-
Notifications
You must be signed in to change notification settings - Fork 0
/
2018-05-14-scopeChain.html
1261 lines (686 loc) · 46.1 KB
/
2018-05-14-scopeChain.html
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
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html class="theme-next mist use-motion" lang="zh-Hans">
<head>
<meta charset="UTF-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1"/>
<meta name="theme-color" content="#222">
<script src="/lib/pace/pace.min.js?v=1.0.2"></script>
<link href="/lib/pace/pace-theme-minimal.min.css?v=1.0.2" rel="stylesheet">
<meta http-equiv="Cache-Control" content="no-transform" />
<meta http-equiv="Cache-Control" content="no-siteapp" />
<link href="/lib/fancybox/source/jquery.fancybox.css?v=2.1.5" rel="stylesheet" type="text/css" />
<link href="/lib/font-awesome/css/font-awesome.min.css?v=4.6.2" rel="stylesheet" type="text/css" />
<link href="/css/main.css?v=5.1.4" rel="stylesheet" type="text/css" />
<link rel="apple-touch-icon" sizes="180x180" href="/images/apple-touch-icon-next.png?v=5.1.4">
<link rel="icon" type="image/png" sizes="32x32" href="/images/favicon-32x32-next.png?v=5.1.4">
<link rel="icon" type="image/png" sizes="16x16" href="/images/favicon-16x16-next.png?v=5.1.4">
<link rel="mask-icon" href="/images/logo.svg?v=5.1.4" color="#222">
<meta name="keywords" content="js," />
<link rel="alternate" href="/atom.xml" title="赖同学" type="application/atom+xml" />
<meta name="description" content="js的作用域作用域作用：访问、操作、调用..域：区域、范围、空间作用域就是变量和函数的可访问范围，或者说是变量或者函数起作用的区域 1.js函数的作用域函数内的区域，就是这个函数的作用域，变量和函数在这个区域内都可以访问操作。最外层函数外的区域叫全局作用域，函数内的区域叫局部作用域 2.js变量的作用域在源代码中变量所在的区域就是这个变量的作用域。变量在这个区域内可以被访问。在全局作用域上定义">
<meta name="keywords" content="js">
<meta property="og:type" content="article">
<meta property="og:title" content="js作用域">
<meta property="og:url" content="http://laibh.top/2018-05-14-scopeChain.html">
<meta property="og:site_name" content="赖同学">
<meta property="og:description" content="js的作用域作用域作用：访问、操作、调用..域：区域、范围、空间作用域就是变量和函数的可访问范围，或者说是变量或者函数起作用的区域 1.js函数的作用域函数内的区域，就是这个函数的作用域，变量和函数在这个区域内都可以访问操作。最外层函数外的区域叫全局作用域，函数内的区域叫局部作用域 2.js变量的作用域在源代码中变量所在的区域就是这个变量的作用域。变量在这个区域内可以被访问。在全局作用域上定义">
<meta property="og:locale" content="zh-Hans">
<meta property="og:updated_time" content="2022-03-04T10:00:38.452Z">
<meta name="twitter:card" content="summary">
<meta name="twitter:title" content="js作用域">
<meta name="twitter:description" content="js的作用域作用域作用：访问、操作、调用..域：区域、范围、空间作用域就是变量和函数的可访问范围，或者说是变量或者函数起作用的区域 1.js函数的作用域函数内的区域，就是这个函数的作用域，变量和函数在这个区域内都可以访问操作。最外层函数外的区域叫全局作用域，函数内的区域叫局部作用域 2.js变量的作用域在源代码中变量所在的区域就是这个变量的作用域。变量在这个区域内可以被访问。在全局作用域上定义">
<script type="text/javascript" id="hexo.configurations">
var NexT = window.NexT || {};
var CONFIG = {
root: '/',
scheme: 'Mist',
version: '5.1.4',
sidebar: {"position":"left","display":"post","offset":12,"b2t":false,"scrollpercent":true,"onmobile":false},
fancybox: true,
tabs: true,
motion: {"enable":true,"async":false,"transition":{"post_block":"fadeIn","post_header":"slideDownIn","post_body":"slideDownIn","coll_header":"slideLeftIn","sidebar":"slideUpIn"}},
duoshuo: {
userId: '0',
author: '博主'
},
algolia: {
applicationID: '1YNH8Y3MP9',
apiKey: '61c189facf700193dfcbb902369ce227',
indexName: 'MyBlog',
hits: {"per_page":10},
labels: {"input_placeholder":"想要找些什么呢","hits_empty":"${query} 没有被找到，再试试","hits_stats":"在 ${time} ms 查找了${hits}个结果"}
}
};
</script>
<link rel="canonical" href="http://laibh.top/2018-05-14-scopeChain.html"/>
<title>js作用域 | 赖同学</title>
</head>
<body itemscope itemtype="http://schema.org/WebPage" lang="zh-Hans">
