🔢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.

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❗

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

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Last updated 3 years ago

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