1+ <!DOCTYPE html> < html > < head > < title > 20170727</ title > < meta charset ='utf-8 '> < link href ='https://dn-maxiang.qbox.me/res-min/themes/marxico.css ' rel ='stylesheet '> < style > .note-content {font-family : 'Helvetica Neue' , Arial, 'Hiragino Sans GB' , STHeiti, 'Microsoft YaHei' , 'WenQuanYi Micro Hei' , SimSun, Song, sans-serif;}
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8+ < h4 id ="20170727 "> 20170727</ h4 >
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10+ < h5 id ="性能优化堆栈内存的销毁问题 "> 性能优化：堆栈内存的销毁问题</ h5 >
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12+ < p > 堆内存释放</ p >
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14+ < blockquote >
15+ < p > 如果有变量(函数名)占用了堆内存的地址，那么当前的堆内存则不能释放</ p >
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17+ < p > 如果当前堆内存的地址，没有被任何的东西所引用，当前的堆内存就没用了，浏览器会在空闲的时候清理掉这些没用的堆内存(谷歌浏览器)</ p >
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19+ < p > IE下的堆内存释放采用的是计数器机制，被一个变量占用，计数器累加1，如果之前的某个占用被移除，计数器减1；但是很多时候IE的计数器计数的时候出现问题，导致“内存泄漏”</ p >
20+ </ blockquote >
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24+ < pre class ="prettyprint hljs-dark "> < code class ="hljs actionscript "> < span class ="hljs-keyword "> var</ span > obj={name:< span class ="hljs-string "> '珠峰培训'</ span > };< span class ="hljs-comment "> //->obj=xxxfff000 此时的obj把堆内存占用了</ span > < br > < br > obj=< span class ="hljs-literal "> null</ span > ; < span class ="hljs-comment "> //->obj不占用堆内存了，浏览器在空闲的时候会销毁这个无用的堆内存</ span > < br > < br > < span class ="hljs-comment "> /*null:空对象指针，不指向任何的堆内存*/</ span > < br > </ code > </ pre >
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26+ < p > 栈内存(作用域)的释放</ p >
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28+ < blockquote >
29+ < p > 全局作用域：浏览器加载页面的时候形成全局作用域，在浏览器中把当前页面关闭的时候，全局作用域销毁</ p >
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31+ < p > 私有作用域：函数执行的时候会形成私有作用域，一般情况下，函数执行完成，形成的这个私有作用域立即释放销毁</ p >
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33+ < p > 特殊情况：当私有作用域中的某一个东西(一般指的都是私有作用域中开辟的那个堆内存)被作用域以外的变量给占用了，当前的私有作用域(栈内存)就不能销毁了；这个私有作用域不销毁，代表着它里面存储的私有变量也不会销毁了；</ p >
34+ </ blockquote >
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36+ < h5 id ="i和i的区别 "> i++和++i的区别</ h5 >
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38+ < p > 都是在自身基础上累加1，不同地方在于和其它值运算的时候，累加1在前还是在后</ p >
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40+ < blockquote >
41+ < p > i++：先运算，在累加 < br >
42+ ++i：先累加，在运算</ p >
43+ </ blockquote >
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45+ < pre class ="prettyprint hljs-dark "> < code class ="hljs xl "> var i=< span class ="hljs-number "> 10</ span > ;< br > < span class ="hljs-function "> < span class ="hljs-title "> console</ span > .< span class ="hljs-built_in "> log</ span > (5+i++); //-></ span > 先计算< span class ="hljs-number "> 5</ span > +i,计算完成后再把i累加< span class ="hljs-number "> 1</ span > =>< span class ="hljs-number "> 15</ span > i=< span class ="hljs-number "> 11</ span > < br > < span class ="hljs-function "> < span class ="hljs-title "> console</ span > .< span class ="hljs-built_in "> log</ span > (5+(i++)); //-></ span > 加上括号也是先运算在累加< br > < br > i=< span class ="hljs-number "> 10</ span > ;< br > < span class ="hljs-function "> < span class ="hljs-title "> console</ span > .< span class ="hljs-built_in "> log</ span > (5+(++i)); //-></ span > 先让i累加< span class ="hljs-number "> 1</ span > ,把累加后的结果和< span class ="hljs-number "> 5</ span > 进行运算 i=< span class ="hljs-number "> 11</ span > =>< span class ="hljs-number "> 16</ span > < br > </ code > </ pre >
