func: Wasm reference instruction

Try it

(module
  (func $double (param i32) (result i32)
    (i32.mul (local.get 0) (i32.const 2))
  )
  ;; Make $double referenceable by ref.func
  (elem declare func $double)

  (func (export "getDouble") (result funcref)
    ref.func $double
  )
)

const url = "{%wasm-url%}";
const { instance } = await WebAssembly.instantiateStreaming(fetch(url));

// The returned funcref can be called like a JavaScript function
const double = instance.exports.getDouble();
console.log(double(21));

WAT syntax

ref.func function

ref.func

The ref.func instruction.

Immediates

function

The function to create a reference to. This can be the function's name, for example $my_function, or its index.

Operand stack

[] -> [output:(ref function_type)]

output

A non-null reference to function. Its type is a reference to the function's type, which can be used anywhere a funcref is expected.

Binary encoding

Description

The ref.func instruction creates a reference to a function defined in or imported into the module. You can store the reference in a table, pass it to another function, or return it to JavaScript, where it can be called like a regular JavaScript function.

A module only validates if every function used with ref.func inside a function body is also referenced somewhere outside the function bodies: for example, in an export, a global initializer, or an elem segment. If a function isn't otherwise referenced, you can declare it with a declarative elem segment, such as (elem declare func $my_function).

Because ref.func is a constant instruction, it can also be used in constant expressions, such as the initializer of a global or an item in an elem segment.

Specifications

Browser compatibility

See also

• funcref
• elem
• ref.null