LazyPromise
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About
LazyPromise is a lightweight async primitive for JavaScript/TypeScript that combines ideas from RxJS, native Promises, and the Effect library. It behaves like a single-shot Observable with lazy evaluation and automatic cancellation, while keeping an API that closely mirrors the native Promise, and adds generator syntax, type-safe errors, and dependency injection without Effect's added complexity.
At a glance
LazyPromise gives JavaScript developers a way to describe a single asynchronous operation that starts later and can pass cancellation along. It is worth exploring if your async code repeatedly wires cancellation through several dependent operations.
Good fit
TypeScript developers who want one abstraction for deferred work, typed errors and cancellation.
Consider something else when
Skip it if native promises already keep the workflow clear or if you need an ongoing stream of values.
Start here
Read the basic usage and native promise interop examples. Then sketch one cancellable operation with two dependent steps and check when each starts and what each receives when cancelled.
Cost and requirements
The documentation covers adding it as a JavaScript library and interoperating with native promises. Budget time for the team to learn its rules for starting and cancelling work.
Limits to know
Cancellation can only stop work that cooperates with it. Check behavior at boundaries with native promises or APIs that have already started an operation.
Sources checked 2026-11-05. Source-based assessment.
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