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1. null or unde fi ned 2024-08-24 Kyoto.ͳΜ͔ #6 id:susisu

2. ͜Μʹͪ͸ • id:susisu • גࣜձࣾ͸ͯͳ
 WebΞϓϦέʔγϣϯΤϯδχΞ • ਓؒ ( TypeScript

   | ESLint | Prettier )

3. JavaScript ͱ͍͑͹

4. null ΋͋Δ unde fi ned ΋͋Δ

5. Α͋͘Δઆ໌ • null: ஋͕ͳ͍ͱઃఆ͞Ε͍ͯΔ • unde fi ned: ஋͕ઃఆ͞Ε͍ͯͳ͍

6. ΫΠζΛ͖࣋ͬͯ·ͨ͠

7. ୈҰ໰

8. > const x = new Map([["foo", "hello"]]); > x.get("foo"); "hello"

   > x.get("bar"); ???

9. > const x = new Map([["foo", "hello"]]); > x.get("foo"); "hello"

   > x.get("bar"); undefined

10. ୈೋ໰

11. > const x = new Headers([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); ???

12. > const x = new Headers([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); null

13. > const x = new Headers([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); null 🤷

14. Α͋͘Δઆ໌ ≠ ݱ࣮ • null: ஋͕ͳ͍ͱઃఆ͞Ε͍ͯΔ • unde fi ned:

    ஋͕ઃఆ͞Ε͍ͯͳ͍

15. ݱ࣮ • લఏ: "JavaScript" ͸͍ͭ͘΋ͷඪ४࢓༷͔Β੒Γཱ͍ͬͯΔ • ECMAScript ͷݴޠ࢓༷Ͱ͸ද޲͖͸΄΅ unde fi

    ned ͕࢖ΘΕΔ • ͨͩ͠ਖ਼نදݱ, JSON, ϓϩτλΠϓͷΈྫ֎తʹ null ͕ొ৔͢Δ • Web ؔ࿈࢓༷ (DOM, Fetch ͳͲ) Ͱ͸׳ྫతʹ null ͕࢖ΘΕΔ͜ͱ͕ଟ͍ • ҙຯతͳ࢖͍෼͚ΑΓ͸, ΄΅࢓༷͝ͱʹҰ؏ͯ͠ͲͪΒ͔Λ࢖͏

16. ΫΠζ࠶։

17. ୈҰ໰

18. > const x = new Map([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); ???

19. > const x = new Map([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); undefined ECMAScript ͷݴޠ࢓༷Ͱఆٛ

20. ୈೋ໰

21. > const x = new Headers([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); ???

22. > const x = new Headers([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); null Fetch Standard Ͱఆٛ

23. > const x = new URLSearchParams([["foo", "hello"]]); > x.get("foo"); "hello"

    > x.get("bar"); null URL Standard Ͱఆٛ

24. ୈࡾ໰

25. > document.getElementById("nonexistent"); ???

26. > document.getElementById("nonexistent"); null DOM Standard Ͱఆٛ

27. ୈ࢛໰

28. > const xs = ["A", "B", "C"]; > xs.find((x) =>

    x === "A"); "A" > xs.find((x) => x === "Z"); ???

29. > const xs = ["A", "B", "C"]; > xs.find((x) =>

    x === "A"); "A" > xs.find((x) => x === "Z"); undefined ECMAScript ͷݴޠ࢓༷Ͱఆٛ

30. ୈޒ໰

31. > const x = "ABC"; > x.match(/A/); ["A", ...] >

    x.match(/Z/); ???

32. > const x = "ABC"; > x.match(/A/); ["A", ...] >

    x.match(/Z/); null ECMAScript ͷݴޠ࢓༷Ͱఆٛ ਖ਼نදݱ͸ྫ֎తʹ null

33. > /A/.exec("ABC"); ["A", ...] > /Z/.exec("ABC"); null

34. ·ͱΊ • null ͱ unde fi ned ͸࢓༷͝ͱʹ֓ͶҰ؏ͨ͠࢖ΘΕํΛ͍ͯ͠Δ • ECMAScript

    ͷݴޠ࢓༷Ͱ͸ද޲͖͸΄΅ unde fi ned ͕࢖ΘΕΔ • Web ؔ࿈࢓༷Ͱ͸׳ྫతʹ null ͕࢖ΘΕΔ͜ͱ͕ଟ͍ • ҙຯతʹ࢖͍෼͚ΒΕ͍ͯΔ͜ͱ͸΄΅ͳͦ͞͏

35. ࢖͍෼͚ͷώϯτ • جຊ͸ unde fi ned Λ࢖͏ͷ͕͓͢͢Ί • ݴޠ૊ΈࠐΈͷػೳͱฒ΂ͯҧ࿨ײ͕ग़ʹ͍͘ •

    Web ͷ API ΍, null Λ࢖͏ϥΠϒϥϦʹڧؔ͘࿈͢Δ෦෼ͳΒ
 null Λ࢖ͬͯ౷ҰײΛग़͢ͷ΋ΞϦ • ͳΜʹͤΑҰ؏ͨ͠࢖͍ํ͕͞Ε͍ͯΔͷ͕Θ͔Γ΍͍ͩ͢Ζ͏

36. ࢀߟจݙ • ECMAScript® 2024 language speci fi cation • https://ecma-international.org/publications-and-standards/standards/

    ecma-262/ • WHATWG Standards • https://spec.whatwg.org

37. ͳΜͰ͜ͷ࿩Λ? • ECMAScript ͷݴޠ࢓༷ʹର͢Δ proposal Ͱ null ͕࢖ΘΕ͍͔ͯͨΒ • https://github.com/arthur

    fi orette/proposal-safe-assignment-operator • > The result should conform to the format [error, null | undefined] or [null, data]. • ͔֬ʹҙຯతʹ͸ؒҧ͍ͬͯͳ͍͕, ࢓༷ͷҰ؏ੑతʹ͸Ͳ͏?