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47+ < p > 思考题</ p >
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51+ < pre class ="prettyprint hljs-dark "> < code class ="hljs matlab "> var < span class ="hljs-built_in "> i</ span > =< span class ="hljs-number "> 4</ span > ;< br > var res=< span class ="hljs-number "> 5</ span > +(++< span class ="hljs-built_in "> i</ span > )+(< span class ="hljs-built_in "> i</ span > ++)+(< span class ="hljs-built_in "> i</ span > ++)+(++< span class ="hljs-built_in "> i</ span > );< br > console.< span class ="hljs-built_in "> log</ span > (res,< span class ="hljs-built_in "> i</ span > );< br > </ code > </ pre >
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53+ < h5 id ="闭包的作用 "> 闭包的作用</ h5 >
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55+ < blockquote >
56+ < p > 你了解过闭包么？ (易车面试题)</ p >
57+ </ blockquote >
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59+ < p > 目前外界普遍认为“形成一个不销毁的私有作用域”才是闭包</ p >
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63+ < pre class ="prettyprint hljs-dark "> < code class ="hljs javascript "> < span class ="hljs-keyword "> var</ span > fn=(< span class ="hljs-function "> < span class ="hljs-keyword "> function</ span > (< span class ="hljs-params "> </ span > )</ span > {< br > < span class ="hljs-keyword "> var</ span > n=< span class ="hljs-number "> 12</ span > ;< br > < span class ="hljs-keyword "> return</ span > < span class ="hljs-function "> < span class ="hljs-keyword "> function</ span > (< span class ="hljs-params "> </ span > )</ span > {< br > < span class ="hljs-built_in "> console</ span > .log(++n);< br > }< br > })();< br > </ code > </ pre >
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65+ < p > 闭包的作用:</ p >
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67+ < ul > < li > < p > 保护里面的私有变量不受外界干扰(里面的变量和全局变量没关系,防止全局变量污染)</ p >
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69+ < pre class ="prettyprint hljs-dark "> < code class ="hljs javascript "> < span class ="hljs-comment "> //->例如:我们封装类库或者组件插件的时候，为了防止和全局变量冲突，我们都使用闭包把代码包裹起来(jQuery就是这样处理的)</ span > < br > ;(< span class ="hljs-function "> < span class ="hljs-keyword "> function</ span > (< span class ="hljs-params "> </ span > )</ span > {< br > < span class ="hljs-keyword "> var</ span > fn=< span class ="hljs-literal "> null</ span > ; < br > < span class ="hljs-keyword "> var</ span > jQuery=< span class ="hljs-function "> < span class ="hljs-keyword "> function</ span > (< span class ="hljs-params "> </ span > )</ span > {< br > ...< br > }< span class ="hljs-comment "> //->jQuery=xxxfff000</ span > < br > < br > < span class ="hljs-built_in "> window</ span > .jQuery=< span class ="hljs-built_in "> window</ span > .$=jQuery;< span class ="hljs-comment "> //->给全局增加了一个jQuery的属性和$的属性，这样在外面就可以使用jQuery或者$了</ span > < br > })();< br > < br > < span class ="hljs-keyword "> var</ span > fn=< span class ="hljs-literal "> undefined</ span > ;< span class ="hljs-comment "> //->和闭包中的fn没关系,防止了冲突和污染</ span > < br > $()< span class ="hljs-comment "> //->$=xxxfff000</ span > < br > </ code > </ pre > </ li >
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