<div class="container sidebar-position-left page-post-detail">
<div class="headband"></div>
<header id="header" class="header" itemscope itemtype="http://schema.org/WPHeader">
<div class="header-inner"><div class="site-brand-wrapper">
<div class="site-meta ">
<div class="custom-logo-site-title">
<a href="/" class="brand" rel="start">
<span class="logo-line-before"><i></i></span>
<span class="site-title">赖同学</span>
<span class="logo-line-after"><i></i></span>
</a>
</div>
<h1 class="site-subtitle" itemprop="description"></h1>
</div>
<div class="site-nav-toggle">
<button>
<span class="btn-bar"></span>
<span class="btn-bar"></span>
<span class="btn-bar"></span>
</button>
</div>
</div>
<nav class="site-nav">
<ul id="menu" class="menu">
<li class="menu-item menu-item-home">
<a href="/" rel="section">
<i class="menu-item-icon fa fa-fw fa-home"></i> <br />
首页
</a>
</li>
<li class="menu-item menu-item-tags">
<a href="/tags/" rel="section">
<i class="menu-item-icon fa fa-fw fa-tags"></i> <br />
标签
</a>
</li>
<li class="menu-item menu-item-categories">
<a href="/categories/" rel="section">
<i class="menu-item-icon fa fa-fw fa-th"></i> <br />
分类
</a>
</li>
<li class="menu-item menu-item-archives">
<a href="/archives/" rel="section">
<i class="menu-item-icon fa fa-fw fa-archive"></i> <br />
归档
</a>
</li>
<li class="menu-item menu-item-sitemap">
<a href="/sitemap.xml" rel="section">
<i class="menu-item-icon fa fa-fw fa-sitemap"></i> <br />
站点地图
</a>
</li>
<li class="menu-item menu-item-guestbook">
<a href="/guestbook" rel="section">
<i class="menu-item-icon fa fa-fw fa-comment"></i> <br />
留言
</a>
</li>
<li class="menu-item menu-item-search">
<a href="javascript:;" class="popup-trigger">
<i class="menu-item-icon fa fa-search fa-fw"></i> <br />
搜索
</a>
</li>
</ul>
<div class="site-search">
<div class="algolia-popup popup search-popup">
<div class="algolia-search">
<div class="algolia-search-input-icon">
<i class="fa fa-search"></i>
</div>
<div class="algolia-search-input" id="algolia-search-input"></div>
</div>
<div class="algolia-results">
<div id="algolia-stats"></div>
<div id="algolia-hits"></div>
<div id="algolia-pagination" class="algolia-pagination"></div>
</div>
<span class="popup-btn-close">
<i class="fa fa-times-circle"></i>
</span>
</div>
</div>
</nav>
</div>
</header>
<main id="main" class="main">
<div class="main-inner">
<div class="content-wrap">
<div id="content" class="content">
<div id="posts" class="posts-expand">
<article class="post post-type-normal" itemscope itemtype="http://schema.org/Article">
<div class="post-block">
<link itemprop="mainEntityOfPage" href="http://laibh.top/2018-05-14-scopeChain.html">
<span hidden itemprop="author" itemscope itemtype="http://schema.org/Person">
<meta itemprop="name" content="赖彬鸿">
<meta itemprop="description" content="">
<meta itemprop="image" content="/images/myPhoto.jpg">
</span>
<span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization">
<meta itemprop="name" content="赖同学">
</span>
<header class="post-header">
<h2 class="post-title" itemprop="name headline">js作用域</h2>
<div class="post-meta">
<span class="post-time">
<span class="post-meta-item-icon">
<i class="fa fa-calendar-o"></i>
</span>
<span class="post-meta-item-text">发表于</span>
<time title="创建于" itemprop="dateCreated datePublished" datetime="2018-05-14T17:54:12+08:00">
2018-05-14
</time>
<span class="post-meta-divider">|</span>
<span class="post-meta-item-icon">
<i class="fa fa-calendar-check-o"></i>
</span>
<span class="post-meta-item-text">更新于:</span>
