Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Scope & Closures in JavaScript
Search
Luciano Battagliero
September 20, 2018
Programming
1
270
Scope & Closures in JavaScript
A deep dive into Scope & Closures in JavaScript.
Presented at the BeerJS Meetup in September, 2018.
Luciano Battagliero
September 20, 2018
Tweet
Share
More Decks by Luciano Battagliero
See All by Luciano Battagliero
BEM
battaglr
5
6.1k
Why you should care about whitespace
battaglr
4
290
SMACSS
battaglr
5
500
Other Decks in Programming
See All in Programming
見えないメモリを観測する: PHP 8.4 `pg_result_memory_size()` とSQL結果のメモリ管理
kentaroutakeda
0
890
rails newと同時に型を書く
aki19035vc
5
610
MCP with Cloudflare Workers
yusukebe
2
260
Kaigi on Railsに初参加したら、その日にLT登壇が決定した件について
tama50505
0
140
Оптимизируем производительность блока Казначейство
lamodatech
0
870
HTML/CSS超絶浅い説明
yuki0329
0
160
数十万行のプロジェクトを Scala 2から3に完全移行した
xuwei_k
0
500
Amazon S3 NYJavaSIG 2024-12-12
sullis
0
130
Go の GC の不得意な部分を克服したい
taiyow
3
980
htmxって知っていますか?次世代のHTML
hiro_ghap1
0
390
ドメインイベント増えすぎ問題
h0r15h0
2
540
テストコード書いてみませんか?
onopon
2
290
Featured
See All Featured
Measuring & Analyzing Core Web Vitals
bluesmoon
5
190
Sharpening the Axe: The Primacy of Toolmaking
bcantrill
38
1.9k
Keith and Marios Guide to Fast Websites
keithpitt
410
22k
Practical Tips for Bootstrapping Information Extraction Pipelines
honnibal
PRO
10
850
10 Git Anti Patterns You Should be Aware of
lemiorhan
PRO
656
59k
I Don’t Have Time: Getting Over the Fear to Launch Your Podcast
jcasabona
29
2k
The Cult of Friendly URLs
andyhume
78
6.1k
Understanding Cognitive Biases in Performance Measurement
bluesmoon
27
1.5k
Templates, Plugins, & Blocks: Oh My! Creating the theme that thinks of everything
marktimemedia
28
2.2k
GraphQLの誤解/rethinking-graphql
sonatard
68
10k
Put a Button on it: Removing Barriers to Going Fast.
kastner
59
3.6k
How To Stay Up To Date on Web Technology
chriscoyier
790
250k
Transcript
None
None
None
None
None
None
var x = 42; console.log(x);
var x = 42; console.log(x); // → 42
var x = 42; console.log(x); // → 42
var x = 42; console.log(x); // → 42
var x = 42; console.log(x); // → 42
None
None
None
None
None
var y = 40; function foo() { var x =
2; return x + y; } foo();
var y = 40; function foo() { var x =
2; return x + y; } foo(); // → 42
var y = 40; function foo() { var x =
2; return x + y; } foo(); // → 42
var y = 40; function foo() { var x =
2; return x + y; } foo(); // → 42
var y = 40; function foo() { var x =
2; return x + y; } foo(); // → 42
None
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo();
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo(); // → shadow
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo(); // → shadow
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo(); // → shadow
None
function foo() { var x = 2; return x +
y; } foo();
function foo() { var x = 2; return x +
y; } foo(); // → ReferenceError: y is not defined
function foo() { var x = 2; return x +
y; } foo(); // → ReferenceError: y is not defined
function foo() { var x = 2; return x +
y; } foo(); // → ReferenceError: y is not defined
function foo() { var x = 2; return x +
y; } foo(); // → ReferenceError: y is not defined
None
None
None
None
window
window === this && window === self && window ===
frames
window === this && window === self && window ===
frames // → true
None
global
global === this
global === this // → true
None
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'); };
╯ □ )╯︵ ┻━┻
None
┬─┬ノ ノ
None
var x = 42; console.log(x) console.log(window.x)
var x = 42; console.log(x) // → 42 console.log(window.x) //
→ 42
var x = 42; window.console.log(this.x)
var x = 42; window.console.log(this.x) // → 42
☉ ☉
None
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo();
var x = 'tree'; function foo() { var x =
'shadow'; return x; } foo(); // → shadow
var x = 'tree'; function foo() { var x =
'shadow'; return window.x; } foo();
var x = 'tree'; function foo() { var x =
'shadow'; return window.x; } foo(); // → tree
None
None
None
None
None
ಥ﹏ಥ
function foo() { var x = 42; console.log(x); }; foo();
console.log(x);
function foo() { var x = 42; console.log(x); // →
42 }; foo(); console.log(x); // → ReferenceError
function foo() { var x = 42; console.log(x); // →
42 }; foo(); console.log(x); // → ReferenceError
function foo() { var x = 42; console.log(x); // →
42 }; foo(); console.log(x); // → ReferenceError
function foo() { var x = 42; console.log(x); // →
42 }; foo(); console.log(x); // → ReferenceError
function foo() { var x = 42; console.log(x); // →
42 }; foo(); console.log(x); // → ReferenceError
None
None
(function () { var x = 42; console.log(x); })(); console.log(x);
(function () { var x = 42; console.log(x); // →
42 })(); console.log(x); // → ReferenceError
(function () { var x = 42; console.log(x); // →
42 })(); console.log(x); // → ReferenceError
(function () { var x = 42; console.log(x); // →
42 })(); console.log(x); // → ReferenceError
(function () { var x = 42; console.log(x); // →
42 })(); console.log(x); // → ReferenceError
(function () { var x = 42; console.log(x); // →
