# Using the compiler

Similar to TypeScript's tsc transpiling to JavaScript, AssemblyScript's asc compiles to WebAssembly.

# Compiler options

The compiler supports various options available on the command line, in a configuration file and programmatically. On the command line, it looks like this:

# Entry file(s)

Non-option arguments are treated as the names of entry files. A single program can have multiple entries, with the exports of each entry becoming the exports of the WebAssembly module. Exports of imported files that are not entry files do not become WebAssembly module exports.

asc entryFile.ts

# General

--version, -v         Prints just the compiler's version and exits.
--help, -h            Prints this message and exits.
--config              Configuration file to apply. CLI arguments take precedence.
--target              Configuration file target to use. Defaults to 'release'.

# Optimization

The compiler can optimize for both speed (-Ospeed) and size (-Osize), as well as produce a debug build (--debug).

--optimize, -O        Optimizes the module. Typical shorthands are:

                        Default optimizations   -O
                        Make a release build    -O --noAssert
                        Make a debug build      --debug
                        Optimize for speed      -Ospeed
                        Optimize for size       -Osize

--optimizeLevel       How much to focus on optimizing code. [0-3]
--shrinkLevel         How much to focus on shrinking code size. [0-2, s=1, z=2]
--converge            Re-optimizes until no further improvements can be made.
--noAssert            Replaces assertions with just their value without trapping.
--uncheckedBehavior   Changes the behavior of unchecked() expressions.
                      Using this option can potentially cause breakage.

                        default  The default behavior: unchecked operations are
                                 only used inside of unchecked().
                        never    Unchecked operations are never used, even when,
                                 inside of unchecked().
                        always   Unchecked operations are always used if possible,
                                 whether or not unchecked() is used.

Optimization levels can also be tweaked manually: --optimizeLevel (0-3) indicates how much the compiler focuses on optimizing the code with --shrinkLevel (0-2, 1=s, 2=z) indicating how much it focuses on keeping the size low during code generation and while optimizing. A shorthand for both is -O[optimizeLevel][shrinkLevel] , with shrink level indicated by optionally appending the letter s (1) or z (2).

# Output

Typical output formats are WebAssembly binary (.wasm, --outFile) and/or text format (.wat, --textFile). Often, both are used in tandem to run and also inspect generated code.

--outFile, -o         Specifies the WebAssembly output file (.wasm).
--textFile, -t        Specifies the WebAssembly text output file (.wat).
--bindings, -b        Specifies the bindings to generate (.js + .d.ts).

                        esm  JavaScript bindings & typings for ESM integration.
                        raw  Like esm, but exports just the instantiate function.
                             Useful where modules are meant to be instantiated
                             multiple times or non-ESM imports must be provided.

# Debugging

For easier debugging during development, a source map can be emitted alongside the WebAssembly binary, and debug symbols can be embedded:

--sourceMap           Enables source map generation. Optionally takes the URL
                      used to reference the source map from the binary file.
--debug               Enables debug information in emitted binaries.

# Features

There are several flags that enable or disable specific WebAssembly or compiler features. By default, only the bare minimum is exposed, and fully standardized WebAssembly features will be used.

--importMemory        Imports the memory from 'env.memory'.
--noExportMemory      Does not export the memory as 'memory'.
--initialMemory       Sets the initial memory size in pages.
--maximumMemory       Sets the maximum memory size in pages.
--sharedMemory        Declare memory as shared. Requires maximumMemory.
--zeroFilledMemory    Assume imported memory is zeroed. Requires importMemory.
--importTable         Imports the function table from 'env.table'.
--exportTable         Exports the function table as 'table'.
--exportStart         Exports the start function using the specified name instead
                      of calling it implicitly. Useful for WASI or to obtain the
                      exported memory before executing any code accessing it.
--runtime             Specifies the runtime variant to include in the program.

                        incremental  TLSF + incremental GC (default)
                        minimal      TLSF + lightweight GC invoked externally
                        stub         Minimal runtime stub (never frees)
                        ...          Path to a custom runtime implementation

--exportRuntime       Always exports the runtime helpers (__new, __collect, __pin etc.).
                      Automatically determined when generation of --bindings is enabled.
--stackSize           Overrides the stack size. Only relevant for incremental GC
                      or when using a custom runtime that requires stack space.
                      Defaults to 0 without and to 16384 with incremental GC.
--enable              Enables WebAssembly features being disabled by default.

                        threads             Threading and atomic operations.
                        simd                SIMD types and operations.
                        reference-types     Reference types and operations.
                        gc                  Garbage collection (WIP).
                        stringref           String reference types.
                        relaxed-simd        Relaxed SIMD operations.

--disable             Disables WebAssembly features being enabled by default.

                        mutable-globals     Mutable global imports and exports.
                        sign-extension      Sign-extension operations
                        nontrapping-f2i     Non-trapping float to integer ops.
                        bulk-memory         Bulk memory operations.