<time title="更新于" itemprop="dateModified" datetime="2022-03-04T18:00:38+08:00">
2022-03-04
</time>
</span>
<span class="post-category" >
<span class="post-meta-divider">|</span>
<span class="post-meta-item-icon">
<i class="fa fa-folder-o"></i>
</span>
<span class="post-meta-item-text">分类于</span>
<span itemprop="about" itemscope itemtype="http://schema.org/Thing">
<a href="/categories/javaScript相关/" itemprop="url" rel="index">
<span itemprop="name">javaScript相关</span>
</a>
</span>
</span>
<span id="/2018-05-14-scopeChain.html" class="leancloud_visitors" data-flag-title="js作用域">
<span class="post-meta-divider">|</span>
<span class="post-meta-item-icon">
<i class="fa fa-eye"></i>
</span>
<span class="post-meta-item-text">阅读次数:</span>
<span class="leancloud-visitors-count"></span>
</span>
<div class="post-wordcount">
<span class="post-meta-item-icon">
<i class="fa fa-file-word-o"></i>
</span>
<span class="post-meta-item-text">字数统计:</span>
<span title="字数统计">
1,415
</span>
</div>
</div>
</header>
<div class="post-body" itemprop="articleBody">
<h1 id="js的作用域"><a href="#js的作用域" class="headerlink" title="js的作用域"></a>js的作用域</h1><h3 id="作用域"><a href="#作用域" class="headerlink" title="作用域"></a>作用域</h3><p>作用：访问、操作、调用..<br>域：区域、范围、空间<br>作用域就是变量和函数的可访问范围，或者说是变量或者函数起作用的区域 </p>
<p>1.js函数的作用域<br>函数内的区域，就是这个函数的作用域，变量和函数在这个区域内都可以访问操作。最外层函数外的区域叫全局作用域，函数内的区域叫局部作用域</p>
<p>2.js变量的作用域<br>在源代码中变量所在的区域就是这个变量的作用域。变量在这个区域内可以被访问。在全局作用域上定义的变量叫做全局变量，在函数内部定义的变量叫做局部变量。</p>
<p>全局变量与局部变量<br>① 全局作用域（Global Scope）<br>最外层函数或者最外层函数定义的变量具有全局作用域</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> outer = <span class="string">'outer'</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(outer);</span><br><span class="line">}</span><br><span class="line">fn(); <span class="comment">//outer</span></span><br></pre></td></tr></table></figure>
<p>② 所有未定义直接赋值的变量自动声明为全局作用域 </p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>{</span><br><span class="line"> inner= <span class="string">'inner'</span>;</span><br><span class="line"> </span><br><span class="line">}</span><br><span class="line">fn();</span><br><span class="line"><span class="built_in">console</span>.log(inner); <span class="comment">//inner</span></span><br></pre></td></tr></table></figure>
<p>③ 所有window对象的属性拥有全局作用域 </p>
<p>局部作用域（Local Scope）<br>局部作用域一般只在固定的代码片段可以访问到，对于函数外部是无法访问的,最常见的例如函数内部 </p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> inner= <span class="string">'inner'</span>;</span><br><span class="line"> </span><br><span class="line">}</span><br><span class="line">fn();</span><br><span class="line"><span class="built_in">console</span>.log(inner); <span class="comment">//undefined</span></span><br></pre></td></tr></table></figure>
<p>注意：只要函数内定义了一个局部变量，函数在解析的时候都会将这个变量提前声明</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> global = <span class="string">'global'</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(global); <span class="comment">//undefined</span></span><br><span class="line"> <span class="keyword">var</span> global = <span class="string">'newGlobal'</span>; </span><br><span class="line"> <span class="built_in">console</span>.log(global); <span class="comment">//newGlobal</span></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.log(global); <span class="comment">//global</span></span><br></pre></td></tr></table></figure>
