🔸new.target

• JS ⟩ value ⟩ object ⟩ create ⟩ new ⟩ new.target

a function can know whether it is invoked with new by checking new.target. new.target is only undefined when the function is invoked without new.

⭐️ 重點

in normal function calls, new.target is undefined.

function User(name) {
    // ⭐️ if called normally, add `new`.
    if (!new.target) { return new User(name); }
    this.name = name;
}

let joe = User('Joe');    // ⭐️ `new` not required now

in class constructors (or function called using new), new.target refers to the constructor/function that was directly invoked by new. for example:

new A(); // new.target === A
new B(); // new.target === B

⚠️ class constructors cannot be invoked without 'new'.

new.target "pseudo-property" is available in all functions.

💈範例

💾 replit：new.target

// ----------------------------------------------------
// in class constructors, `new.target` refers to 
// the constructor that was directly invoked by `new`.
// for example:
//     new A();     // new.target === A
//     new B();     // new.target === B
// ----------------------------------------------------
class A {
    constructor() {
        console.log(`new ${new.target.name}(): new.target === A ? ${new.target === A}`);
    }
}

class B extends A {
    constructor() {
        super();
    }
}

const a = new A();         // new.target === A ? true
const b = new B();         // new.target === A ? false

// A();
// ⛔️ TypeError: Class constructor `A` cannot be invoked without 'new'

// ----------------------------------------------------
// in normal function calls, `new.target` is `undefined`.
// ----------------------------------------------------
function User(name) {
    // ⭐ if called without `new`, add `new` automatically.
    if (!new.target) { return new User(name) }
    this.name = name;
}

let joe = User('Joe');    // ⭐ `new` not required now
console.log(joe.name);    // 'Joe'

📗 參考

• JS.info ⟩ Constructor, operator "new"

📘 手冊

• new operator

• new.target - detect whether a function/constructor was called using new.