--use, -u             Aliases a global object under another name, e.g., to switch
                      the default 'Math' implementation used: --use Math=JSMath
                      Can also be used to introduce an integer constant.
--lowMemoryLimit      Enforces very low (<64k) memory constraints.

# Linking

Specifying the base offsets of compiler-generated memory respectively the table leaves some space for other data in front. In its current form this is mostly useful to link additional data into an AssemblyScript binary after compilation, be it by populating the binary itself or initializing the data externally upon initialization. One good example is leaving some scratch space for a frame buffer.

--memoryBase          Sets the start offset of emitted memory segments.
--tableBase           Sets the start offset of emitted table elements.


To integrate with the compiler, for example to post-process the AST, one or multiple custom transforms can be specified.

--transform           Specifies the path to a custom transform to load.

# Other

Other options include those forwarded to Binaryen and various flags useful in certain situations.

# Binaryen

--trapMode            Sets the trap mode to use.

                       allow  Allow trapping operations. This is the default.
                       clamp  Replace trapping operations with clamping semantics.
                       js     Replace trapping operations with JS semantics.

--runPasses           Specifies additional Binaryen passes to run after other
                      optimizations, if any. See: Binaryen/src/passes/pass.cpp
--noValidate          Skips validating the module using Binaryen.

# And the kitchen sink

--baseDir             Specifies the base directory of input and output files.
--noColors            Disables terminal colors.
--noUnsafe            Disallows the use of unsafe features in user code.
                      Does not affect library files and external modules.
--disableWarning      Disables warnings matching the given diagnostic code.
                      If no diagnostic code is given, all warnings are disabled.
--noEmit              Performs compilation as usual but does not emit code.
--stats               Prints statistics on I/O and compile times.
--pedantic            Make yourself sad for no good reason.
--lib                 Adds one or multiple paths to custom library components and
                      uses exports of all top-level files at this path as globals.
--path                Adds one or multiple paths to package resolution, similar
                      to node_modules. Prefers an 'ascMain' entry in a package's
                      package.json and falls back to an inner 'assembly/' folder.
--wasm                Uses the specified Wasm binary of the compiler.
-- ...                Specifies node.js options (CLI only). See: node --help

# Configuration file

Instead of providing the options outlined above on the command line, a configuration file typically named asconfig.json can be used. It may look like in the following example, excluding comments:

  "extends": "./path/to/other/asconfig.json", // (optional) base config
  "entries": [
    // (optional) entry files, e.g.
  "options": {
    // common options, e.g.
    "importTable": true
  "targets": {
    // (optional) targets
    "release": {
      // additional options for the "release" target, e.g.
      "optimize": true,
      "outFile": "myModule.release.wasm"
    "debug": {
      // additional options for the "debug" target, e.g.
      "debug": true,
      "outFile": "myModule.debug.wasm"

Per-target options, e.g. targets.release, add to and override top-level options. Options provided on the command line override any options in the configuration file. Usage is, for example:

asc --config asconfig.json --target release

# Programmatic usage

The compiler API can also be used programmatically:

import asc from "assemblyscript/asc";

const { error, stdout, stderr, stats } = await asc.main([
  // Command line options
  "--outFile", "myModule.wasm",
], {
  // Additional API options
  stdout?: ...,
  stderr?: ...,
  readFile?: ...,
  writeFile?: ...,
  listFiles?: ...,
  reportDiagnostic?: ...,
  transforms?: ...
if (error) {
  console.log("Compilation failed: " + error.message);
} else {

The compiler runs in browsers as well. The simplest way to set it up is to include the generated web.js (opens new window) so the compiler can be used with an import on the Web:

<script src="https://cdn.jsdelivr.net/npm/assemblyscript@x.x.x/dist/web.js"></script>
<script type="module">
import asc from "assemblyscript/asc";

Here, x.x.x must be replaced with the respective version to use (opens new window), or latest to always use the latest version (not recommended in production). By default, the script installs the necessary import map (opens new window) and, for browsers that do not yet support import maps, an import map shim (opens new window). It also accepts the following options in case there is a need to only perform part of the setup:

Script URL Effect
web.js?noinstall Does not install the import map.
web.js?noshim Does not install the import map shim.
web.js?noinstall,noshim Does not install the import map or shim.

Regardless of the options used, the script always declares the following global variables:

Variable Description
ASSEMBLYSCRIPT_VERSION Version string of the compiler release used
ASSEMBLYSCRIPT_IMPORTMAP The respective import map of the release as JSON

# Host bindings

WebAssembly alone cannot yet transfer higher level data types like strings, arrays and objects over module boundaries, so for now some amount of glue code is required to exchange these data structures with the host / JavaScript.