<p>相当于</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> global = <span class="string">'global'</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> global; <span class="comment">//提前声明了局部变量；</span></span><br><span class="line"> <span class="built_in">console</span>.log(global); <span class="comment">//undefined</span></span><br><span class="line"> <span class="keyword">var</span> global = <span class="string">'newGlobal'</span>; </span><br><span class="line"> <span class="built_in">console</span>.log(global); <span class="comment">//newGlobal</span></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.log(global); <span class="comment">//global</span></span><br></pre></td></tr></table></figure>
<h3 id="执行环境（Execution-Contexts）"><a href="#执行环境（Execution-Contexts）" class="headerlink" title="执行环境（Execution Contexts）"></a>执行环境（Execution Contexts）</h3><p>也可以翻译为执行上下文，当解释器进入到ECMAScript的可执行代码，解释器就进入一个执行环境，活动的执行环境组成了逻辑上的一个栈，在这个逻辑栈顶部的执行环境是当前运行的执行环境。<br>js为每一个执行环境关联了一个变量对象，环境中定义的所有变量和函数都保存在这个对象中。js的执行顺序是根据函数的调用来决定的，当一个函数的变量对象弹出，把控制权交给了之前的执行环境变量对象。 </p>
<p>变量对象(Variable Object)、活动对象(Activation Object)和Arguments对象(Arguments Object)当前执行环境的变量对象始终在作用域链的第0位。 </p>
<p>解析器处理代码时的两个阶段：</p>
<p>我们都知道javascript解析器是一段一段解析处理代码的，为什么？这就要涉及解析器处理代码时的两个阶段，解析代码和执行代码。</p>
<p>当解析器进入执行环境时，变量对象就会添加执行环境中声明的变量和函数作为它的属性，这就意味着变量和函数在声明之前已经可用，变量值为undefined，这就是变量和函数声明提升(Hoisting)的原因，与此同时作用域链和this确定，此过程为解析阶段，俗称预解析。接着解析器开始执行代码，为变量添加相应值的引用，得到执行结果，此过程为执行阶段。</p>
<h3 id="作用域链（Scope-Chain）"><a href="#作用域链（Scope-Chain）" class="headerlink" title="作用域链（Scope Chain）"></a>作用域链（Scope Chain）</h3><p>作用域链是js内部中一种变量、函数查找机制，它决定了变量和函数的作用范围，即作用域。函数对象和其他对象一样，拥有可以通过代码访问的属性和一系列仅供js引擎访问的内部属性。其中一个内部属性就是scope，该内部属性包含了函数被创建的作用域中对象的集合，这个集合就是被称为函数的作用域链，决定了那些数据能被函数访问。</p>
<p>在函数执行过程中，没遇到一个变量，都会经历一次标识符解析过程以决定从哪里获取和存储数据。该过程从作用域链头部，也就是从活动对象开始搜索，查找同名的标识符，如果找到了就使用这个标识符对应的变量，如果没找到继续搜索作用域链中的下一个对象，如果搜索完所有对象都未找到，则认为该标识符未定义。函数执行过程中，每个标识符都要经历这样的搜索过程。</p>
<p>四、作用域链和代码优化<br>从作用域链的结构可以看出，在运行期上下文的作用域链中，标识符所在的位置越深，读写速度就会越慢。如上图所示，因为全局变量总是存在于运行期上下文作用域链的最末端，因此在标识符解析的时候，查找全局变量是最慢的。所以，在编写代码的时候应尽量少使用全局变量，尽可能使用局部变量。一个好的经验法则是：如果一个跨作用域的对象被引用了一次以上，则先把它存储到局部变量里再使用</p>
<p>优化前：</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">changeColor</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">document</span>.getElementById(<span class="string">"btnChange"</span>).onclick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">document</span>.getElementById(<span class="string">"targetCanvas"</span>).style.backgroundColor=<span class="string">"red"</span>;</span><br><span class="line"> };</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>优化后：</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">changeColor</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> doc=<span class="built_in">document</span>;</span><br><span class="line"> doc.getElementById(<span class="string">"btnChange"</span>).onclick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> doc.getElementById(<span class="string">"targetCanvas"</span>).style.backgroundColor=<span class="string">"red"</span>;</span><br><span class="line"> };</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</div>
<div>
<ul class="post-copyright">
<li class="post-copyright-author">
<strong>本文作者：</strong>
赖彬鸿
</li>
<li class="post-copyright-link">
<strong>本文链接：</strong>
<a href="http://laibh.top/2018-05-14-scopeChain.html" title="js作用域">http://laibh.top/2018-05-14-scopeChain.html</a>
</li>
<li class="post-copyright-license">
<strong>版权声明： </strong>
本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/3.0/" rel="external nofollow" target="_blank">CC BY-NC-SA 3.0</a> 许可协议。转载请注明出处！
</li>
</ul>
</div>
<footer class="post-footer">
<div class="post-tags">
<a href="/tags/js/" <i class="fa fa-tag"></i> js</a>
</div>
<div class="post-nav">
<div class="post-nav-next post-nav-item">
