🔢NaN

not a number.

• JS ⟩ value ⟩ primitive ⟩ number ⟩ special ⟩ NaN

• JS ⟩ scope ⟩ global ⟩ global object ⟩ property ⟩ NaN

( non-configurable, non-writable global object property )❗

• a property of the global object. (that is, a variable in global scope).

• equivalent to Number.NaN.

🧨 雷區

NaN is the only value that doesn't equal to itself❗

// ⭐️ `NaN` is the ONLY ONE that doesn't equal to itself.
NaN === NaN,            // false❗️
NaN !== NaN,            // true❗️

// equivalent forms
Number.isNaN(x) === (x !== x)

trichotomy law： a < b, a = b, a > b doesn't apply to NaN❗

3 <  NaN,      // false❗
3 == NaN,      // false❗
3 >  NaN,      // false❗

⭐️ 重點

• isNaN(x) will try to convert x to a number first❗

• Number.isNaN(x) never do conversions.

for Number.isNaN(x) to be true, x must be a number first❗️

x !=== x is equivalent to Number.isNaN(x)

arithmetic operator(s)

• 0/0 evaluates to NaN (no error)❗

• non-numeric operand(s) in arithmetic operations that cannot convert to numbers convert to NaN.

comparison operator

if either operand is (or converts to) NaN, the result is false.

NaN, ±Infinity all converts to 0 when used as operands of bitwise operator.

👥 相關

• 👉 Infinity

💈範例

• replit： NaN

// ⭐️ `NaN` is the ONLY ONE that doesn't equal to itself❗️
// -------------------------------------------------------
// 
//     `x !=== x` is equivalent to `Number.isNaN(x)`
//
NaN === NaN,            // false❗️
NaN !== NaN,            // true❗️
Number.isNaN(NaN),      // true    (⭐️ the ONLY value that makes this true❗️)

// ⭐️ isNaN() tries to convert to a number first❗️
Number('foo'),          // NaN
isNaN('foo'),           // true❗️  ('foo' -> NaN)
Number('123abc'),       // NaN
isNaN('123abc'),        // true❗️  ('123abc' -> NaN)

// ⭐️ Number.isNaN() NEVER converts anything❗️
// --------------------------------------------
// for `Number.isNaN(x)` to be true, `x` must be a number first❗️
Number.isNaN('foo'),    // false❗️ (⭐️ `false` for non-numeric values❗️)
Number.isNaN(2 / 'foo'),   // true

📗 參考

• JavaScript: The Definitive Guide ⟩ 4.8

📘 手冊

• NaN
• Number.NaN - equivalent to NaN.
• isNaN() - with type conversion. (attempts to convert to a number)
• Number.isNaN() ⟩ - without type conversion.

  • Difference between Number.isNaN() and global isNaN()