📘yield*

the "done" value of the iterator

JS ⟩ objects ⟩ built-in ⟩ Generator ⟩ generator function ⟩ yield* expression

the value of yield* expression is the "done" value of the iterator. (undefined by default)

💈範例

• replit：value of yield* expression

const { log } = console;

// generator: a()
function* a() {
    yield* [1, 2, 3];
    return 'foo';                // ⭐️ "done" value of `a`
}

// generator: b()
function* b() {
    
    // --------------------------------------------------------------
    const value = yield* a();    // ⭐️ "done" value of `a` returned
                                 //     after it's finished yielding.
    // --------------------------------------------------------------
    
    console.log(value)           // console: 'foo'
    
    return value + value;        // ⭐️ "done" value of `b`
}

// (iterable) iterators
const it1 = a();

log(it1.next());    // {value: 1, done: false}
log(it1.next());    // {value: 2, done: false}
log(it1.next());    // {value: 3, done: false}
log(it1.next());    // {value: 'foo'⭐️, done: true}

log(it1.next());    // {value: undefined❗, done: true}
                    // ⭐️ "done" value is only used once❗

const it2 = b();

log(it2.next());    // {value: 1, done: false}
log(it2.next());    // {value: 2, done: false}
log(it2.next());    // {value: 3, done: false}

log(it2.next());    // console: 'foo' <--- ⭐️ "done" value of `a` returned here
                    // {value: 'foofoo'⭐️, done: true}

⭐️ 重點

yield* is an expression (not a statement) that evaluates to a value.

🧨 雷區

yield and yield* operator can only be used within generator functions❗️

• replit：yield must be in generator functions

// this seems like a generator function, but there's a catch ...
function* sequence(...iterables) {

    // --------------------------------------------------------------
    // ⭐ `yield/yield*` only available within generator functions❗
    // --------------------------------------------------------------
    
    // ❌ but this `yield*` is within an "arrow function"❗
    //
    //                 ╭─── 🔸 arrow function ───╮
    iterables.forEach( iterable => yield* iterable );
    //                             ^^^^^^
    // ⛔ ReferenceError: yield is not defined 
    
}

📘 手冊

• yield* ⟩

  • value of yield* expression