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var, let, const and immutable of JavaScript
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Araya
August 20, 2018
Programming
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var, let, const and immutable of JavaScript
JavaScriptのvar, let, constによる変数宣言、immutableについて、社内LTで話しました。
Araya
August 20, 2018
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Transcript
ઌͣconstΑΓ࢝ΊΑ @arayaryoma 2018/08/20 - ϔϚλΠτࣾLT
ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ มએݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let͑Δʁ varมͩ͠letఆͬΆ͍…ʁ Μɺconst͋Δʁ ͔ͯ͠͠ࢲͷJavaScriptɺ3ΕͯΔʁ
ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ มએݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let͑Δʁ varมͩ͠letఆͬΆ͍…ʁ Μɺconst͋Δʁ ͔ͯ͠͠ࢲͷJavaScriptɺ3ΕͯΔʁ ࠓͦΜͳ͋ͳͨͷͨΊʹ varͱletͱconstͷҧ͍Λ ղઆ͠·͢
ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ มએݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let͑Δʁ varมͩ͠letఆͬΆ͍…ʁ Μɺconst͋Δʁ ͔ͯ͠͠ࢲͷJavaScriptɺ3ΕͯΔʁ ͜ͷεϥΠυΛݟऴΘΔࠒ ͋ͳͨvarΛॻ͔ͳ͍ମʹ ͳ͍ͬͯΔͰ͠ΐ͏
͢͜ͱͱ݁ JSͷvar, let, constͷҧ͍ͱ͍ॴ ϞμϯͳJSͷॻ͖ํ(ྲྀߦ)ʹ͍ͭͯগ͚ͩ͠ varશʹෆཁ ·ͣconstͰॻ͘
redeclaration(࠶એݴ)
var let const
ಈ͘ʁ
ಈ͘(variable shadowing)
constಉ͡ ಈ͕͘Մಡੑ͕Լ͕ΔͷͰආ͚Δ͖
scope(มείʔϓ)
varͰએݴͨ͠มͷείʔϓfunction (τοϓϨϕϧʹએݴͨ͠߹άϩʔόϧ)
if(){}ͳͲͷϒϩοΫͰએݴͨ͠มΛ ϒϩοΫͷ֎͔ΒࢀরͰ͖Δ
letͰએݴͨ͠มείʔϓ͕ϒϩοΫ
constಉ͘͡ϒϩοΫείʔϓ
switch-caseͷ᠘
blockʹͯ͠ղܾ
reassign(࠶ೖ)
var࠶ೖՄೳ
let࠶ೖՄೳ
const࠶ೖෆՄೳ
ͨͩ͠constͰએݴͯ͠immutableͰͳ͍ͷͰҙɻ ObjectͷpropertyArrayͷཁૉ͍͘ΒͰॻ͖ΘΔɻ
var let const ࠶એݴ είʔϓ function block block ࠶ೖ immutable?
(primitiveͳσʔλܕҎ֎) NO NO NO
࣮ફྫͰݟΔ ݱ෩JavaScript
࣮ફྫ1 Array [1,2,3,4,5]ͷ֤ཁૉΛ10ഒͯ͠ग़ྗ
Bad. i͕for blockͷ֎ʹӨڹΛٴ΅͢
iͷείʔϓ͕for blockʹͳͬͨɻ Ͱ͏ͪΐͬͱ؆ܿʹॻ͖͍ͨɻ arrͷཁૉΛॻ͖͑ͯΔͷා͍ɻ
࣮1ߦ arrΛมߋ͠ͳ͍ɻGood
࣮ફྫ2 Array [1,2,3,4,5]ͷ֤ཁૉͷ߹ܭΛग़ྗ
letͱfor loop
for-ofΛ͏
Array.prototype.reduceΛ͏ɻ sumΛ࠶ೖෆՄʹͰ͖ͯ҆શɻ
ݱ෩JS มείʔϓͰ͖Δ͚ͩখ͘͞ɺͰ͖Δ͚ͩ࠶ೖෆՄʹ͢Δ Ұ࡞ͬͨobjectมԽͤ͞ͳ͍ͷ͕ਖ਼ٛ ෭࡞༻ͷൃੜΛආ͚Δ Immutable.jsΛͬͯcollectionͯ͢immutableͰ͋Δ͜ͱΛલఏʹ͢Δ ྲྀ ArrayͳͲʹศརͳmethod༷ͰՃ͞Ε͍ͯΔ(https://github.com/tc39/ proposals)
࣮ફྫ3 ඇಉظ+varͷා͞Λମݧ(ϥΠϒίʔσΟϯά)
ArrayObjectimmutableʹ͍ͨ͠ਓ
Object.freeze()ͰखܰʹObjectArrayΛimmutableʹ
Immutable.jsͷclassΛ͏
Immutable.jsͬͯʁ https://facebook.github.io/immutable-js/ WebϑϩϯτΤϯυͰ͓ೃછΈͷFacebook JavaScriptͰimmutableͳܕΛѻ͑ΔΑ͏ʹ͢ΔͨΊͷϥΠϒϥϦ ObjectArrayͳͲɺJavaScriptͷbuilt-inͳσʔλܕΛimmutableʹ ͢ΔΞϓϩʔνͰͳ͘ɺؔܕݴޠ͔Β༌ೖͨ͠ಠࣗͷσʔλܕ Λѻ͏ List, Stack, Map,
OrderedMap, Set, OrderedSet,
·ͱΊ var͏͍Βͳ͍ ·ͣconstͰॻ͍ͯɺͲ͏ͯ͠ඞཁͳΒletΛ͏ ෭࡞༻Λආ͚Δ͜ͱ͕ਖ਼ٛͩͱ͍͏࣌ ؔܕݴޠͷύϥμΠϜ͕JSͷੈքʹ༌ೖ͞ΕͯΔ TypeScript + Rx.js + Immutable.jsͱ͔Γ͡ΊΔͱશʹผݴޠ