Lock in $30 Savings on PRO—Offer Ends Soon! ⏳

Scope & Closures in JavaScript

Avatar for Luciano Battagliero Luciano Battagliero
September 20, 2018
Programming
1
330

Scope & Closures in JavaScript

A deep dive into Scope & Closures in JavaScript.
Presented at the BeerJS Meetup in September, 2018.

Avatar for Luciano Battagliero

Luciano Battagliero

September 20, 2018
Tweet

More Decks by Luciano Battagliero

See All by Luciano Battagliero
BEM
Avatar for Luciano Battagliero battaglr
5
6.3k
Why you should care about whitespace
Avatar for Luciano Battagliero battaglr
4
310
SMACSS
Avatar for Luciano Battagliero battaglr
5
550

Other Decks in Programming

See All in Programming
Findy AI+の開発、運用におけるMCP活用事例
Avatar for starfish719 starfish719
0
1.7k
perlをWebAssembly上で動かすと何が嬉しいの？？？ / Where does Perl-on-Wasm actually make sense?
Avatar for mackee mackee
0
120
Rubyで鍛える仕組み化プロヂュース力
Avatar for muryoimpl muryoimpl
0
160
新卒エンジニアのプルリクエスト with AI駆動
Avatar for Yuta Fukunaga fukunaga2025
0
230
Pythonではじめるオープンデータ分析〜書籍の紹介と書籍で紹介しきれなかった事例の紹介〜
Avatar for Ryo YOSHI welliving
2
570
大規模Cloud Native環境におけるFalcoの運用
Avatar for Chihiro Hasegawa owlinux1000
0
190
20251212 AI 時代的 Legacy Code 營救術 2025 WebConf
Avatar for mouson(墨嗓) mouson
0
210
公共交通オープンデータ × モバイルUX 複雑な運行情報を 『直感』に変換する技術
Avatar for Tsubasa SEKIGUCHI tinykitten
PRO
0
160
LLMで複雑な検索条件アセットから脱却する！！ 生成的検索インタフェースの設計論
Avatar for po3rin po3rin
4
960
Claude Codeの「Compacting Conversation」を体感50%減！ CLAUDE.md + 8 Skills で挑むコンテキスト管理術
Avatar for Kazuki Murahama kmurahama
1
630
The Art of Re-Architecture - Droidcon India 2025
Avatar for Sid Patil siddroid
0
120
AI 駆動開発ライフサイクル(AI-DLC):ソフトウェアエンジニアリングの再構築 / AI-DLC Introduction
Avatar for kanamsasa kanamasa
11
3.8k

Featured

See All Featured
Templates, Plugins, & Blocks: Oh My! Creating the theme that thinks of everything
Avatar for Michelle Schulp Hunt marktimemedia
31
2.6k
Joys of Absence: A Defence of Solitary Play
Avatar for Sebastian Deterding codingconduct
1
260
How to Get Subject Matter Experts Bought In and Actively Contributing to SEO & PR Initiatives.
Avatar for Liv Day livdayseo
0
29
Helping Users Find Their Own Way: Creating Modern Search Experiences
Avatar for Dan Newman danielanewman
31
3k
Put a Button on it: Removing Barriers to Going Fast.
Avatar for kastner kastner
60
4.1k
Building AI with AI
Avatar for Ines Montani inesmontani
PRO
1
570
Building Experiences: Design Systems, User Experience, and Full Site Editing
Avatar for Michelle Schulp Hunt marktimemedia
0
330
Mind Mapping
Avatar for Hélio Medeiros helmedeiros
PRO
0
38
How to Create Impact in a Changing Tech Landscape [PerfNow 2023]
Avatar for Tammy Everts tammyeverts
55
3.2k
How to optimise 3,500 product descriptions for ecommerce in one day using ChatGPT
Avatar for Katarina Dahlin katarinadahlin
PRO
0
3.4k
Conquering PDFs: document understanding beyond plain text
Avatar for Ines Montani inesmontani
PRO
4
2.1k
Connecting the Dots Between Site Speed, User Experience & Your Business [WebExpo 2025]
Avatar for Tammy Everts tammyeverts
10
750

Transcript

  1. None
  2. None
  3. None
  4. None
  5. None
  6. None

  7. var x = 42; console.log(x);

  8. var x = 42; console.log(x); // → 42

  9. var x = 42; console.log(x); // → 42

  10. var x = 42; console.log(x); // → 42

  11. var x = 42; console.log(x); // → 42

  12. None
  13. None
  14. None
  15. None
  16. None

  17. var y = 40; function foo() { var x =

    2; return x + y; } foo();

  18. var y = 40; function foo() { var x =

    2; return x + y; } foo(); // → 42

  19. var y = 40; function foo() { var x =

    2; return x + y; } foo(); // → 42

  20. var y = 40; function foo() { var x =

    2; return x + y; } foo(); // → 42

  21. var y = 40; function foo() { var x =

    2; return x + y; } foo(); // → 42
  22. None

  23. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo();