<a href="/2018-05-13-saveStorage.html" rel="next" title="Cookie LocalStorage sessionStorage">
<i class="fa fa-chevron-left"></i> Cookie LocalStorage sessionStorage
</a>
</div>
<span class="post-nav-divider"></span>
<div class="post-nav-prev post-nav-item">
<a href="/2018-05-15-eventDelegation.html" rel="prev" title="事件委派">
事件委派 <i class="fa fa-chevron-right"></i>
</a>
</div>
</div>
</footer>
</div>
</article>
<div class="post-spread">
<script>
window._bd_share_config = {
"common": {
"bdText": "",
"bdMini": "1",
"bdMiniList": false,
"bdPic": ""
},
"image": {
"viewList": ["tsina", "douban", "sqq", "qzone", "weixin", "twi", "fbook"],
"viewText": "分享到：",
"viewSize": "16"
},
"slide": {
"bdImg": "5",
"bdPos": "left",
"bdTop": "100"
}
}
</script>
<script>
with(document)0[(getElementsByTagName('head')[0]||body).appendChild(createElement('script')).src='/static/api/js/share.js?v=89860593.js?'+~(-new Date()/36e5)];;
</script>
</div>
</div>
</div>
<div class="comments" id="comments">
<div id="lv-container" data-id="city" data-uid="MTAyMC8zOTcwMy8xNjIzMA"></div>
</div>
</div>
<div class="sidebar-toggle">
<div class="sidebar-toggle-line-wrap">
<span class="sidebar-toggle-line sidebar-toggle-line-first"></span>
<span class="sidebar-toggle-line sidebar-toggle-line-middle"></span>
<span class="sidebar-toggle-line sidebar-toggle-line-last"></span>
</div>
</div>
<aside id="sidebar" class="sidebar">
<div class="sidebar-inner">
<ul class="sidebar-nav motion-element">
<li class="sidebar-nav-toc sidebar-nav-active" data-target="post-toc-wrap">
文章目录
</li>
<li class="sidebar-nav-overview" data-target="site-overview-wrap">
站点概览
</li>
</ul>
<section class="site-overview-wrap sidebar-panel">
<div class="site-overview">
<div class="site-author motion-element" itemprop="author" itemscope itemtype="http://schema.org/Person">
<img class="site-author-image" itemprop="image"
src="/images/myPhoto.jpg"
alt="赖彬鸿" />
<p class="site-author-name" itemprop="name">赖彬鸿</p>
<p class="site-description motion-element" itemprop="description"></p>
</div>
<nav class="site-state motion-element">
<div class="site-state-item site-state-posts">
<a href="/archives/">
<span class="site-state-item-count">135</span>
<span class="site-state-item-name">日志</span>
</a>
</div>
<div class="site-state-item site-state-categories">
<a href="/categories/index.html">
<span class="site-state-item-count">32</span>
<span class="site-state-item-name">分类</span>
</a>
</div>
<div class="site-state-item site-state-tags">
<a href="/tags/index.html">
<span class="site-state-item-count">40</span>
<span class="site-state-item-name">标签</span>
</a>
</div>
</nav>
<div class="feed-link motion-element">
<a href="/atom.xml" rel="alternate">
<i class="fa fa-rss"></i>
RSS
</a>
</div>
<div class="links-of-author motion-element">
<span class="links-of-author-item">
<a href="https://github.com/LbhFront-end" target="_blank" title="GitHub">
<i class="fa fa-fw fa-github"></i>GitHub</a>
</span>
<span class="links-of-author-item">
<a href="https://www.cnblogs.com/lbh2018/" target="_blank" title="博客园">
<i class="fa fa-fw fa-globe"></i>博客园</a>
</span>
<span class="links-of-author-item">
<a href="https://yq.aliyun.com/users/1802204154913774?spm=a2c4e.11153940.blogcont662526.4.6c0a34f6E2lR5o" target="_blank" title="云栖">
<i class="fa fa-fw fa-globe"></i>云栖</a>
</span>
<span class="links-of-author-item">
<a href="mailto:544289495@qq.com" target="_blank" title="E-Mail">
<i class="fa fa-fw fa-envelope"></i>E-Mail</a>
</span>
<span class="links-of-author-item">
