➕await

turns Promise (future value) into a value/error.

• JS ⟩ async ⟩ await

• JS ⟩ statement ⟩ expression ⟩ operator ⟩ unary ⟩ await

(⭐️ ES2017) (primary expression) (unary operator) turns a Promise / thenable into a return value / thrown exception.

• if resolves normally, await returns the result.

• if rejected, it throws error (as if there were a throw statement at that line).

// works only inside async functions❗️ 
let value = await promise;

⭐️ 重點

• await can only be used inside an async function❗

• modern browsers allow top-level await in modules.

// when top-level await not supported, use async IIFE instead.
(async () => {
  let value = await promise;
  // ...
})();

🔴 主題

• await promises (in parallel or in series)

  • await in parallel

  • await sequentially

• await "thenable" - objects that await supports.

• error handling for await.

📗 參考

• JS.info ⟩ async/await

• JavaScript: The Definitive Guide ⟩ Ch. 13

📘 手冊

• await