The compiler can generate the necessary bindings using the --bindings command line option (either as an ES module or a raw instantiate function), enabling exchange of:

Type Strategy Description
Number By value Basic numeric types except 64-bit integers.
BigInt By value 64-bit integers via js-bigint-integration.
Boolean By value Coerced to true or false.
Externref By reference Using reference-types.
String Copy
ArrayBuffer Copy
TypedArray Copy Any Int8Array, Float64Array etc.
Array Copy Any Array<T>
StaticArray Copy Any StaticArray<T>
Object Copy If a plain object. That is: Has no constructor or non-public fields.
Object By reference If not a plain object. Passed as an opaque reference counted pointer.

Note the two different strategies used for Object: In some situations, say when calling a Web API, it may be preferable to copy the object as a whole, field by field, which is the strategy chosen for plain objects with no constructor or non-public fields:

// Copied to a JS object
class PlainObject {
  field: string;

export function getObject(): PlainObject {
  return {
    field: "hello world"

However, copying may not be desirable in every situation, say when individual object properties are meant to be modified externally where serializing/deserializing the object as a whole would result in unnecessary overhead. To support this use case, the compiler can pass just an opaque reference to the object, which can be enforced by providing an empty constructor (not a plain object anymore):

// Not copied to a JS object
class ComplexObject {
  constructor() {} // !
  field: string | null;

export function newObject(): ComplexObject {
  return new ComplexObject();

export function setObjectField(target: ComplexObject, field: string | null): void {
  target.field = field;

export function getObjectField(target: ComplexObject): string | null {
  return target.field;

Also note that exporting an entire class has no effect at the module boundary (yet), and it is instead recommended to expose only the needed functionality as shown in the example above. Supported elements at the boundary are globals, functions and enums.

# Using ESM bindings

Bindings generated with --bindings esm perform all the steps from compilation over instantiation to exporting the final interface. To do so, a few assumptions had to be made:

  • The WebAssembly binary is located next to the JavaScript bindings file using the same name but with a .wasm extension.

    import * as myModule from "./build/mymodule.js"
  • JavaScript globals in globalThis can be accessed directly via the env module namespace. For example, console.log can be manually imported through:

    @external("env", "console.log")
    declare function consoleLog(s: string): void

    Note that this is just an example and console.log is already provided by the standard library when called from an AssemblyScript file. Other global functions not already provided by the standard library may require an import as of this example, though.

  • Imports from other namespaces than env, i.e. (import "module" "name"), become an import { name } from "module" within the binding. Importing a custom function from a JavaScript file next to the bindings file can be achieved through:

    @external("./otherfile.js", "myFunction")
    declare function myFunction(...): ...

    Similarly, importing a custom function from, say, a Node.js dependency can be achieved through:

    @external("othermodule", "myFunction")
    declare function myFunction(...): ...

These assumptions cannot be intercepted or customized since, to provide static ESM exports from the bindings file directly, instantiation must start immediately when the bindings file is imported. If customization is required, --bindings raw can be used instead.

# Using raw bindings

The signature of the single instantiate function exported by --bindings raw is:

export async function instantiate(module: WebAssembly.Module, imports?: WebAssembly.Imports): AdaptedExports

Note that the function does not make any assumptions on how the module is to be compiled, but instead expects a readily compiled WebAssembly.Module as in this example:

import { instantiate } from "./module.js"
const exports = await instantiate(await WebAssembly.compileStreaming(fetch("./module.wasm")), { /* imports */ })

Unlike --bindings esm, raw bindings also do not make any assumptions on how imports are resolved, so these must be provided manually as part of the imports object. For example, to achieve a similar result as with ESM bindings, but now customizable:

import { instantiate } from "./module.js"
import * as other from "./otherfile.js"
export const {
} = await instantiate(await WebAssembly.compileStreaming(fetch("./module.wasm")), {
  "./otherfile.js": other

# Debugging

The debugging workflow is similar to debugging JavaScript since both Wasm and JS execute in the same engine, and the compiler provides various options to set up additional WebAssembly-specific debug information. Note that any sort of optimization should be disabled in debug builds.

# Debug symbols

When compiling with the --debug option, the compiler appends a name section to the binary, containing names of functions, globals, locals and so on. These names will show up in stack traces.

# Source maps

The compiler can generate a source map alongside a binary using the --sourceMap option. By default, a relative source map path will be embedded in the binary which browsers can pick up when instantiating a module from a fetch response. In environments that do not provide fetch or an equivalent mechanism, like in Node.js, it is alternatively possible to embed an absolute source map path through --sourceMap path/to/source/map.

# Breakpoints

Some JavaScript engines also support adding break points directly in WebAssembly code. Please consult your engine's documentation: Chrome (opens new window), Firefox (opens new window), Node.js (opens new window), Safari (opens new window).

