A subset of TypeScript that compiles to WebAssembly.

AssemblyScript defines a subset of TypeScript that it compiles to WebAssembly. It aims to provide everyone with an existing background in TypeScript and standard JavaScript-APIs with a comfortable way to compile to WebAssembly, eliminating the need to switch between languages or to learn new ones just for this purpose.

Try it out in your browser!

Contents

• How it works
  A few insights to get an initial idea.

• What to expect
  General remarks on design decisions and trade-offs.

• Example
  Basic examples to get you started.

• Usage
  An introduction to the environment and its provided functionality.

• Command line
  How to use the command line utility.

• API
  How to use the API programmatically.

• Additional documentation
  A list of available documentation resources.

• Building
  How to build the compiler and its components yourself.

How it works

Under the hood, AssemblyScript rewires TypeScript's compiler API to Binaryen's compiler backend. The compiler itself is written in (and based upon) TypeScript and no binary dependencies are required to get started.

Every AssemblyScript program is valid TypeScript syntactically, but not necessarily semantically. The definitions required to start developing in AssemblyScript are provided by assembly.d.ts. See also: Usage

The compiler is able to produce WebAssembly binaries (.wasm) as well as their corresponding text format. Both Binaryen's s-expression format (.wast) and, with a little help of WABT, official linear text format (.wat) are supported. See also: CLI

What to expect

Instead of reimplementing TypeScript as closely as possible at the expense of performance, AssemblyScript tries to support its features as closely as reasonable while not supporting certain dynamic constructs intentionally:

• All types must be annotated to avoid possibly unwanted implicit type conversions
• Optional function parameters require an initializer expression
• Union types (except classType | null representing a nullable), any and undefined are not supported by design
• The result of logical && / || expressions is always bool

Also note that AssemblyScript is a rather new and ambitious project developed by one guy and a hand full of occasional contributors. Expect bugs and breaking changes. Prepare to fix stuff yourself and to send a PR for it, unless you like the idea enough to consider sponsoring development.

Example

export function add(a: i32, b: i32): i16 {
  return (a + (b as i32)) as i16;
}

Compiles to:

(module
 (type $iFi (func (param i32 f64) (result i32)))
 (memory $0 256)
 (export "memory" (memory $0))
 (export "add" (func $add))
 (func $add (type $iFi) (param $0 i32) (param $1 f64) (result i32)
  (return
   (i32.shr_s
    (i32.shl
     (i32.add
      (get_local $0)
      (i32.trunc_s/f64
       (get_local $1)
      )
     )
     (i32.const 16)
    )
    (i32.const 16)
   )
  )
 )
)

See the examples repository for more.

Running a module

The stand-alone loader component provides an easy way to run and work with compiled WebAssembly modules:

$> npm install assemblyscript-loader

import load from "assemblyscript-loader"; // JS: var load = require("assemblyscript-loader").load;

load("path/to/module.wasm", {
  imports: {
    ...
  }
}).then(module => {
  ...
  // i.e. call module.exports.main()
});

Usage

$> npm install assemblyscript --save-dev

The environment is configured by either referencing assembly.d.ts directly or by using a tsconfig.json that simply extends tsconfig.assembly.json, like so:

{
  "extends": "./node_modules/assemblyscript/tsconfig.assembly.json",
  "include": [
    "./*.ts"
  ]
}

The tsconfig.json-approach is recommended to inherit other important settings as well.

Once configured, the following AssemblyScript-specific types become available:

While generating a warning to avoid type confusion, the JavaScript types number and boolean resolve to f64 and bool respectively.

WebAssembly-specific operations are available as built-in functions that translate to the respective opcode directly:

