⚖️callable vs. constructable

• JS ⟩ value ⟩ object ⟩ create ⟩ new ⟩ callable vs. constructable

• if a function can be invoked in the form F(), it's callable.

• if a function can be invoked in the form new F(), it's constructable.

⭐️ 重點

not every function is callable, for example, Map is a function, but calling it with Map() throws a TypeError❗

not every function is constructable, for example, Symbol is a function, but calling it with new Symbol() throws a TypeError❗

👥 相關

• 👉 [[Call]], [[Construct]] internal methods.

• constructor is a constructable function.

💈範例

• replit ⟩ callable vs. constructable

// Array is callable & constructable, same effect.
let arr1 = Array(3);
let arr2 = new Array(3);

let map = Map();            // Map is not callable.
//        ^^^
// ⛔ TypeError: Constructor `Map` requires 'new'

let sym = new Symbol();     // Symbol is not constructable.
//        ^^^
// ⛔ TypeError: `Symbol` is not a constructor

arr1,    // [ , , ] (3 empthy items)
arr2,    // [ , , ] (3 empthy items)

Map instanceof Function,    // true
Symbol instanceof Function, // true

📗 參考

• JS.info ⟩ Constructor, operator "new"

📘 手冊

• new operator ⭐️

function	F()	new F()	note
Array, Error, Function	✅	✅	same effect
Boolean, String, Number	primitive	wrapper object
Date	string	object
Symbol, BigInt	✅	⛔️ TypeError
Proxy, Map	⛔️ TypeError	✅
class	⛔️ TypeError	✅