Custom Thenable

Thu, 20 Aug 2026 16:44:56 +0545

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const thenable: PromiseLike<string> = {
 then<TResult1 = string, TResult2 = void>(
 resolve: (value: string) => TResult1 | PromiseLike<TResult1>,
 reject: (reason: any) => TResult2 | PromiseLike<TResult2>,
 ): PromiseLike<TResult1 | TResult2> {
 return Promise.resolve("hello").then(resolve, reject);
 },
};

console.log("isPromise", thenable instanceof Promise); // false
const result = await thenable;
console.log("result", result); // hello

Cloudflare RPC mention an interesting construct in the JS promises here. They are using custom promise objects (that can be awaited) to batch (over the network) multiple RPC calls sequentially without awaiting the result of the first.

Javascript on aayushprime

Custom Thenable