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7 月前

為什麼要用 isNaN 函式

與其他 JavaScript 的值不同，你不可能靠等號運算符（== 與 ===）來判斷某個值是不是 NaN ，因為連 NaN == NaN 與 NaN === NaN 的結果都是 false 。因此， isNaN 函式是必要的。

NaN 值的來源

NaN 會在算術運算（arithmetic operations）出現 undefined 或是 unrepresentable 值的結果時產生。這些值不一定是溢出條件。 NaN 亦為試圖給毫無可用數字的原始值、予以強制運算之結果。

例如，零除以零的結果會是 NaN ——不過把其他數字除以零則不是 NaN 。

令人困惑的特殊狀況行為

從最早的 isNaN 函式版本規範始，其針對非數值之行為，不斷教人困惑至極。當 isNaN 函式的參數並非 數字 型別時，此值會先強制轉換到數字。該值接著會測定此值是否為 NaN 。因此，當被強制轉換的非數字，給出了有效的非 NaN 值（經典案例為空的字串與布林原始值：它們在強制轉換時，會給予數字結果 0 或 1）時，會回傳不如預期的「false」值：以空的字串為例，它很明顯地「非數字」。這段教人糾結的點，乃出於「非數字」術語的「數字」一詞、由 IEEE-754 浮點值定義之事實而來。這個函式要解釋為「當這個值，被強制轉換為數值時，它還是 IEEE-754 的『非數字』值嗎？」的答案。

最新的 ECMAScript（ES2015）版本導入了 Number.isNaN() 函式。儘管 Number.isNaN 的 NaN 依舊維持了數字上的意義、而不是簡單的「非數字」， Number.isNaN(x) 在偵測 x 為 NaN 與否時比較可靠。另外，如果在缺少 Number.isNaN 的情況下，通過表達式 (x != x) 來檢測變量 x 是否是 NaN 會更加的可靠。

一個 isNaN 的 polyfill 可以理解為（這個 polyfill 利用了 NaN 自身永不等於自身這一特性）：

js 
var isNaN = function (value) {
  var n = Number(value);
  return n !== n;
    
js
isNaN(NaN); // true
isNaN(undefined); // true
isNaN({}); // true
isNaN(true); // false
isNaN(null); // false
isNaN(37); // false
// 字串
isNaN("37"); // false: "37" 轉換成數字的 37 後就不是 NaN 了
isNaN("37.37"); // false: "37.37" 轉換成數字的 37.37 後就不是 NaN 了
isNaN("123ABC"); // true:  parseInt("123ABC") 是 123 但 Number("123ABC") 是 NaN
isNaN(""); // false: 空字串轉換成數字的 0 後就不是 NaN 了
isNaN(" "); // false: 有空白的字串轉換成數字的 0 後就不是 NaN 了
// 日期
isNaN(new Date()); // false
isNaN(new Date().toString()); // true
// 這個偵測的錯誤是不能完全信賴 isNaN 的理由
isNaN("blabla"); // true: "blabla" 被轉換為數字，將其解析為數字失敗後回傳了 NaN
    
實用的特殊狀況行為

    
當然，你能以更用途導向的方法去思考 isNaN()：如果 isNaN() 回傳 false，那麼把 x 用在任何算術表達式都不會回傳 NaN。相反地，如果回傳 true，那麼把 x 用在任何算術表達式都會是 NaN。這在 JavaScript 的意義是 isNaN(x) == true 等於 x - 0 回傳 NaN（儘管在 JavaScript 裡面 x - 0 == NaN 永遠回傳 false，你因而無法測試）── 事實上，isNaN(x)、isNaN(x - 0)、isNaN(Number(x))、Number.isNaN(x - 0)、Number.isNaN(Number(x)) 在 JavaScript 裡面，都會回傳一樣的東西。而 isNaN(x) 是所有表達式裡面最短的一種。