• rotl(value: i32, shift: i32): i32
  Performs the sign-agnostic rotate left operation on a 32-bit integer.
• rotll(value: i64, shift: i64): i64
  Performs the sign-agnostic rotate left operation on a 64-bit integer.
• rotr(value: i32, shift: i32): i32
  Performs the sign-agnostic rotate right operation on a 32-bit integer.
• rotrl(value: i64, shift: i64): i64
  Performs the sign-agnostic rotate right operation on a 64-bit integer.
• clz(value: i32): i32
  Performs the sign-agnostic count leading zero bits operation on a 32-bit integer. All zero bits are considered leading if the value is zero.
• clzl(value: i64): i64
  Performs the sign-agnostic count leading zero bits operation on a 64-bit integer. All zero bits are considered leading if the value is zero.
• ctz(value: i32): i32
  Performs the sign-agnostic count tailing zero bits operation on a 32-bit integer. All zero bits are considered trailing if the value is zero.
• ctzl(value: i64): i64
  Performs the sign-agnostic count trailing zero bits operation on a 64-bit integer. All zero bits are considered trailing if the value is zero.
• popcnt(value: i32): i32
  Performs the sign-agnostic count number of one bits operation on a 32-bit integer.
• popcntl(value: i64): i64
  Performs the sign-agnostic count number of one bits operation on a 64-bit integer.
• abs(value: f64): f64
  Computes the absolute value of a 64-bit float.
• absf(value: f32): f32
  Computes the absolute value of a 32-bit float.
• ceil(value: f64): f64
  Performs the ceiling operation on a 64-bit float.
• ceilf(value: f32): f32
  Performs the ceiling operation on a 32-bit float.
• floor(value: f64): f64
  Performs the floor operation on a 64-bit float.
• floorf(value: f32): f32
  Performs the floor operation on a 32-bit float.
• sqrt(value: f64): f64
  Calculates the square root of a 64-bit float.
• sqrtf(value: f32): f32
  Calculates the square root of a 32-bit float.
• trunc(value: f64): f64
  Rounds to the nearest integer towards zero of a 64-bit float.
• truncf(value: f32): f32
  Rounds to the nearest integer towards zero of a 32-bit float.
• nearest(value: f64): f64
  Rounds to the nearest integer tied to even of a 64-bit float.
• nearestf(value: f32): f32
  Rounds to the nearest integer tied to even of a 32-bit float.
• min(left: f64, right: f64): f64
  Determines the minimum of two 64-bit floats. If either operand is NaN, returns NaN.
• minf(left: f32, right: f32): f32
  Determines the minimum of two 32-bit floats. If either operand is NaN, returns NaN.
• max(left: f64, right: f64): f64
  Determines the maximum of two 64-bit floats. If either operand is NaN, returns NaN.
• maxf(left: f32, right: f32): f32
  Determines the maximum of two 32-bit floats. If either operand is NaN, returns NaN.
• copysign(x: f64, y: f64): f64
  Composes a 64-bit float from the magnitude of x and the sign of y.
• copysignf(x: f32, y: f32): f32
  Composes a 32-bit float from the magnitude of x and the sign of y.
• reinterpreti(value: f32): i32
  Reinterprets the bits of a 32-bit float as a 32-bit integer.
• reinterpretl(value: f64): i64
  Reinterprets the bits of a 64-bit float as a 64-bit integer.
• reinterpretf(value: i32): f32
  Reinterprets the bits of a 32-bit integer as a 32-bit float.
• reinterpretd(value: i64): f64
  Reinterprets the bits of a 64-bit integer as a 64-bit double.
• current_memory(): i32
  Returns the current memory size in units of pages. One page is 64kb.
• grow_memory(value: i32): i32
  Grows linear memory by a given unsigned delta of pages. One page is 64kb. Returns the previous memory size in units of pages or -1 on failure.
• unreachable(): void
  Emits an unreachable operation that results in a runtime error when executed.
• load<T>(offset: usize): T
  Loads a value of the specified type from memory.
• store<T>(offset: usize, value: T): void
  Stores a value of the specified type to memory.

The following AssemblyScript-specific operations are implemented as built-ins as well:

• sizeof<T>(): usize
  Determines the byte size of the specified core or class type. Compiles to a constant.
• unsafe_cast<T1,T2>(value: T1): T2
  Casts a value of type T1 to a value of type T2. Useful for casting classes to pointers and vice-versa. Does not perform any checks.
• isNaN(value: f64): bool
  Tests if a 64-bit float is a NaN.
• isNaNf(value: f32): bool
  Tests if a 32-bit float is a NaN.
• isFinite(value: f64): bool
  Tests if a 64-bit float is finite.
• isFinitef(value: f32): bool
  Tests if a 32-bit float is finite.

These constants are present as immutable globals (note that optimizers might inline them):

• NaN: f64
  NaN (not a number) as a 64-bit float.
• NaNf: f32
  NaN (not a number) as a 32-bit float.
• Infinity: f64
  Positive infinity as a 64-bit float.
• Infinityf: f32
  Positive infinity as a 32-bit float.

By default, AssemblyScript's memory management runtime will be linked statically:

• memcpy(dest: usize, src: usize, size: usize): usize
  Copies data from one chunk of memory to another.
• memset(dest: usize, c: i32, size: usize): usize
  Sets a chunk of memory to the provided value c. Usually used to reset it to all 0s.
• memcmp(vl: usize, vr: usize, n: usize): i32
  Compares a chunk of memory to another. Returns 0 if both are equal, otherwise vl[i] - vr[i] at the first difference's byte offset i.
• malloc(size: usize): usize
  Allocates a chunk of memory of the specified size.
• realloc(ptr: usize, size: usize): usize
  Changes the size of an allocated memory block.
• free(ptr: usize): void
  Frees a previously allocated chunk of memory.

Linking in the runtime adds up to 14kb to a module, but the optimizer is able to eliminate unused runtime code. Once WebAssembly exposes the garbage collector natively, there'll be other options as well. If the runtime has been excluded through --noRuntime, its methods will be imported where referenced (i.e. when using new). Also note that manually calling grow_memory where the runtime is present will most likely break it.

