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Reference Types

Reference types let Wasm refer to functions and host objects.

Types

Section titled “Types”
  • funcref — reference to a function inside the module or table.
  • externref — opaque reference to a host value (e.g. a JS object).

ref.null, ref.func, ref.is_null

Section titled “ref.null, ref.func, ref.is_null”
(module
  (func $double (param $x i32) (result i32)
    (i32.mul (local.get $x) (i32.const 2)))


  (table 2 funcref)
  (elem (i32.const 0) $double)


  (func (result i32)
    (ref.is_null (ref.null funcref)))    ;; -> 1


  (func (result i32)
    (ref.is_null (ref.func $double)))    ;; -> 0
)

Tables and call_indirect

Section titled “Tables and call_indirect”
(module
  (type $t0 (func (param i32) (result i32)))
  (func $double (type $t0) (param $x i32) (result i32)
    (i32.mul (local.get $x) (i32.const 2)))


  (table 1 funcref)
  (elem (i32.const 0) $double)


  (func (param $n i32) (result i32)
    (call_indirect (type $t0) (local.get $n) (i32.const 0)))
)

The last argument to call_indirect is the table element index. The preceding arguments are passed to the function.

externref

Section titled “externref”

From JS, you can pass host references as externref via imports or function parameters.

Instruction Reference

Section titled “Instruction Reference”