Write AssemblyScript tests at blazing fast speeds

wasm, assemblyscript, testing, cli, typescript, webassembly
npm install @as-pect/assembly@3.1.2



This packages is a monorepo that contains the cli and the core for the @as-pect packages.

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Write your module in AssemblyScript and get blazing fast bootstrapped tests with WebAssembly speeds!

Table of contents

  1. Philosophy
  2. Usage
  3. Comparisons
  4. Configuration File
  5. Types And Tooling
  6. CI Usage
  7. AssemblyScript Compiler Options
  8. Reporters
  9. Portability
  10. RTrace and Memory Leaks
  11. Performance Testing
  12. Custom Imports Using CLI
  13. Using as-pect as a Package
  14. Contributors


Testing is the first step of every project and you have a responsibility to make sure that the software you write works as intended. The as-pect project was created to help quickly scaffold and bootstrap AssemblyScript tests so that you can be confident in yourself and the software you write.

One of the goals of this project is 100% portability to jest so that tests can be run in two different environments.

The most important reason why as-pect exists is to set up and bootstrap AssemblyScript tests otherwise would be a hastle to new developers. For instance, the following steps need to be compelted for each individual test module.

  1. Create a set of testing primitives or rely on assert() to validate module state
  2. Compile the module itself
  3. Bootstrap imports
  4. Write tests
  5. Log test output

This process alone could take hours of your time, and as-pect covers this need for testing AssemblyScript modules by simply being an npm install away.


To install as-pect, install the latest version(s) from npm.

$ npm install @as-pect/cli assemblyscript/assemblyscript

To initialize a test suite, run npx asp --init. It will create the following folders and files.

$ npx asp --init

# It will create the following folders if they don't exist
C ./assembly/
C ./assembly/__tests__/

# The as-pect types file will be created here if it doesn't exist
C ./assembly/__tests__/as-pect.d.ts

# An example test file will be created here if the __tests__ folder does not exist
C ./assembly/__tests__/example.spec.ts

# The default configuration file will be created here if it doesn't exist
C ./as-pect.config.js

If you want asp's boilerplate located somewhere other than in assembly/, you can move it yourself, and update as-pect.config.js to point to the new location accordingly.

To run as-pect, use the command line: npx asp, or create an npm script.

  "scripts": {
    "test": "asp"

The command line defaults to using ./aspect.config.js, otherwise you can specify all the configuration options using the command line interface.

To change the location of the as-pect configuration, use the --config option.

$ npx asp --config as-pect.config.js


There are a set of comparison functions defined in the types/as-pect.d.ts types definition. These comparison functions allow you to inspect object and memory state.


This comparison is used for comparing data using the == operator. In AssemblyScript this operator is used for comparing strings, numbers, and exact reference equality (or pointer comparison.)

For example, the following statements are valid toBe assertions:

let a = new Vec3(1, 2, 3);

This method is safe to use portably with jest.


This method performs a single memory.compare() on two blocks of data. This is useful for references and strings. For example, using a toBe() assertion on two different references results in a failed assertion:

let a = new Vec3(1, 2, 3);
let b = new Vec3(1, 2, 3);
expect<Vec3>(a).toBe(b); // fails!

Instead, it's posible to compare two different references like this:

expect<Vec3>(a).toStrictEqual(b); // passes!

The following snippet an approximate the JavaScript equivalent for the toStrictEqual comparison:

// loop over each property (properties are the same at compile time)
for (let prop in a) {
  if (a[prop] === b[prop]) {
    // exact equality check
  } else {

If the object has child references, like strings or pointers to other blocks of memory, the comparison will fail because the pointers are different. This happens because as-pect cannot perform object traversal. Instead, a custom method should be used to traverse child references to compare equality.

