JavaScript Promise Integration

concepts

1. JSPI

   JSPI lets WebAssembly suspend on JS Promises directly. C/C++ ports get async semantics without manually pumping a state machine.

2. Wasm fetch()

   The other JSPI shape: Wasm-as-Promise-producer. A wasm function calls a synchronous-looking fetch host import and the wrapped export hands back a Promise the JS side can await.

3. C-loop Unblocker

   Two lanes for a ported C library that calls in a loop: synchronous shim (blocks the page) vs JSPI-suspended (UI keeps ticking). Live RAF pulse meter shows dropped frames in the legacy lane.

4. Stack-switching trace

   Step or play through a JSPI call; the two stacks (JS + suspendable Wasm) render frame-by-frame with a chronological event log of entered / suspended / resumed events.

why it shipped

In order to support responsive applications written using WebAssembly it is necessary to provide functionality that allows WebAssembly programs to be suspended and resumed. The primary initial use case is to allow WebAssembly programs whose source relies on synchronous APIs to use asynchronous APIs that are increasingly common on the Web platform. The Promise integration proposal allows this to be done without changing the WebAssembly specification itself and without allowing JavaScript programs to be suspended (other than via existing async mechanisms).

references

• ChromeStatus entry
• Spec