var, let, const and immutable of JavaScript

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ઌͣconstΑΓ࢝ΊΑ @arayaryoma 2018/08/20 - ϔϚλΠτࣾ಺LT

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ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏΍ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ ม਺એݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let΋࢖͑Δʁ var͸ม਺ͩ͠let͸ఆ਺ͬΆ͍…ʁ Μɺconst΋͋Δʁ ΋͔ͯ͠͠ࢲͷJavaScriptɺ3೥஗ΕͯΔʁ

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ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏΍ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ ม਺એݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let΋࢖͑Δʁ var͸ม਺ͩ͠let͸ఆ਺ͬΆ͍…ʁ Μɺconst΋͋Δʁ ΋͔ͯ͠͠ࢲͷJavaScriptɺ3೥஗ΕͯΔʁ ࠓ೔͸ͦΜͳ͋ͳͨͷͨΊʹ  varͱletͱconstͷҧ͍Λ ղઆ͠·͢

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ٱʑʹJavaScriptॻ͚͘ͲES2015ͱ͔ ͍͏΍ͭͰ͍Ζ͍ΖมΘͬͯΔͬΆ͍ʁ ม਺એݴ࣌ɺvar ͚ͩ͡Όͳͯ͘let΋࢖͑Δʁ var͸ม਺ͩ͠let͸ఆ਺ͬΆ͍…ʁ Μɺconst΋͋Δʁ ΋͔ͯ͠͠ࢲͷJavaScriptɺ3೥஗ΕͯΔʁ ͜ͷεϥΠυΛݟऴΘΔࠒ ͋ͳͨ͸varΛॻ͔ͳ͍਎ମʹ ͳ͍ͬͯΔͰ͠ΐ͏

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࿩͢͜ͱͱ݁࿦ JSͷvar, let, constͷҧ͍ͱ࢖͍ॴ ϞμϯͳJSͷॻ͖ํ(ྲྀߦ)ʹ͍ͭͯগ͚ͩ͠ var͸׬શʹෆཁ ·ͣ͸constͰॻ͘

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redeclaration(࠶એݴ)

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var let const

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ಈ͘ʁ

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ಈ͘(variable shadowing)

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const΋ಉ͡ ಈ͕͘Մಡੑ͕Լ͕ΔͷͰආ͚Δ΂͖

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scope(ม਺είʔϓ)

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varͰએݴͨ͠ม਺ͷείʔϓ͸function಺ (τοϓϨϕϧʹએݴͨ͠৔߹͸άϩʔόϧ)

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if(){}ͳͲͷϒϩοΫ಺Ͱએݴͨ͠ม਺Λ ϒϩοΫͷ֎͔ΒࢀরͰ͖Δ

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letͰએݴͨ͠ม਺͸είʔϓ͕ϒϩοΫ಺

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const΋ಉ͘͡ϒϩοΫείʔϓ

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switch-caseͷ᠘

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blockʹͯ͠ղܾ

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reassign(࠶୅ೖ)

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var͸࠶୅ೖՄೳ

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let΋࠶୅ೖՄೳ

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const͸࠶୅ೖෆՄೳ

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ͨͩ͠constͰએݴͯ͠΋immutableͰ͸ͳ͍ͷͰ஫ҙɻ Objectͷproperty΍Arrayͷཁૉ͸͍͘ΒͰ΋ॻ͖׵ΘΔɻ

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var let const ࠶એݴ είʔϓ function block block ࠶୅ೖ immutable?  (primitiveͳσʔλܕҎ֎) NO NO NO

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࣮ફྫͰݟΔ  ݱ୅෩JavaScript

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࣮ફྫ1 Array [1,2,3,4,5]ͷ֤ཁૉΛ10ഒͯ͠ग़ྗ

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Bad. i͕for blockͷ֎ʹ΋ӨڹΛٴ΅͢

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iͷείʔϓ͕for block಺ʹͳͬͨɻ  Ͱ΋΋͏ͪΐͬͱ؆ܿʹॻ͖͍ͨɻ  arrͷཁૉΛॻ͖׵͑ͯΔͷ΋ා͍ɻ

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࣮૷͸1ߦ  arrΛมߋ͠ͳ͍ɻGood

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࣮ફྫ2 Array [1,2,3,4,5]ͷ֤ཁૉͷ߹ܭ஋Λग़ྗ

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letͱfor loop

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for-ofΛ࢖͏

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Array.prototype.reduceΛ࢖͏ɻ  sumΛ࠶୅ೖෆՄʹͰ͖ͯ҆શɻ

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ݱ୅෩JS ม਺είʔϓ͸Ͱ͖Δ͚ͩখ͘͞ɺͰ͖Δ͚ͩ࠶୅ೖෆՄʹ͢Δ Ұ౓࡞ͬͨobject͸มԽͤ͞ͳ͍ͷ͕ਖ਼ٛ ෭࡞༻ͷൃੜΛආ͚Δ Immutable.jsΛ࢖ͬͯcollection͸͢΂ͯimmutableͰ͋Δ͜ͱΛલఏʹ͢Δ ྲྀ೿ ArrayͳͲʹศརͳmethod΋࢓༷Ͱ௥Ճ͞Ε͍ͯΔ(https://github.com/tc39/ proposals)

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࣮ફྫ3  ඇಉظ+varͷා͞Λମݧ(ϥΠϒίʔσΟϯά)

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Array΋Object΋immutableʹ͍ͨ͠ਓ΁

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Object.freeze()ͰखܰʹObject΍ArrayΛimmutableʹ

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Immutable.jsͷclassΛ࢖͏

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Immutable.jsͬͯʁ https://facebook.github.io/immutable-js/ WebϑϩϯτΤϯυͰ͸͓ೃછΈͷFacebook੡ JavaScriptͰimmutableͳܕΛѻ͑ΔΑ͏ʹ͢ΔͨΊͷϥΠϒϥϦ Object΍ArrayͳͲɺJavaScriptͷbuilt-inͳσʔλܕΛimmutableʹ ͢ΔΞϓϩʔνͰ͸ͳ͘ɺؔ਺ܕݴޠ͔Β༌ೖͨ͠ಠࣗͷσʔλܕ Λѻ͏ List, Stack, Map, OrderedMap, Set, OrderedSet,

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·ͱΊ var͸΋͏͍Βͳ͍ ·ͣconstͰॻ͍ͯɺͲ͏ͯ͠΋ඞཁͳΒletΛ࢖͏ ෭࡞༻Λආ͚Δ͜ͱ͕ਖ਼ٛͩͱ͍͏࣌୅ ؔ਺ܕݴޠͷύϥμΠϜ͕JSͷੈքʹ༌ೖ͞ΕͯΔ TypeScript + Rx.js + Immutable.jsͱ͔΍Γ͸͡ΊΔͱ׬શʹผݴޠ