Skip to content
Switch branches/tags

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

Interface Types Polyfill


This repo holds some sample / test code for the development of the WebAssembly Interface Types Proposal. Its purpose is to iterate on the proposal by letting us experiment with running code.

Repo structure

Individual demos live in folders. Given a folder named Foo:

  • Foo/Foo_base.wat : Wasm code
  • Foo/Foo_interface.wat : Interface annotations
  • Foo/Foo.js : JS code to load and run Foo demo

Top-level scripts:

  • webIDL.js : runtime polyfill, reads custom section and interprets it
  • : script to parse @interface declarations from .wat and build the "interface-types" custom section
  • Makefile : rules to build .wasm files from .wats

Interface Types

By convention, Interface Types are capitalized, where Wasm types are not (e.g. Int vs i32)

Type Encoding Description
Int 0x7fff integer number
Float 0x7ffe floating-point number
Any 0x7ffd opaque reference
String 0x7ffc string data

Adapter Instructions

Instruction Opcode Immediates Signature Description
arg.get 0x00 param-index [] -> [ty(param)] Access the function's arguments
call 0x01 import-index sig(import) Call a function import (Interface types)
call-export 0x02 export-name sig(export) Call a function export (Wasm types)
read-utf8 0x03 [i32, i32] -> [String] Read a string out of memory, given a pointer + length
write-utf8 0x04 export-name [String] -> [i32, i32] Write a string into memory, return pointer + length
as-wasm 0x05 wasm-type [interfaceTy(wasm)] -> [wasm] Cast an Interface type to a Wasm type
as-interface 0x06 interface-type [wasmTy(interface)] -> [interface] Cast a Wasm type to an Interface type
table-ref-add 0x07 [Any] -> [i32] Add a reference to a table, returns an index for that reference
table-ref-get 0x08 [i32] -> [Any] Given an index, return the reference associated
call-method 0x09 import-index sig(import) Call a function import, with the first argument bound as the this parameter
make-record 0x0a type-index fields(record) -> [record] Creates a record object of a given type
get-field 0x0c type, field-index [record] -> [field(record)] Gets a field value from a record object
const 0x0d type, value [] -> [type] Pushes a constant value. Really only i32 at the moment
fold-seq 0x0e import-index [Seq] + sig(import) -> result(import) Calls a function on every element of a sequence, carrying an accumulator value across
add 0x0f type [T, T] -> [T] Add two numbers together
mem-to-seq 0x10 type, memory-name [i32, i32] -> [Seq] Loads a pointer+length into a sequence
load 0x11 type, memory-name [i32] -> [T] Loads a scalar value from memory 0x12
list.push 0x13
repeat-until 0x14
seq-to-mem 0x15
store 0x16


repeat-until takes two functions, a continuation predicate, and a next-step predicate. Given an $cond of type [b] -> [i32], and a $iter of type [a, b] -> [a, b], then the whole expression repeat-until $cond $iter has type [a, b] -> [a], where a is an accumulator, and b is a state parameter.

Binary Format

The current binary format in this repo is a placeholder format. In the future it should align with the proposed binary format.

There are five subsections:

  1. Exported function declarations
  2. Type declarations
  3. Imported function declarations
  4. Adapter functions
  5. Forwarded exports

Common patterns

leb_u32(x) is an unsigned LEB with value no larger than a 32 bit number.

string(x) is the length of the string x (leb_u32-encoded), followed by the bytes of the string.

Custom section preamble

The bindings are represented as a custom section with the name interface-types. Custom sections follow a standard preamble:

0x00 ;; section ID, 0=custom
leb_u32(size) ;; size of the section
string("interface-types") ;; name of the section

Exported function declarations

Exported functions are declared here with a signature. This is redundant information, but makes polyfilling more convenient because we don't need to parse the whole wasm module to determine exported function signatures.

TODO: more

Imported functions


Adapter functions


Forwarded exports



Makes use of the following V8 flags:

  • --experimental-wasm-anyref

Run on command line: node --experimental-wasm-anyref [.js file]

In Chrome: chrome --js-flags="--experimental-wasm-anyref"


No description, website, or topics provided.




No releases published


No packages published