Function js_sys::Atomics::add

pub fn add(
    typed_array: &JsValue,
    index: u32,
    value: i32
) -> Result<i32, JsValue>

The static Atomics.add() method adds a given value at a given position in the array and returns the old value at that position. This atomic operation guarantees that no other write happens until the modified value is written back.

You should use add_bigint to operate on a BigInt64Array or a BigUint64Array.

MDN documentation