:rgb(102,255,153)">64位的浮点数</span><br> ,和java的double一样. <span style="background-color:rgb(51,255,51)">NaN是一个特殊的数值.它不等于它自己.请不要使用== 和===来判断一个数字是否是Number,请使用 Number.isNaN() 或者isNaN方法</span></td> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)"><code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">var myVariable = 10;</code></td> 
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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> <a href="https://developer.mozilla.org/en-US/docs/Glossary/Boolean" title="Boolean: In computer science, a&nbsp;boolean&nbsp;is a logical data type that can have only the values&nbsp;true or false." style="margin:0px; padding:0px; border-width:0px 0px 1px; border-bottom-style:dashed; border-bottom-color:rgb(170,170,170); text-decoration:none" rel="nofollow">Boolean</a></th> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)">布尔型，一个 True/False （真 / 假）值。&nbsp;<code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">true</code>/<code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">false</code>&nbsp;是 JS 里的特殊关键字，不需要引号。</td> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)"><code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">var myVariable = true;</code></td> 
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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> <a href="https://developer.mozilla.org/en-US/docs/Glossary/Array" title="Array: An array is an ordered collection of data (either primitive or object). Based on its place in the array, each data item has a numeric index through which you can access the corresponding value.&nbsp;In JavaScript, arrays are also objects that can be manipulated with various methods." style="margin:0px; padding:0px; border-width:0px 0px 1px; border-bottom-style:dashed; border-bottom-color:rgb(170,170,170); text-decoration:none" rel="nofollow">Array</a></th> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)">数组，一种允许你存储多个值在一个引用里的结构。</td> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)"><code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">var myVariable = [1,'Bob','Steve',10];</code><br> 调用数组的元素只需:&nbsp;<code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">myVariable[0]</code>,<code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">myVariable[1]</code>, 等等.</td> 
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   <th style="margin:0px; padding:2px 8px 4px; border-width:2px; border-style:solid; border-color:rgb(255,255,255) rgb(255,255,255) rgb(212,221,228); font-style:inherit; font-family:'Open Sans Light',Helvetica,Arial,sans-serif; font-size:14px"> <a href="https://developer.mozilla.org/en-US/docs/Glossary/Object" title="Object: Object refers to a data structure containing data and instructions for working with the data. Objects sometimes refer to real-world things, for example a car or map object in a racing game. JavaScript, Java, C++,&nbsp;Python, and Ruby are examples of object-oriented programming languages." style="margin:0px; padding:0px; border-width:0px 0px 1px; border-bottom-style:dashed; border-bottom-color:rgb(170,170,170); text-decoration:none" rel="nofollow">Object</a></th> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)">对象，基本上 JavaScript 里的任何东西都是对象，而且都可以被储存在变量里。将这个记在脑子里。</td> 
   <td style="margin:0px; padding:6px 8px; border:2px solid rgb(255,255,255)"><code style="margin:0px; padding:0px; border:0px; font-style:inherit; font-weight:inherit; font-family:Consolas,'Liberation Mono',Courier,monospace">var myVariable = document.querySelector('h1');</code><br> 上面所有示例都是对象。</td> 
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undefined 是特殊的一类,typeof(undefined)                  输出结果为defined

null  在w3c里面被归纳为特殊的一类,但是 typeof(null) 输出为 object ，如何判断是否为null呢，判定变量 temp请看下面的代码：

if(temp&&typeof(temp)=="object"){
    
    //temp是一个数组或者对象
    
    }

那如何判断一个传进来的参数是数组呢,typeof运算符不能判断是对象或者数组,我们就需要判定他的构造函数,请看下面的代码:

if(temp&&typeof(temp)==='object'&&temp.constructor===Array){
     	//temp是一个数组
     }

上面的测试在不同的帧或者窗口创建的数组检测下会给出false,当数组有可能在其他帧中被创建时，下面的检测可能会更有效

if(temp&&typeof(temp)==='object'&&typeof(temp.length)==='number'&&!temp.propertyEnumerable('length')){
     	
     }

arguments数组不是一个数组,它是一个带有length成员元素的对象.上面的检测会将arguments数组识别为一个数组,有时候这是你希望得到的结果,尽管arguments不包含数组的方法.无论如何,如果propertyIsEumarable方法被覆盖,该检测仍然可能失败.

hasOwnProperty()  判断是否是实例属性

注意：仅仅通过判断值是否为  [undefined][]  还不足以检测一个属性是否存在，一个属性可能存在而其值恰好为 `undefined` 。

delete " xxx"                        删除实例属性

我们可以通过hasOwnProperty() 和in 来判断是否是原型属性