Futures, Promises, Tasks — Middle¶

At middle level, focus on this question:

Does creating a future start the underlying work immediately (eager), or only once something actively drives it (lazy)?

Prerequisite: junior.md.

JavaScript Promises: eager — work starts immediately¶

const promise = fetch(url);   // the actual network request STARTS
                                // RIGHT NOW, even before anyone
                                // calls .then() or awaits it
console.log("This runs while the fetch is already in flight");

flowchart LR Create["new Promise(...)\nor calling fetch()"] --> Starts["Underlying work STARTS\nIMMEDIATELY - the promise\nis just a handle to\nALREADY-RUNNING work"]

Rust Futures: lazy — nothing happens until polled¶

let future = fetch(url);  // NOTHING happens yet - this just
                           // constructs the future VALUE
// The actual network request only starts once something
// polls this future (typically via .await inside an async
// context being driven by an executor)
some_async_fn(future).await;  // NOW the work actually begins

flowchart LR Create2["let future = fetch(url)"] --> Inert["INERT - just a value,\nno work started"] --> Poll["Only when POLLED\n(by an executor/await)\ndoes the actual work begin"]

	JavaScript Promise	Rust Future
When does work start	Immediately upon creation	Only when first polled
A future you never await	Work still happens (fire-and-forget)	Work NEVER happens at all

🎓 Takeaway: this eager-vs-lazy distinction has a real, surprising consequence: an unawaited JavaScript promise's work still runs (a common "fire and forget" pattern, sometimes accidental); an unawaited/ unpolled Rust future's work never runs at all — dropping a Rust future without ever polling it is equivalent to never having called the function, a frequent source of confusion for engineers moving between these two languages' async models.

Test yourself¶

Why does an unawaited JavaScript promise's underlying operation still execute, while an unawaited Rust future's operation never runs at all?
Give a real bug scenario in JavaScript caused by accidentally not awaiting a promise (hint: think about error handling and ordering).
Give a real bug scenario in Rust caused by constructing a future but never polling/awaiting it.

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