  24. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo(); // → shadow

  25. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo(); // → shadow

  26. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo(); // → shadow
  27. None

  28. function foo() { var x = 2; return x +

    y; } foo();

  29. function foo() { var x = 2; return x +

    y; } foo(); // → ReferenceError: y is not defined

  30. function foo() { var x = 2; return x +

    y; } foo(); // → ReferenceError: y is not defined

  31. function foo() { var x = 2; return x +

    y; } foo(); // → ReferenceError: y is not defined

  32. function foo() { var x = 2; return x +

    y; } foo(); // → ReferenceError: y is not defined
  33. None
  34. None
  35. None
  36. None

  37. window

  38. window === this && window === self && window ===

    frames

  39. window === this && window === self && window ===

    frames // → true
  40. None

  41. global

  42. global === this

  43. global === this // → true

  44. None

  45. function getGlobal() { if (typeof self !== 'undefined') { return

    self; } if (typeof window !== 'undefined') { return window; } if (typeof global !== 'undefined') { return global; } throw new Error('Unable to locate global object'); };

  46. ╯ □ ）╯︵ ┻━┻

  47. None

  48. ┬─┬ノ ノ

  49. None

  50. var x = 42; console.log(x) console.log(window.x)

  51. var x = 42; console.log(x) // → 42 console.log(window.x) //

    → 42

  52. var x = 42; window.console.log(this.x)

  53. var x = 42; window.console.log(this.x) // → 42

  54. ☉ ☉

  55. None

  56. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo();

  57. var x = 'tree'; function foo() { var x =

    'shadow'; return x; } foo(); // → shadow

  58. var x = 'tree'; function foo() { var x =

    'shadow'; return window.x; } foo();

  59. var x = 'tree'; function foo() { var x =

    'shadow'; return window.x; } foo(); // → tree
  60. None
  61. None
  62. None
  63. None
  64. None

  65. ಥ﹏ಥ

  66. function foo() { var x = 42; console.log(x); }; foo();

    console.log(x);

  67. function foo() { var x = 42; console.log(x); // →

    42 }; foo(); console.log(x); // → ReferenceError

  68. function foo() { var x = 42; console.log(x); // →

    42 }; foo(); console.log(x); // → ReferenceError

  69. function foo() { var x = 42; console.log(x); // →

    42 }; foo(); console.log(x); // → ReferenceError

  70. function foo() { var x = 42; console.log(x); // →

    42 }; foo(); console.log(x); // → ReferenceError

  71. function foo() { var x = 42; console.log(x); // →

    42 }; foo(); console.log(x); // → ReferenceError
  72. None
  73. None

  74. (function () { var x = 42; console.log(x); })(); console.log(x);

  75. (function () { var x = 42; console.log(x); // →

    42 })(); console.log(x); // → ReferenceError

  76. (function () { var x = 42; console.log(x); // →

    42 })(); console.log(x); // → ReferenceError

  77. (function () { var x = 42; console.log(x); // →

    42 })(); console.log(x); // → ReferenceError

  78. (function () { var x = 42; console.log(x); // →

    42 })(); console.log(x); // → ReferenceError

  79. (function () { var x = 42; console.log(x); // →

    42 })(); console.log(x); // → ReferenceError
  80. None
  81. None

  82. if (true) { var x = 42; console.log(x); } console.log(x);

  83. if (true) { var x = 42; console.log(x); // →

    42 } console.log(x); // → 42

  84. if (true) { var x = 42; console.log(x); // →

    42 } console.log(x); // → 42

  85. if (true) { var x = 42; console.log(x); // →

    42 } console.log(x); // → 42

  86. if (true) { var x = 42; console.log(x); // →

    42 } console.log(x); // → 42

  87. if (true) { var x = 42; console.log(x); // →

    42 } console.log(x); // → 42

  88. // “Under the hood” var x; if (true) { x

    = 42; console.log(x); } console.log(x);

  89. // “Under the hood” var x; if (true) { x

    = 42; console.log(x); // → 42 } console.log(x); // → 42

  90. // “Under the hood” var x; if (true) { x

    = 42; console.log(x); } console.log(x);

  91. // “Under the hood” var x; if (true) { x

    = 42; console.log(x); // → 42 } console.log(x); // → 42
  92. None
  93. None

  94. try { throw 42; } catch(x) { console.log(x); } console.log(x);

  95. try { throw 42; } catch(x) { console.log(x); // →

    42 } console.log(x); // → ReferenceError

  96. try { throw 42; } catch(x) { console.log(x); // →

    42 } console.log(x); // → ReferenceError

  97. try { throw 42; } catch(x) { console.log(x); // →

    42 } console.log(x); // → ReferenceError

  98. try { throw 42; } catch(x) { console.log(x); // →

    42 } console.log(x); // → ReferenceError

  99. try { throw 42; } catch(x) { console.log(x); // →

    42 } console.log(x); // → ReferenceError

  100. ☞ ⌐ ☞

  101. None
  102. None
  103. None
  104. None
  105. None

  106. if (true) { let x = 42; console.log(x); } console.log(x);