The toStrictEqual comparison, however, does perform a == comparison before opting into using a full memory comparison. If the @operator("==") is overridden, then it's possible for two references to be compared using this method:

class Vec3 {
  constructor(public a: f64 = 0.0, public b: f64 = 0.0, public c: f64 = 0.0) {}

  // override the operator
  protected __equals(ref: Vec3 | null): bool {
    // === is a strict pointer comparison in AssemblyScript
    if (this === ref) return true;
    if (ref === null) return false;
    return this.a == ref.a && this.b == ref.b && this.c == ref.c;

This method is not safe to use portably with jest yet. Once Reflection is supported by AssemblyScript, as-pect will support compatibility between jest's version of this function.


This comparison is the same comparison used on ArrayBuffer and Strings. It compares the bytes of the heap allocations by obtaining the exact size of the block and then performing a memcompare if the actual and expected blocks match.

Only use this comparison when comparing ArrayBuffer references.

let buffer = new ArrayBuffer(100); // 100 bytes long heap allocation
let buffer2 = new ArrayBuffer(100); // another buffer


toBeTruthy and toBeFalsy

These comparisons are used to determine if a value is truthy or falsy in the JavaScript sense. In JavaScript there are only six falsy values:

  • false
  • 0
  • ""
  • null
  • undefined
  • NaN

In AssemblyScript, there is no undefined, so as-pect will treat each of those values as falsy. Truthy values are anything that is not falsy,

expect<Vec3>(new Vec3(1, 2, 3)).toBeTruthy();

These methods are safe to use with jest.


This comparison is only used for float values to determine if the value is a NaN value.

expect<f32>(NaN).toBeNaN(); // passes
expect<f64>(1.0).not.toBeNaN(); // passes

/** This results in a runtime error, despite not being NaN. */
expect<Vec3>(new Vec3()).not.toBeNaN();

This method is technically safe to use with jest with the assumption that as-pect will fail if used with a reference type.


This comparison looks specifically for a null value.

expect<Vec3>(null).toBeNull(); // valid assertion

In the case of numeric values, numbers cannot be null in AssemblyScript. Thus, the following example will throw a runtime error.


This method is safe to use with jest assuming you explicitly return null and avoid use of undefined which does not exist in AssemblyScript.


This comparison is used to detect if float values are finite. The following values are not finite in JavaScript or AssemblyScript.

  • Infinity
  • -Infinity
  • NaN

The following assertions are true.


As long as the number values are always f32 or f64 (or number in JavaScript or AssemblyScript,) toBeFinite is a safe assertion to use portably with jest.


This comparison is used to test and see if a function throws an error. In the case of AssemblyScript and as-pect, the function will be called from within a JavaScript try block, and if the function throws, the assertion is valid, unless it is negated with the not property.

expect<() => void>(() => {
  throw new Error("Whoops!");
}).toThrow(); // valid assertion

// alternative shorter convenience syntax
expectFn(() => {
  throw new Error("Whoops!");

Closure is not supported in AssemblyScript yet. Also, any references that are left dangling on the stack will hang around un__release()ed by AssemblyScript.

This function is safe to use with jest.

toBeGreaterThan and toBeLessThan

This set of comparisons validate that a value is greater than, less than, or equal to another value. The following assertions are true.


These assertions also work with reference types when the @operator(">" | "<" | ">=" | "<=") is used on a method in the class.

class Vec3 {
  constructor(public x: f64 = 0.0, public y: f64 = 0.0, public z: f64 = 0.0) {}

  protected __gt(other: Vec3): bool {
    return (
      this.x * this.x + this.y * this.y + this.z * this.z >
      other.x * other.x + other.y * other.y + other.z * other.z

// valid assertion because `@operator` was overloaded
expect<Vec3>(new Vec3(1, 2, 3)).toBeGreaterThan(new Vec3(0, 0, 0));

These methods are safe to use portably with jest, provided they aren't used with reference types.


When doing floating point math, it's possible that values will not be exactly as expected because of floating point error.

expect<f64>(0.1 + 0.2).toBe(0.3); // fails

> 0.1 + 0.2

Instead, use expect().toBeCloseTo() to validate an expected floating point value.

expect<f64>(0.1 + 0.2).toBeCloseTo(0.3); // passes!