比方說，你可以用這個式子，去測試函式的參數能不能透過算術處理（也就是能「像」數字一樣被利用）、否則就提供預設值之類的。你可以透過上下文的根據以隱式數值轉換（implicitly converting value），以使用 JavaScript 提供的全部功能。


js
function increment(x) {
  if (isNaN(x)) x = 0;
  return x + 1;
// 與 Number.isNaN() 一樣：
function increment(x) {
  if (Number.isNaN(Number(x))) x = 0;
  return x + 1;
// 以下範例的函式參數 x，isNaN(x) 都會回傳 false，
// 儘管 x 不是數字，依舊能用在算術表達式。
increment(""); // 1: "" 被轉換成 0
increment(new String()); // 1: 空字串的新字串物件被轉換成 0
increment([]); // 1: [] 被轉換成 0
increment(new Array()); // 1: 空陣列的新陣列物件被轉換成 0
increment("0"); // 1: "0" 被轉換成 0
increment("1"); // 2: "1" 被轉換成 1
increment("0.1"); // 1.1: "0.1" 被轉換成 0.1
increment("Infinity"); // Infinity: "Infinity" 被轉換成 Infinity
increment(null); // 1: null 被轉換成 0
increment(false); // 1: false 被轉換成 0
increment(true); // 2: true 被轉換成 1
increment(new Date()); // 回傳以毫秒為單位加 1，當今的日期/時間
// 以下範例的函式參數 x，isNaN(x) 都會回傳 false，而 x 的確是數字。
increment(-1); // 0
increment(-0.1); // 0.9
increment(0); // 1
increment(1); // 2
increment(2); // 3
// …等等…
increment(Infinity); // Infinity
// 以下範例的函式參數 x，isNaN(x) 都會回傳 true，x 也的確不是數字。
// 使得函式會被 0 取代，並回傳 1
increment(String); // 1
increment(Array); // 1
increment("blabla"); // 1
increment("-blabla"); // 1
increment(0 / 0); // 1
increment("0/0"); // 1
increment(Infinity / Infinity); // 1
increment(NaN); // 1
increment(undefined); // 1
increment(); // 1
// isNaN(x) 與 isNaN(Number(x)) 永遠一樣，不過這裡的 x 是強制存在的！
isNaN(x) == isNaN(Number(x)); // 針對所有 x 的值都是 true，x == undefined 也不例外，
// 因為 isNaN(undefined) == true 且 Number(undefined) 回傳 NaN，
// 不過……
isNaN() == isNaN(Number()); // false，因為 isNaN() == true 且 Number() == 0
        Specification
      
  1. JavaScript
  2. Tutorials and guides
  3. Beginner's tutorials
    1. Your first website: Adding interactivity
    2. Dynamic scripting with JavaScript
    3. JavaScript frameworks and libraries
  4. JavaScript Guide
    1. Introduction
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    4. Loops and iteration
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    6. Expressions and operators
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    8. Representing dates & times
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    11. Keyed collections
    12. Working with objects
    13. Using classes
    14. Using promises
    15. JavaScript typed arrays
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    17. Resource management
    18. Internationalization
    19. JavaScript modules
  5. Intermediate
    1. Advanced JavaScript objects
    2. Asynchronous JavaScript
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    4. Language overview
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    6. Equality comparisons and sameness
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    8. Closures
  6. Advanced
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    2. Meta programming
    3. Memory Management
  7. References
  8. Built-in objects
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      1. import.meta.resolve()
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  10. Statements & declarations
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      1. Import attributes
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      2. callee

