There are two ways of interacting with WebAssembly memory from JS, 1. exporting memory to JS and 2. importing memory from JS. Demonstrated below is the somewhat easier way (method 1). Memory in WebAssembly is linear[1], so byte sizes of data has to be taken into account with the offset:
// mod.wat
(module
(memory (export "memory") 1) ;; create one page of memory and export it
(export "read" (func $read (param $index i32) (result i32)
local.get $index ;; get the index to be read
i32.const 4 ;; i32 uses 4 bytes per number
i32.mul
i32.load ;; load that index
))
)
After exporting the memory, the memory can be accessed as an exported member on the WASM instance and combine it with a Uint32Array to enable direct reading and writing directly to the instance's memory:
const module = await WebAssembly.compile(readFileSync('path/to/mod.wasm'));
const instance = await WebAssembly.instantiate(module);
const memory = new Uint32Array(instance.exports.memory.buffer, 0, 1);
memory[0] = 42;
console.log(instance.exports.read(0));
// => 42
This is useful in avoiding the overhead that comes with crossing the boundary between JS and WASM during function invocations. Instead of loading values one-by-one in function calls, they can be put directly into the buffer in JS.
Some further reading that helped me along is WebAssembly by Hand and WASM by Hand (but it's a different one).
[1] Linear memory