  107. if (true) { let x = 42; console.log(x); // →

    42 } console.log(x); // → ReferenceError

  108. ノ ∀ ノ⌒･ ｡ ｡ ･゜ﾟ･ ☆

  109. None

  110. { let x = 42; console.log(x); } console.log(x);

  111. { let x = 42; console.log(x); // → 42 }

    console.log(x); // → ReferenceError

  112. ヽ 〇 ﾉ

  113. None
  114. None

  115. console.log(x);

  116. console.log(x); // → ReferenceError

  117. ┐ ､ ┌

  118. console.log(x); var x;

  119. console.log(x); // → undefined var x;

  120. ・・

  121. console.log(x); var x = 42;

  122. console.log(x); // → undefined var x = 42;

  123. ლ ಠ ಠ ლ

  124. None
  125. None

  126. var x = 42;

  127. var x; x = 42;

  128. var x; // Declaration x = 42; // Assignment

  129. None
  130. None
  131. None
  132. None

  133. console.log(x); var x;

  134. console.log(x); // → undefined var x;

  135. // “Under the hood” var x; console.log(x);

  136. // “Under the hood” var x; console.log(x); // → undefined

  137. None

  138. console.log(x); var x = 42;

  139. console.log(x); // → undefined var x = 42;

  140. // “Under the hood” var x; console.log(x); x = 42;

  141. // “Under the hood” var x; console.log(x); // → undefined

    x = 42;
  142. None
  143. None

  144. foo(); function foo() { return 42; }

  145. foo(); // → 42 function foo() { return 42; }

  146. ｏ

  147. // “Under the hood” function foo() { return 42; }

    foo();

  148. // “Under the hood” function foo() { return 42; }

    foo(); // → 42

  149. function foo() { return 2; } foo(); function foo() {

    return 4; }

  150. function foo() { return 2; } foo(); // → 4

    function foo() { return 4; }

  151. // “Under the hood” function foo() { return 2; }

    function foo() { return 4; } foo();

  152. // “Under the hood” function foo() { return 2; }

    function foo() { return 4; } foo(); // → 4

  153. function foo() { return 2; } foo(); var foo =

    function () { return 4; };

  154. function foo() { return 2; } foo(); // → 2

    var foo = function () { return 4; };

  155. งಠ ಠ ง

  156. // “Under the hood” function foo() { return 2; }

    var foo; foo(); foo = function () { return 4; };

  157. // “Under the hood” function foo() { return 2; }

    var foo; foo(); // → 2 foo = function () { return 4; };

  158. foo(); var foo = function () { return 42; };

  159. foo(); // → TypeError: foo is not a function var

    foo = function () { return 42; };

  160. ╰ 益 ╯

  161. // “Under the hood” var foo; foo(); foo = function

    () { return 42; };

  162. // “Under the hood” var foo; foo(); // → TypeError:

    foo is not a function foo = function () { return 42; };
  163. None
  164. None

  165. console.log(x); let x = 42;

  166. console.log(x); // → ReferenceError let x = 42;

  167. ಠ ಠ

  168. None

  169. let x = 2; { console.log(x); let x = 4;

    }

  170. let x = 2; { console.log(x); // → ReferenceError let

    x = 4; }
  171. None
  172. None
  173. None
  174. None
  175. None
  176. None
  177. None

  178. function foo() { return 'I am a Closure!’; }

  179. ☉ ☉

  180. None
  181. None
  182. None

  183. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); console.log(x);

  184. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  185. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  186. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  187. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  188. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  189. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError

  190. function foo() { let x = 42; return function ()

    { console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
  191. None
  192. None
  193. None

  194. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo);

  195. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  196. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  197. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  198. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  199. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  200. let x = 'FOO_SCOPE'; function foo() { console.log(x); } function

    bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE

  201. function foo(x) { setTimeout(function () { console.log(x); }, 1000); }

    foo(42);

  202. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);

  203. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);

  204. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);

  205. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);

  206. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);

  207. // “Deep down in the JavaScript Engine” function setTimeout(callback, delay)

    { // Works using magic! isItTimeAlready(delay) && callback(); }

  208. // “Deep down in the JavaScript Engine” function setTimeout(callback, delay)

    { // Works using magic! isItTimeAlready(delay) && callback(); }

  209. // “Deep down in the JavaScript Engine” function setTimeout(callback, delay)

    { // Works using magic! isItTimeAlready(delay) && callback(); }

  210. ノ ∀ つ──☆ ・ﾟ

  211. function foo(x) { setTimeout(function () { console.log(x); // → 42

    }, 1000); } foo(42);
  212. None
  213. None
  214. None
  215. None
  216. None
  217. None
  218. None
  219. None