await "thenable"

⟩ ⟩ ⟩ thenable

"thenable" objects： those with a .then() method.

If await gets a "thenable" object, it calls .then() and waits for the resolved value.

replit：

// -----------------------------
//     ⭐ "thenable" objects
// -----------------------------
// those with a `.then()` method
class FutureValue {

    constructor(value, seconds) {
        this.value = value;
        this.seconds = seconds;
    }

    // 🔸 then()
    then(resolve, reject) {
        console.log(new Date(), `then() method called`);
        setTimeout(resolve, this.seconds * 1000, this.value);
    }
}

// ⭐ normal Promise
function futureValue(value, seconds) {
    console.log(new Date(), `futureValue('${value}', ${seconds})`)
    return new Promise(resolve => {
        setTimeout(resolve, seconds * 1000, value);
    });
};


// main
(async () => {
  let result = await new FutureValue('hello', 2);   // await "thenable" ⭐  
  console.log(new Date(), result);
})();


(async () => {
  let result = await futureValue('there', 2);       // await Promise
  console.log(new Date(), result);
})();

// outcomes:
// -----------------------------------------------
// 2022-08-31T05:04:45.193Z futureValue('there', 2)
// 2022-08-31T05:04:45.197Z then() method called
// 2022-08-31T05:04:47.198Z there
// 2022-08-31T05:04:47.198Z hello

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