42 })(); console.log(x); // → ReferenceError
None
None
if (true) { var x = 42; console.log(x); } console.log(x);
if (true) { var x = 42; console.log(x); // →
42 } console.log(x); // → 42
if (true) { var x = 42; console.log(x); // →
42 } console.log(x); // → 42
if (true) { var x = 42; console.log(x); // →
42 } console.log(x); // → 42
if (true) { var x = 42; console.log(x); // →
42 } console.log(x); // → 42
if (true) { var x = 42; console.log(x); // →
42 } console.log(x); // → 42
// “Under the hood” var x; if (true) { x
= 42; console.log(x); } console.log(x);
// “Under the hood” var x; if (true) { x
= 42; console.log(x); // → 42 } console.log(x); // → 42
// “Under the hood” var x; if (true) { x
= 42; console.log(x); } console.log(x);
// “Under the hood” var x; if (true) { x
= 42; console.log(x); // → 42 } console.log(x); // → 42
None
None
try { throw 42; } catch(x) { console.log(x); } console.log(x);
try { throw 42; } catch(x) { console.log(x); // →
42 } console.log(x); // → ReferenceError
try { throw 42; } catch(x) { console.log(x); // →
42 } console.log(x); // → ReferenceError
try { throw 42; } catch(x) { console.log(x); // →
42 } console.log(x); // → ReferenceError
try { throw 42; } catch(x) { console.log(x); // →
42 } console.log(x); // → ReferenceError
try { throw 42; } catch(x) { console.log(x); // →
42 } console.log(x); // → ReferenceError
☞ ⌐ ☞
None
None
None
None
None
if (true) { let x = 42; console.log(x); } console.log(x);
if (true) { let x = 42; console.log(x); // →
42 } console.log(x); // → ReferenceError
ノ ∀ ノ⌒・ 。 。 ・゜゚・ ☆
None
{ let x = 42; console.log(x); } console.log(x);
{ let x = 42; console.log(x); // → 42 }
console.log(x); // → ReferenceError
ヽ 〇 ノ
None
None
console.log(x);
console.log(x); // → ReferenceError
┐ 、 ┌
console.log(x); var x;
console.log(x); // → undefined var x;
・・
console.log(x); var x = 42;
console.log(x); // → undefined var x = 42;
ლ ಠ ಠ ლ
None
None
var x = 42;
var x; x = 42;
var x; // Declaration x = 42; // Assignment
None
None
None
None
console.log(x); var x;
console.log(x); // → undefined var x;
// “Under the hood” var x; console.log(x);
// “Under the hood” var x; console.log(x); // → undefined
None
console.log(x); var x = 42;
console.log(x); // → undefined var x = 42;
// “Under the hood” var x; console.log(x); x = 42;
// “Under the hood” var x; console.log(x); // → undefined
x = 42;
None
None
foo(); function foo() { return 42; }
foo(); // → 42 function foo() { return 42; }
o
// “Under the hood” function foo() { return 42; }
foo();
// “Under the hood” function foo() { return 42; }
foo(); // → 42
function foo() { return 2; } foo(); function foo() {
return 4; }
function foo() { return 2; } foo(); // → 4
function foo() { return 4; }
// “Under the hood” function foo() { return 2; }
function foo() { return 4; } foo();
// “Under the hood” function foo() { return 2; }
function foo() { return 4; } foo(); // → 4
function foo() { return 2; } foo(); var foo =
function () { return 4; };
function foo() { return 2; } foo(); // → 2
var foo = function () { return 4; };
งಠ ಠ ง
// “Under the hood” function foo() { return 2; }
var foo; foo(); foo = function () { return 4; };
// “Under the hood” function foo() { return 2; }
var foo; foo(); // → 2 foo = function () { return 4; };
foo(); var foo = function () { return 42; };
foo(); // → TypeError: foo is not a function var
foo = function () { return 42; };
╰ 益 ╯
// “Under the hood” var foo; foo(); foo = function
() { return 42; };
// “Under the hood” var foo; foo(); // → TypeError:
foo is not a function foo = function () { return 42; };
None
None
console.log(x); let x = 42;
console.log(x); // → ReferenceError let x = 42;
ಠ ಠ
None
let x = 2; { console.log(x); let x = 4;
}
let x = 2; { console.log(x); // → ReferenceError let
x = 4; }
None
None
None
None
None
None
None
function foo() { return 'I am a Closure!’; }
☉ ☉
None
None
None
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); console.log(x);
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
function foo() { let x = 42; return function ()
{ console.log(x); }; } let bar = foo(); bar(); // → 42 console.log(x); // → ReferenceError
None
None
None
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo);
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
let x = 'FOO_SCOPE'; function foo() { console.log(x); } function
bar(callback) { let x = 'BAR_SCOPE'; callback(); } bar(foo); // → FOO_SCOPE
function foo(x) { setTimeout(function () { console.log(x); }, 1000); }
foo(42);
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
// “Deep down in the JavaScript Engine” function setTimeout(callback, delay)
{ // Works using magic! isItTimeAlready(delay) && callback(); }
// “Deep down in the JavaScript Engine” function setTimeout(callback, delay)
{ // Works using magic! isItTimeAlready(delay) && callback(); }
// “Deep down in the JavaScript Engine” function setTimeout(callback, delay)
{ // Works using magic! isItTimeAlready(delay) && callback(); }
ノ ∀ つ──☆ ・゚
function foo(x) { setTimeout(function () { console.log(x); // → 42
}, 1000); } foo(42);
None
None
None
None
None
None
None
None