Reference values and integer values will result in a runtime error, because toBeCloseTo comparisons require a floating point number to work.

This method is safe to use portably with jest.


This comparison verifies the length of a given object. This includes Arrays, TypedArrays, ArrayBuffers, and custom classes that have a length property.

class LengthExample {
  constructor(public length: i32 = 0) {}

const array = new Array<Vec3>(100);
const typedarray = new Uint8Array(42);
const buffer = new ArrayBuffer(29);
const custom = new LengthExample(50);


This method is safe to use with jest, with the exception of using ArrayBuffer.

toContain and toInclude

This comparison is used to determine if an Array contains a value.

All the values returned by T[index] will be compared using the == operator, so overloading the class @operator("==") can be used in conjunction with this comparison. The index must be a number value, and there must be a length property that matches the index type. All values from 0 to length - 1 will be checked.

const data = new Uint8Array(100);
data[5] = 255;


This method is portable with jest using the toContain() method.

toContainEqual and toIncludeEqual

This comparison is used to determine if an Array contains a reference that equals another reference.

All the values returned by T[index] will be compared using the == operator, and if that comparison does not work, a memcompare will be used. Overloading the class @operator("==") can be used in conjunction with this comparison. The index must be a number value, and there must be a length property that matches the index type. All values from 0 to length - 1 will be checked.

const reference = new Vec3(1, 2, 3);
const data = new Array<Vec3>(0);
data.push(new Vec(0, 0, 0));
data.push(new Vec(1, 2, 3));
data.push(new Vec(4, 5, 6));


This method is portable with jest using the toContainEqual() method.

Configuration File

Currently as-pect will compile each file that matches the globs in the include property of your configuration. The default include is "assembly/__tests__/**/*.spec.ts". It must compile each file, and run each binary separately inside it's own TestContext. This is a limitation of AssemblyScript, not of as-pect.

A typical configuration is provided when you use asp --init and is located here.

Types And Tooling

The as-pect cli comes with a way to generate the types for all the globals used by the framework. Simply use the --init or --types flag. When a new version of as-pect is released, simply run the npx asp --types command to get the latest version of these function definitions. This will greatly increase your productivity because it comes with lots of documentation, and adds a lot of intellisense to your development experience.

It is also possible to reference the types manually. Use the following reference at the top of your assembly/index.ts file to include these types in your project automatically. If you use this method for your types, feel free to delete the auto-generated types file in your test folder.

/// <reference path="../node_modules/@as-pect/core/types/as-pect.d.ts" />
/// <reference path="../node_modules/@as-pect/core/types/as-pect.portable.d.ts" />

CI Usage

If any module fails during compilation, the utility will exit immediately with code 1 so it can be used for quicker ci builds.

Adding this line to your .travis.yml will allow you to specify a custom script to your CI build.

  - npm run test:ci

Then in your package.json file, you can instruct the "test:ci" script to run the asp command line tool to use the SummaryTestReporter like this:

  "scripts": {
    "test:ci": "asp --summary"

AssemblyScript Compiler Options

--compiler CLI Argument

By default as-pect will use node's resolver to look for an AssemblyScript module. If you want to specify a different version of the compiler, use --compiler ../relative/path/to/compiler/folder. Note that it expects the following to be the same __folder__/dist/asc.js, __folder__/cli/util/options.js, and __folder__/lib/loader.js.

Compiler Flags

Regardless of the installed version, all the compiler flags will be passed to the asc command line tool.

const compiler = require(path.join(path.cwd(), options.compiler, "cli/asc"));

Inside the callback, any files that are generated, except for the .wasm file will be output using the {testFolder}/{testName}.{ext} format. This includes source maps, .wat files, .js files, and types files generated by the compiler.


