You can't cancel a JavaScript promise (except sometimes you can)

Summary

You can't cancel a JavaScript promise. There's no .cancel() method, no AbortController integration, no built-in way to say "never mind, stop." The TC39 committee considered adding cancellation in 2016, but the proposal was withdrawn after heated debate. Part of the problem is that cancelling arbitrary code mid-execution can leave resources in a dirty state (open handles, half-written data), so true cancellation requires cooperative cleanup, which undermines the simplicity people want from a .cancel() method. But you can do something weirder: return a promise that never resolves, await it, and let the garbage collector clean up the suspended function. No exceptions, no try/catch, no special return values. The function just stops. This is how the Inngest TypeScript SDK interrupts async workflow functions. But the technique is general-purpose, and the JavaScript semantics behind it are worth understanding on their own. Why you'd want to interrupt a function Sometimes you need to stop someone else's async function at an exact point, without their code doing anything special. The function's author writes normal async/await code. Your runtime decides when and where to interrupt it. The concrete case we hit: running workflow functions on serverless infrastructure where each invocation has a hard timeout. A workflow might have dozens of steps that take hours to complete end-to-end, but each invocation can only run for seconds or minutes. The runtime (in our case, the SDK itself) needs to interrupt the function, save progress, and re-invoke it later to pick up where it left off, all without the user's code knowing it happened. That requires interrupting an await without throwing. Interrupting with errors When implementing interruption, the obvious approach is to throw an exception. Imagine a run function that executes a callback and then throws a special error to stop the caller from continuing: class InterruptError extends Error {} async function run(callback) { const resul...