      3. length
  12. Classes
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  13. Regular expressions
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    6. Disjunction: |
    7. Input boundary assertion: ^, $
    8. Literal character: a, b
    9. Lookahead assertion: (?=...), (?!...)
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    11. Modifier: (?ims-ims:...)
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    15. Quantifier: *, +, ?, {n}, {n,}, {n,m}
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  14. Errors
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    2. Error: Permission denied to access property "x"
    3. InternalError: too much recursion
    4. RangeError: argument is not a valid code point
    5. RangeError: BigInt division by zero
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    7. RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'
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    50. SyntaxError: invalid identity escape in regular expression
    51. SyntaxError: invalid named capture reference in regular expression
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    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
    55. SyntaxError: invalid regular expression flag "x"
    56. SyntaxError: invalid unicode escape in regular expression
    57. SyntaxError: JSON.parse: bad parsing
    58. SyntaxError: label not found
    59. SyntaxError: missing : after property id
    60. SyntaxError: missing ) after argument list
    61. SyntaxError: missing ) after condition
    62. SyntaxError: missing ] after element list
    63. SyntaxError: missing } after function body
    64. SyntaxError: missing } after property list
    65. SyntaxError: missing = in const declaration
    66. SyntaxError: missing formal parameter
    67. SyntaxError: missing name after . operator
    68. SyntaxError: missing variable name
    69. SyntaxError: negated character class with strings in regular expression
    70. SyntaxError: new keyword cannot be used with an optional chain
    71. SyntaxError: nothing to repeat
    72. SyntaxError: numbers out of order in {} quantifier.
    73. SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code
    74. SyntaxError: parameter after rest parameter
    75. SyntaxError: private fields can't be deleted
    76. SyntaxError: property name __proto__ appears more than once in object literal
    77. SyntaxError: raw bracket is not allowed in regular expression with unicode flag
    78. SyntaxError: redeclaration of formal parameter "x"
    79. SyntaxError: reference to undeclared private field or method #x
    80. SyntaxError: rest parameter may not have a default
    81. SyntaxError: return not in function
    82. SyntaxError: setter functions must have one argument
    83. SyntaxError: string literal contains an unescaped line break
    84. SyntaxError: super() is only valid in derived class constructors
    85. SyntaxError: tagged template cannot be used with optional chain
    86. SyntaxError: Unexpected '#' used outside of class body
    87. SyntaxError: Unexpected token
    88. SyntaxError: unlabeled break must be inside loop or switch
    89. SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'
    90. SyntaxError: use of super property/member accesses only valid within methods or eval code within methods
    91. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    92. TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed
    93. TypeError: 'x' is not iterable
    94. TypeError: "x" is (not) "y"
    95. TypeError: "x" is not a constructor
    96. TypeError: "x" is not a function
    97. TypeError: "x" is not a non-null object
    98. TypeError: "x" is read-only
    99. TypeError: already executing generator
    100. TypeError: BigInt value can't be serialized in JSON
    101. TypeError: calling a builtin X constructor without new is forbidden
    102. TypeError: can't access/set private field or method: object is not the right class
    103. TypeError: can't assign to property "x" on "y": not an object
    104. TypeError: can't convert BigInt to number
    105. TypeError: can't convert x to BigInt
    106. TypeError: can't define property "x": "obj" is not extensible
    107. TypeError: can't delete non-configurable array element
    108. TypeError: can't redefine non-configurable property "x"
    109. TypeError: can't set prototype of this object
    110. TypeError: can't set prototype: it would cause a prototype chain cycle
    111. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    112. TypeError: class constructors must be invoked with 'new'
    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
    115. TypeError: getting private setter-only property
    116. TypeError: Initializing an object twice is an error with private fields/methods
    117. TypeError: invalid 'instanceof' operand 'x'
    118. TypeError: invalid Array.prototype.sort argument
    119. TypeError: invalid assignment to const "x"
    120. TypeError: Iterator/AsyncIterator constructor can't be used directly
    121. TypeError: matchAll/replaceAll must be called with a global RegExp
    122. TypeError: More arguments needed
    123. TypeError: null/undefined has no properties
    124. TypeError: property "x" is non-configurable and can't be deleted
    125. TypeError: Reduce of empty array with no initial value
    126. TypeError: setting getter-only property "x"
    127. TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol
    128. TypeError: X.prototype.y called on incompatible type
    129. URIError: malformed URI sequence
    130. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    131. Warning: unreachable code after return statement
  15. Misc
    1. JavaScript technologies overview
    2. Execution model
    3. Lexical grammar
    4. Iteration protocols
    5. Strict mode
    6. Template literals
    7. Trailing commas
    8. Deprecated features
 
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