Reporters are the way tests get reported. When running the CLI, the SummaryReporter is used and all the values will be logged to the console. The test suite itself does not log out test results. If you want to use a custom reporter, you can create your own by extending the abstract Reporter class.

export abstract class Reporter {
  public abstract onStart(suite: TestSuite): void;
  public abstract onGroupStart(group: TestGroup): void;
  public abstract onGroupFinish(group: TestGroup): void;
  public abstract onTestStart(group: TestGroup, result: TestResult): void;
  public abstract onTestFinish(group: TestGroup, result: TestResult): void;
  public abstract onFinish(suite: TestSuite): void;
  public abstract onTodo(group: TestGroup, todo: string): void;

Each test suite run will use the provided reporter and call onStart(suite: TestSuite) to notify a consumer that a test has started. This happens once per test file. Since a file can have multiple describe function calls, these are logically placed into TestGroups. Each TestGroup has it's own description and contains a list of TestResults that were run.

If no reporter is provided to the configuration, one will be provided that uses stdout and chalk to provide colored output.

If performance is enabled, then the times array will be populated with the runtime values measured in milliseconds.


This reporter only outputs failed tests and is the default TestReporter used by the as-pect cli. It can be used directly from the configuration file.

const { SummaryReporter } = require("@as-pect/core");

// export your configuration
module.exports = {
  reporter: new SummaryReporter({
    // enableLogging: false, // disable logging

It can also be used from the cli using the --summary flag.

npx asp --summary
npx asp --summary=enableLogging=false

Note: When using parameters for the builtin reporters, the = is required to parse the querystring parameters correctly.


This reporter outputs a lot of information, including:

  • All Test Groups and Test Names for each test
  • RTrace Info (reference allocations vs deallocations)
  • Performance Statistics
  • Logging Information

It can be used directly from the configuration file.

const { VerboseReporter } = require("@as-pect/core");

module.exports = {
  reporter: new VerboseReporter(),

It can also be used from the cli using the --verbose flag.

npx asp --verbose


The JSONReporter can be used to create json files that contain the test output. The file output location is {testname}.spec.json. It can be used directly from the configuration file.

// as-pect.config.js
const { JSONReporter } = require("@as-pect/core");

// export your configuration
module.exports = {
  reporter: new JSONReporter(),

It can also be used from the cli using the --json flag.

npx asp --json

The object ouput definition is shaped like this:

// Test Results are compiled into an array
  // For each test, there is an object with the following shape
    // The Test Group
    group: group.name,
    // The Test Name
    name: result.name,
    // If it ran
    ran: result.ran,
    // If it passed
    pass: result.pass,
    // The total test runtime
    runtime: result.runTime,
    // The error message
    message: result.message,
    // Actual value message if an expectation failed
    actual: result.actual ? result.actual.message : null,
    // Expected value message if an expectation failed
    expected: result.expected ? result.expected.message : null,
    // The average run time (performance)
    average: result.average,
    // The median run time (performance)
    median: result.median,
    // The maximum run time (performance)
    max: result.max,
    // The minimum run time (performance)
    min: result.min,
    // The standard deviation of the run times (performance)
    stdDev: result.stdDev,
    // The variance of the run times (performance)
    variance: result.variance,


The CSVReporter can be used to create csv files that contain the test output. The file output location is {testname}.spec.csv. It can be used directly from the configuration file.

const { CSVReporter } = require("@as-pect/core");

// export your configuration
module.exports = {
  reporter: new CSVReporter(),

It can also be used from the cli using the --csv flag.

npx asp --csv

This is a list of all the columns in the exported csv file.

const csvColumns = [
  "Group", // The Test Group
  "Name", // The Test Name
  "Ran", // If it ran
  "Pass", // If it passed
  "Runtime", // The total test runtim
  "Message", // The error message
  "Actual", // Actual value message if an expectation failed
  "Expected", // Expected value message if an expectation failed
  "Average", // The average run time (performance)
  "Median", // The median run time (performance)
  "Max", // The maximum run time (performance)
  "Min", // The minimum run time (performance)
  "StdDev", // The standard deviation of the run times (performance)
  "Variance", // The variance of the run times (performance)


It is possible to write as-pect tests that run in jest as well. The compatible functions are documented in the @as-pect/core/types/as-pect.portable.d.ts file and can be added to your project by using the --portable flag. Instead of using the types provided by @types/jest, use the portable ones provided by as-pect. This is the recommended way to setup testing when trying to write portable tests.

npm install --save-dev jest @as-pect/cli typescript assemblyscript/assemblyscript
npx ts-jest config:init
npx asp --portable

Then change the jest configuration to match these properties as you see fit:

module.exports = {
  preset: "ts-jest",
  testEnvironment: "node",
  testMatch: ["assembly/__tests__/**/*.spec.ts"],

This is the compatibility table.