# Transforms

AssemblyScript is compiled statically, so code transformation cannot be done at runtime but must instead be performed at compile-time. To enable this, the compiler frontend (asc) provides a mechanism to hook into the compilation process before, while and after the module is being compiled.

Specifying --transform ./myTransform.js on the command line will load the node module pointed to by ./myTransform.js.

import * as assemblyscript from "assemblyscript"
import { Transform } from "assemblyscript/transform"
class MyTransform extends Transform {
export default MyTransform

# Properties

A transform is an ES6 class/node module with the following inherited properties:

  • readonly program: Program

    Reference to the Program instance.

  • readonly baseDir: string

    Base directory used by the compiler.

  • readonly stdout: OutputStream

    Output stream used by the compiler.

  • readonly stderr: OutputStream

    Error stream uses by the compiler.

  • readonly log: typeof console.log

    Logs a message to console.

  • function writeFile(filename: string, contents: string | Uint8Array, baseDir: string): boolean

    Writes a file to disk.

  • function readFile(filename: string, baseDir: string): string | null

    Reads a file from disk.

  • function listFiles(dirname: string, baseDir: string): string[] | null

    Lists all files in a directory.

# Hooks

The frontend will call several hooks, if present on the transform, during the compilation process:

  • function afterParse(parser: Parser): void

    Called when parsing is complete, before a program is initialized from the AST. Note that types are not yet known at this stage and there is no easy way to obtain them.

  • function afterInitialize(program: Program): void

    Called once the program is initialized, before it is being compiled. Types are known at this stage, respectively can be resolved where necessary.

  • function afterCompile(module: Module): void

    Called with the resulting module before it is being emitted. Useful to modify the IR before writing any output, for example to replace imports with actual functionality or to add custom sections.

Transforms are a very powerful feature, but may require profound knowledge of the compiler to utilize them to their full extent, so reading through the compiler sources is a plus.

# Portability

With AssemblyScript being very similar to TypeScript, there comes the opportunity to compile the same code to JavaScript with tsc and WebAssembly with asc. The AssemblyScript compiler itself is portable code. Writing portable code is largely a matter of double-checking that the intent translates to the same outcome in both the strictly typed AssemblyScript and the types-stripped-away TypeScript worlds.

# Portable standard library

Besides the full standard library, AssemblyScript provides a portable variant of the functionality that is present in both JavaScript and WebAssembly. In addition to that, the portable library lifts some of the functionality that is only available with asc to JavaScript, like the portable conversions mentioned below.

Also note that some parts of JavaScript's standard library function a little more loosely than how they would when compiling to WebAssembly. While the portable definitions try to take care of this, one example where this can happen is Map#get returning undefined when a key cannot be found in JavaScript, while resulting in an abort in WebAssembly, where it is necessary to first check that the key exists using Map#has.

To use the portable library, extend assemblyscript/std/portable.json instead of assemblyscript/std/assembly.json within tsconfig.json and add the following somewhere along your build step so the portable features are present in the environment:

import "assemblyscript/std/portable.js"

Note that the portable standard library is still a work in progress and so far focuses on functionality useful to make the compiler itself portable, so if you need something specific, feel free to improve its definitions and feature set (opens new window).

# Portable conversions

While asc understands the meaning of

// non-portable
let someFloat: f32 = 1.5
let someInt: i32 = <i32>someFloat

and then inserts the correct conversion steps, tsc does not because all numeric types are just aliases of number. Hence, when targeting JavaScript with tsc, the above will result in

var someFloat = 1.5
var someInt = someFloat

which is obviously wrong. To account for this, portable conversions can be used, resulting in actually portable code. For example

// portable
let someFloat: f32 = 1.5
let someInt: i32 = i32(someFloat)

will essentially result in

var someFloat = 1.5
var someInt = someFloat | 0

which is correct. The best way of dealing with this is asking yourself the question: What would this code do when compiled to JavaScript?

# Portable overflows

Likewise, again because asc knows the meaning but tsc does not, overflows must be handled explicitly:

// non-portable
let someU8: u8 = 255
let someOtherU8: u8 = someU8 + 1
// portable
let someU8: u8 = 255
let someOtherU8: u8 = u8(someU8 + 1)

essentially resulting in

let someU8 = 255
let someOtherU8 = (someU8 + 1) & 0xff

# Non-portable code

In JavaScript, all numeric values are IEEE754 doubles that cannot represent the full range of values fitting in a 64-bit integer (max. safe integer (opens new window) is 2^53 - 1). Hence i64 and u64 are not portable and not present in std/portable. There are several ways to deal with this. One is to use an i64 polyfill like in this example (opens new window).

Other than that, portable code (JavaScript) does not have a concept of memory, so there are no load and store implementations in the portable standard library. Technically this can be polyfilled in various ways, but no default is provided since actual implementations are expected to be relatively specific (for instance: the portable compiler accesses Binaryen's memory).