🔰type conversion

JS ⟩ value ⟩ type ⟩ conversion

When JS expects a boolean, a value of any type can be supplied, and JS will convert it as needed. The same is true for all types.

// ⭐ explicit type conversion
Number(' 012'),    // 12
String(34),        // '34'
Boolean({}),       // true

// ⭐ implicit type conversion
+' 012',    // === Number(' 012')
34 + '',    // === String(34)
!!{},       // === Boolean({})

🗺️

• replit： type conversion table (require： TableMaker)

┌── (primitive?)
│                              (-> number)   (-> string)
│  x          desc                    +x     String(x)
────────────────────────────────────────────────────────────────────
✅ null                                  0   "null"                 
────────────────────────────────────────────────────────────────────
✅ undefined                           NaN   "undefined"            
────────────────────────────────────────────────────────────────────
✅ 0                                     0   "0"                    
✅ Infinity                       Infinity   "Infinity"             
✅ NaN                                 NaN   "NaN"                  
────────────────────────────────────────────────────────────────────
✅ ""          empty str                 0   ""                     
✅ "1.2"       numeric                 1.2   "1.2"                  
✅ "one"       non-numeric             NaN   "one"                  
────────────────────────────────────────────────────────────────────
✅ true                                  1   "true"                 
✅ false                                 0   "false"                
────────────────────────────────────────────────────────────────────
❌ {a:1}                               NaN   "[object Object]"      
❌ []          empty arr                 0   ""                     
❌ [6]         one numeric               6   "6"                    
❌ ['a']       any other               NaN   "a"                    
❌ new Date                  1667314617317   "Tue Nov 01 2022 14:...
❌ /regex/                             NaN   "/regex/"              
────────────────────────────────────────────────────────────────────
❌ () => {}                            NaN   "() => {}"             
❌ class {}                            NaN   "class {}"             
────────────────────────────────────────────────────────────────────

🧨

can't convert❗

• BigInt -> Number❗(👉 TypeError )

• Symbol -> Number❗(👉 TypeError )

⭐️

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

• Number.isNaN(x) never do conversions.

convert any value to Boolean.

!!x    // any -> true/false, same as Boolean(x)

convert any value to Number.

+x    // any -> number, same as `Number(x)`

any non-nullish value has .toString() method, usually equivalent to String().

🔴

• operators

  • unary plus (+) - any -> number (except symbol, bigint)

  • double tilde (~~) - nearest integer towards zero.

  • not not (!!) - any -> bool.

• type conversions

  • converting objects

    • 🔰 object -> primitive conversion

  • 💍 falsy

👥

• ℹ️ type conversion is involved in sloppy equality (==)

• ℹ️ type conversion is involved in comparison operator.

• ⚠️ values that try to convert to a number may result in NaN❗

• ⛔ possible errors：

  • TypeError：cannot convert BigInt to Number❗

  • TypeError：cannot convert Symbol to Number❗

💈

replit： type conversion examples

let n = NaN;

8 + " cats",    // "8 cats"   (8 -> '8')
n + " dogs",    // "NaN dogs" (NaN -> 'NaN')

'3' * '8',      // 24         ('3' -> 3, '8' -> 8)
4 - "a",        // NaN        ('a' -> NaN)

📗

• JavaScript: The Definitive Guide ⟩

  • 3.4 Boolean Values

  • 3.9 Type Conversions ⟩ 3.9.2 Explicit Conversions

📘

• JavaScript data types and data structures ⟩ Type coercion

• String ⟩ String coercion

• Object.prototype.toString() - object to string.

• Object.prototype.valueOf() - object to (primitive) value, if exists.

🗣

• In JavaScript, why is "0" equal to false, but when tested by 'if' it is not false by itself?