How to use the compiler from the command line or as an API.

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

Command line options

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


The compiler can optimize for both speed and size. --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 convenient shorthand is -O[optimizeLevel][shrinkLevel] , with shrink level indicated by appending the letter s (1) or z (2) to the optimize level.

--optimize, -O Optimizes the module. Typical shorthands are:
Default optimizations -O / -O3s
Make a release build -O --noAssert
Make a debug build --debug
Optimize for speed -O3
Optimize for size -O3z --converge
--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.

Also noteworthy: The standard library provides memory manager and garbage collector variants for various use cases. From largest/most sophisticated to smallest/simplest:

--runtime Specifies the runtime variant to include in the program.
full Default runtime based on TLSF and reference counting.
half Same as 'full', but not exported to the host.
stub Minimal stub implementation without free/GC support.
none Same as 'stub', but not exported to the host.

If external allocation is not required, choosing either half or none can significantly reduce the module's size.


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

--outFile, -o Specifies the output file. File extension indicates format.
--binaryFile, -b Specifies the binary output file (.wasm).
--textFile, -t Specifies the text output file (.wat).

There are several other output formats as well for tooling purposes with varying levels of maturity.

--asmjsFile, -a Specifies the asm.js output file (.js).
--idlFile, -i Specifies the WebIDL output file (.webidl).
--tsdFile, -d Specifies the TypeScript definition output file (.d.ts).


For easier debugging a source map can be emitted alongside the WebAssembly binary.

--sourceMap Enables source map generation. Optionally takes the URL
used to reference the source map from the binary file.

It is also often useful to emit debug information, like function names, alongside the binary.

--debug Enables debug information in emitted binaries.


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 provided as 'env.memory'.
--sharedMemory Declare memory as shared by settings the max shared memory.
--importTable Imports the function table provided as 'env.table'.
--exportTable Exports the function table as 'table'.
--explicitStart Exports an explicit '_start' function to call.
--enable Enables WebAssembly features being disabled by default.
sign-extension Sign-extension operations
bulk-memory Bulk memory operations.
simd SIMD types and operations.
threads Threading and atomic operations.
reference-types Reference types and operations.
--disable Disables WebAssembly features being enabled by default.
mutable-globals Mutable global imports and exports.
--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.


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 'require'.


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


--validate, -c Validates the module using Binaryen. Exits if invalid.
--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

And the kitchen sink

--baseDir Specifies the base directory of input and output files.
--noUnsafe Disallows the use of unsafe features in user code.
Does not affect library files and external modules.
--noEmit Performs compilation as usual but does not emit code.
--measure Prints measuring information 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.
--traceResolution Enables tracing of package resolution.
--listFiles Lists files to be compiled and exits.
--printrtti Prints the module's runtime type information to stderr.
-- ... Specifies node.js options (CLI only). See: node --help


The compiler can also be used programmatically.