data: Wasm definition

Try it

(module
  (memory (export "memory") 1)
  (data $greeting1 (i32.const 0) "Hello ")
  (data $greeting2 "World")

  (func (export "init")
    i32.const 6       ;; destination offset in memory
    i32.const 0       ;; offset into the data segment
    i32.const 5       ;; number of bytes to copy
    memory.init $greeting2
  )
)

WebAssembly.instantiateStreaming(fetch("{%wasm-url%}")).then((result) => {
  result.instance.exports.init();
  const memBuffer = result.instance.exports.memory.buffer;
  const memArray = new Uint8Array(memBuffer, 0, 11);
  console.log(new TextDecoder().decode(memArray));
});

In the above example, we specify two data definitions with identifiers $greeting1 and $greeting2, containing the strings "Hello " and "World", respectively. The first data definition has a memory offset specified ((i32.const 0)), so immediately writes the string to the specified memory at bytes 0–4. The second data definition doesn't have a memory offset specified, so isn't written to memory until the memory.init instruction is executed later on.

In the JavaScript, we call the exported init() function to write the second data definition into memory, then decode the exported memory buffer and log the result ot the console.

Syntax

;; Active form, written to memory immediately
data name memory_index offset data_string

;; Passive form, written later via memory.init
data name data_string

data: The data definition type. Must always be included first.
name Optional: An optional identifying name for the table. This must begin with a $ symbol, for example $my_table. If this is omitted, the data can be identified (for example when calling memory.init) by its index, for example 0 for the first data in the wasm script, 1 for the second, etc.
memory_index Optional: An integer representing the index number of the memory instance to write the data to, in the case of Wasm modules with multiple memories.
offset Optional: An integer representing the offset at which to start writing the data in the memory.
data_string: A string of literal bytes defining the data represented by this data instance.