<a href="tencent://AddContact/?fromId=45&fromSubId=1&subcmd=all&uin=544289495&website=www.oicqzone.com" target="_blank" title="QQ">
<i class="fa fa-fw fa-qq"></i>QQ</a>
</span>
<span class="links-of-author-item">
<a href="https://www.google.com.hk/search?safe=strict&source=hp&ei=JtLCXIriJ8G4-gS_-4qABQ&q=site%3Alaibh.top&btnK=Google+%E6%90%9C%E7%B4%A2&oq=site%3Alaibh.top&gs_l=psy-ab.3...1158.6834..7051...5.0..1.246.3720.2-17......0....1..gws-wiz.....0..0j0i10.rJMUHvdrbds" target="_blank" title="Google">
<i class="fa fa-fw fa-google"></i>Google</a>
</span>
</div>
<iframe frameborder="no" border="0" marginwidth="0" marginheight="0" width=330 height=90 src="//music.163.com/outchain/player?type=0&id=2353471182&auto=0&height=90"></iframe>
<div class="links-of-blogroll motion-element links-of-blogroll-inline">
<div class="links-of-blogroll-title">
<i class="fa fa-fw fa-link"></i>
友情链接
</div>
<ul class="links-of-blogroll-list">
<li class="links-of-blogroll-item">
<a href="http://www.chjtx.com/JRoll/" title="醉萝卜" target="_blank">醉萝卜</a>
</li>
<li class="links-of-blogroll-item">
<a href="http://hzd.plus/" title="Zhendong" target="_blank">Zhendong</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.cnblogs.com/cnyball" title="cnyballk" target="_blank">cnyballk</a>
</li>
<li class="links-of-blogroll-item">
<a href="http://johnzz.top/" title="John" target="_blank">John</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://xiaojun1994.top/" title="xiaojun1994" target="_blank">xiaojun1994</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://me.ursb.me" title="Airing" target="_blank">Airing</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.iyouhun.com" title="游魂" target="_blank">游魂</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://icoty.github.io/" title="荒野之萍" target="_blank">荒野之萍</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://im-one.github.io/" title="imOne" target="_blank">imOne</a>
</li>
<li class="links-of-blogroll-item">
<a href="http://blog.hourxu.com/" title="Ambre" target="_blank">Ambre</a>
</li>
<li class="links-of-blogroll-item">
<a href="http://www.huyujs.com" title="胡雨" target="_blank">胡雨</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.andou.live" title="安逗" target="_blank">安逗</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.jianshu.com/u/701a8bbf4f7e" title="陈健斌" target="_blank">陈健斌</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://itobys.github.io/" title="汤姆Tom酱" target="_blank">汤姆Tom酱</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://breeze2.github.io/blog/" title="林毅锋" target="_blank">林毅锋</a>
</li>
<li class="links-of-blogroll-item">
<a href="http://www.qzroc.com/" title="大鹏博客" target="_blank">大鹏博客</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://lyreal666.com/" title="余腾靖的博客" target="_blank">余腾靖的博客</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://buzuosheng.com/" title="不作声" target="_blank">不作声</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.baidu.com/s?ie=UTF-8&wd=site%3Alaibh.top" title="百度" target="_blank">百度</a>
</li>
<li class="links-of-blogroll-item">
<a href="https://www.google.com.hk/search?safe=strict&source=hp&ei=zXdWXfemLJbO0PEP8qyXyA0&q=site%3Alaibh.top&oq=site%3Alaibh.top&gs_l=psy-ab.3...580.8501..8767...0.0..0.397.934.2-1j2......0....2j1..gws-wiz.QESXfWGadT0&ved=0ahUKEwi3wbusiofkAhUWJzQIHXLWBdkQ4dUDCAU&uact=5" title="谷歌" target="_blank">谷歌</a>
</li>
</ul>
</div>
</div>
</section>
<!--noindex-->
<section class="post-toc-wrap motion-element sidebar-panel sidebar-panel-active">
<div class="post-toc">
