ArrayBuffer ranged views

[ghost]

Consider an application using JavaScript and WebAssembly, communicating through the Wasm heap.

If the JS wants to pass around a read-only slice of the Wasm heap (a JS ArrayBuffer object), the JS must either pass around a TypedArray and always make sure to use it's .byteLength and .byteOffset properties to ensure correct usage, or copy the memory in the slice into a new ArrayBuffer.

One wants the slice to be read-only in order to prevent writing to the memory, and one doesn't want to move around the entire ArrayBuffer object, as that would allow reading into the memory at practically arbitrary locations.

Without being able to do both, view a section of an ArrayBuffer, and mark it as read-only, developers will have to find a workaround, which will likely come to creating ad-hoc wrapper classes or copying.

Could sliced views be introduced into the proposal alongside read-only views?

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The only two questions that I would have regarding an introduction of the idea would be the following:

• Would the views have alignment requirements? Would they expose their alignment? (E.g. TypedArrays need to know alignment)
• What would be the construction API?

[Jack-Works]

The "read-only view" in this proposal includes both TypedArray family and DataView. Can a read-only DataView satisfy your needs?

[ghost]

@Jack-Works I'm not sure if we're on the same page with the problem.

Consider this example:

const over_allocated_buffer = Uint8Array.of(0, 1, 2, 3, 4, 5);

function get_pair() {
    const ro_data_view = new DataView(over_allocated_buffer, 0, 2, { readonly: true });

    return ro_data_view;
}

The function get_pair should return a read-only DataView pointing to a pair of bytes, and we could confirm this ourselves,

assert(get_pair().byteLength === 2);

But, we can easily access the DataView#buffer property...

// Accessing implementation detail
const { buffer } = get_pair();

const view = new Uint8Array(buffer):

// reads past 2 bytes
console.log(view);

... leading us back around to the problem with using a TypedArray.

[Jack-Works]

Ok, I understand your question, the point here is not read-only or not.
A truly limited slice view (cannot be escaped by .buffer) of ArrayBuffer is really useful (as you said in the WebAssembly case). But I think it is beyond the current focused topic "read-only".
Maybe I can re-consider this proposal to focus on both "limited view" and read-only view, they're two orthogonal features.

A possible solution is like this:

const slice = new DataView(over_allocated_buffer, 0, 2, { slice: true });
slice.buffer // undefined, therefore no way to escape

Then you can read/write on the slice of the buffer but no way to escape.

[Jack-Works]

A new proposal 🎉