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JavaScriptの特別な値メモ

JavaScript
  • undefined
    • typeof undefined → "undefined"
    • undefined === undefined → true
    • undefined == null → true
    • undefined == 0 → false
    • undefined == false → false
    • void x → undefined
  • null
    • typeof null → "object"
    • null === null → true
    • null == 0 → false
    • null == false → false
  • NaN
    • typeof NaN → "number"
    • NaN === NaN → false
    • -NaN → NaN
  • Infinity
    • typeof Infinity → "number"
    • Infinity === Infinity → true
    • -Infinity === -Infinity → true
    • Infinity === -Infinity → false
    • Infinity - Infinity, 0*Infinity, Infinity/Infinity → NaN
    • Infinity + 3, Infinity - 10, 2*Infinity, Infinity*Infinity → Infinity
  • true, false
    • false == 0 → true
    • true == 1 → true
    • true == 2 → false (真偽値と数値は、数値に変換して比較する)

このうち、undefined, NaN, Infinity には値を代入できてしまう!