<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#js的作用域"><span class="nav-number">1.</span> <span class="nav-text">js的作用域</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#作用域"><span class="nav-number">1.0.1.</span> <span class="nav-text">作用域</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#执行环境（Execution-Contexts）"><span class="nav-number">1.0.2.</span> <span class="nav-text">执行环境（Execution Contexts）</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#作用域链（Scope-Chain）"><span class="nav-number">1.0.3.</span> <span class="nav-text">作用域链（Scope Chain）</span></a></li></ol></li></ol></li></ol></div>
</div>
</section>
<!--/noindex-->
</div>
</aside>
</div>
</main>
<footer id="footer" class="footer">
<div class="footer-inner">
<script async src="//busuanzi.ibruce.info/busuanzi/2.3/busuanzi.pure.mini.js"></script>
<div class="copyright">© 2018 — <span itemprop="copyrightYear">2022</span>
<span class="with-love">
<i class="fa fa-heart throb" style="color: #d43f57;"></i>
</span>
<span class="author" itemprop="copyrightHolder">赖彬鸿</span>
</div>
<script src='https://unpkg.com/mermaid@7.1.2/dist/mermaid.min.js'></script>
<script>
if (window.mermaid) {
mermaid.initialize("");
}
</script>
<!--
<div class="powered-by">由 <a class="theme-link" target="_blank" href="https://hexo.io">Hexo</a> 强力驱动</div>
<span class="post-meta-divider">|</span>
<div class="theme-info">主题 — <a class="theme-link" target="_blank" href="https://github.com/iissnan/hexo-theme-next">NexT.Mist</a> v5.1.4</div>
-->
<div>
<span id="busuanzi_container_site_pv" title="访问量">
<i class="fa fa fa-eye"></i> <span id="busuanzi_value_site_pv"></span>
</span>
<span id="busuanzi_container_site_uv" title="访问人数">
<i class="fa fa-user"></i> <span id="busuanzi_value_site_uv"></span>
</span>
<span class="post-count" title="博客总字数"><i class="fa fa-pagelines" aria-hidden="true"></i>745.9k</span>
</div>
<i class="fa fa-shield" aria-hidden="true" title="本站安全运行时间"></i> <span id="timeDate">载入天数...</span><span id="times">载入时分秒...</span>
<script>
var now = new Date();
function createtime() {
var grt= new Date("01/29/2018 09:47:04");//此处修改你的建站时间或者网站上线时间
now.setTime(now.getTime()+250);
days = (now - grt ) / 1000 / 60 / 60 / 24; dnum = Math.floor(days);
hours = (now - grt ) / 1000 / 60 / 60 - (24 * dnum); hnum = Math.floor(hours);
if(String(hnum).length ==1 ){hnum = "0" + hnum;} minutes = (now - grt ) / 1000 /60 - (24 * 60 * dnum) - (60 * hnum);
mnum = Math.floor(minutes); if(String(mnum).length ==1 ){mnum = "0" + mnum;}
seconds = (now - grt ) / 1000 - (24 * 60 * 60 * dnum) - (60 * 60 * hnum) - (60 * mnum);
snum = Math.round(seconds);
if(String(snum).length ==1 ){snum = "0" + snum;}
document.getElementById("timeDate").innerHTML = dnum+" 天 ";
document.getElementById("times").innerHTML = hnum + " 小时 " + mnum + " 分 " + snum + " 秒";
}
setInterval("createtime()",250);
</script>
</div>
</footer>
<div class="back-to-top">
<i class="fa fa-arrow-up"></i>
<span id="scrollpercent"><span>0</span>%</span>
</div>
</div>
<script type="text/javascript">
if (Object.prototype.toString.call(window.Promise) !== '[object Function]') {
window.Promise = null;
}
</script>
<script type="text/javascript" src="//cdn.bootcss.com/jquery/2.1.3/jquery.min.js"></script>
<script type="text/javascript" src="/lib/fastclick/lib/fastclick.min.js?v=1.0.6"></script>
<script type="text/javascript" src="/lib/jquery_lazyload/jquery.lazyload.js?v=1.9.7"></script>
<script type="text/javascript" src="/lib/velocity/velocity.min.js?v=1.2.1"></script>
<script type="text/javascript" src="/lib/velocity/velocity.ui.min.js?v=1.2.1"></script>