Type coercion requires an explicit cast where precision or signage is lost respectively is implicit where it is maintained. For example, to cast a f64 to an i32:

function example(value: f64): i32 {
  return value as i32; // translates to the respective opcode
}

Global WebAssembly imports can be declared anywhere while WebAssembly exports are exported from the entry file (the file specified when calling asc or Compiler.compileFile). Aside from that, imports and exports work just like in TypeScript.

// entry.ts

import { myOtherExportThatDoesntBecomeAWebAssemblyExport } from "./imported";

declare function myImport(): void;

export function myExport(): void {
  myOtherExportThatDoesntBecomeAWebAssemblyExport();
}

Currently, imports can also be pulled from different namespaces by separating the namespace and the function with a $ character.

declare function Math$random(): double;

Command line

The command line compiler asc works similar to TypeScript's tsc:

Syntax: asc [options] entryFile

Options:

 --config, -c       Specifies a JSON configuration file with command line options.
                    Will look for 'asconfig.json' in the entry's directory if omitted.

 --outFile, -o      Specifies the output file name. Emits text format if ending with .wast
                    (sexpr) or .wat (linear). Prints to stdout if omitted.

 --optimize, -O     Runs optimizing binaryen IR passes.

 --validate, -v     Validates the module.

 --quiet, -q        Runs in quiet mode, not printing anything to console.

 --target, -t       Specifies the target architecture:

                    wasm32  Compiles to 32-bit WebAssembly [default]
                    wasm64  Compiles to 64-bit WebAssembly

 --textFormat, -f   Specifies the format to use for text output:

                    sexpr   Emits s-expression syntax (.wast) [default]
                    linear  Emits official linear syntax (.wat)

                    Text format only is emitted when used without --textFile.

 --textFile         Can be used to save text format alongside a binary in one command.

 --noTreeShaking    Whether to disable built-in tree-shaking.

 --noImplicitConversion  Whether to disallow implicit type conversions.

 --noRuntime        Whether to exclude the runtime.

 --exportRuntime, -e  Runtime functions to export, defaults to 'malloc' and 'free'. [multiple]

 --help, -h         Displays this help message.

A configuration file (usually named asconfig.json) using the long option keys above plus a special key entryFile specifying the path to the entry file can be used to reuse options between invocations.

API

It's also possible to use the API programmatically:

• Compiler.compileFile(filename: string, options?: CompilerOptions): binaryen.Module | null
  Compiles the specified entry file to a WebAssembly module. Returns null on failure.

• Compiler.compileString(source: string, options?: CompilerOptions): binaryen.Module | null
  Compiles the specified entry file source to a WebAssembly module. Returns null on failure.

• Compiler.lastDiagnostics: typescript.Diagnostic[]
  Contains the diagnostics generated by the last invocation of compilerFile or compileString.

• CompilerOptions
  AssemblyScript compiler options.

  • silent: boolean
    Whether compilation shall be performed in silent mode without writing to console. Defaults to false.
  • target: CompilerTarget | string
    Specifies the target architecture. Defaults to CompilerTarget.WASM32.
  • noTreeShaking: boolean
    Whether to disable built-in tree-shaking. Defaults to false.
  • noImplicitConversion: boolean
    Whether to disallow implicit type conversions. Defaults to false.
  • noRuntime: boolean
    Whether to exclude the runtime.
  • exportRuntime: string[]
    Runtime functions to export, defaults to 'malloc' and 'free'.

• CompilerTarget
  Compiler target.

  • WASM32
    32-bit WebAssembly target using uint pointers.
  • WASM64
    64-bit WebAssembly target using ulong pointers.

Example

import { Compiler, CompilerTarget, CompilerMemoryModel, typescript } from "assemblyscript";

const module = Compiler.compileString(`
export function add(a: i32, b: i32): i32 {
  return a + b;
}
`, {
  target: CompilerTarget.WASM32,
  silent: true
});

console.error(typescript.formatDiagnostics(Compiler.lastDiagnostics));
if (!module)
  throw Error("compilation failed");

module.optimize();

if (!module.validate())
  throw Error("validation failed");

const textFile = module.emitText();
const wasmFile = module.emitBinary();

...

module.dispose();

Remember to call binaryen.Module#dispose() once you are done with a module to free its resources. This is necessary because binaryen.js has been compiled from C/C++ and doesn't provide automatic garbage collection.

Additional documentation

AssemblyScript

• Standard Library Documentation
• API Documentation

WebAssembly

• WebAssembly Design Documents

Building

Clone the GitHub repository including submodules and install the development dependencies:

$> git clone --recursive https://github.com/AssemblyScript/assemblyscript.git
$> cd assemblyscript
$> npm install

Afterwards, to build the distribution files to dist/, run:

$> npm run build

Note that the first invocation of build also builds the TypeScript submodule (lib/typescript) and may take some time.

To run the tests (ideally on node.js >= 8):

$> npm test

To build the documentation to the website repostory checked out next to this repository, run:

$> npm run docs

License: Apache License, Version 2.0