RTrace and Memory Leaks

If an expectation fails and hits an unreachable() instruction, any unreleased references in the function call stack will be held indefinitely as a memory leak. Test Suites don't stop running if they fail the test callback. However, tests will stop if they fail inside the beforeEach(), beforeAll(), afterEach(), and afterAll() callbacks.

Typically, a throws() test will leave at least a single Expectation on the heap. This is to be expected, because the unreachable() instruction unwinds the stack, and prevents the ability for each function to __release a reference pointer properly. Your test suite output may look like this:

[Describe]: toHaveLength TypedArray type: Uint32Array

 [Success]: ✔ should assert expected length
  [Throws]: ✔ when expected length should not equal the same value RTrace: +3
 [Success]: ✔ should verify the length is not another value
  [Throws]: ✔ when the length is another expected value RTrace: +3

The RTrace: +3 corresponds to an Expectation, a Uint32Array, and a single backing ArrayBuffer that was left on the heap because of the fact that the expectation failed. This was expected because these two tests were annotated with the throws(desc, callback) function. If you see a function that is expected to pass and RTrace returns a very large value, it might be an indicator of a very serious memory leak, and the DefaultTestReporter can be your best friend when it comes to finding these sorts of problems.

Among other solutions, the following methods are exposed to you as a way to inspect how many allocations and frees occurred during the course of function execution. Every one of these functions exist in the RTrace namespace and will call into JavaScript to query the state of the heap relative to the overall test file, the test group, and each individual test depending on the function.

This can be disabled with the --nortrace cli option.

Performance Testing

To increase performance on testing, do not use the log() function and reduce the amount of IO that as-pect must do to compile your tests. The biggest bottleneck in Web Assembly testing, is compilation. This means that using things like @inline many times will cause your module to compile more slowly, and as a result the testing file will run slower.

Providing these values inside an as-pect.config.js configuration will set these as the global defaults.

Note that when using the cli, the cli flag inputs will override the as-pect.config.js configured values.

// in as-pect.config.js
module.exports = {
  performance: {
    /** Enable performance statistics gathering for *every* test. */
    enabled: false,
    /** Set the maximum number of samples to run for every test. */
    maxSamples: 10000,
    /** Set the maximum test run time in milliseconds for every test. */
    maxTestRunTime: 2000,
    /** Report the median time in the default reporter for every test. */
    reportMedian: true,
    /** Report the average time in milliseconds for every test. */
    reportAverage: true,
    /** Report the standard deviation for every test. */
    reportStandardDeviation: false,
    /** Report the maximum run time in milliseconds for every test. */
    reportMax: false,
    /** Report the minimum run time in milliseconds for every test. */
    reportMin: false,

Custom Imports Using CLI

If a set of custom imports are required for your test module, it's possible to provide a set of imports for a given test file.

If your test is located at assembly/__tests__/customImports.spec.ts, then use filename assembly/__tests__/customImports.spec.imports.js to export the test module's imports. This file will be required by the cli before the module is instantiated.


Please see the provided example located in assembly/__tests__/customImports.spec.ts.

Using as-pect as a Package

It's possible that running your tests requires a browser environment. Instead of running as-pect from the command line, use the --output-binary flag along with the --norun flag and this will cause as-pect to output the *.spec.wasm file. This binary can be fetch()ed and instantiate like the following example.

// browser-test.ts
import { instantiateBuffer } from "assemblyscript/lib/loader";
import {
  // EmptyReporter,
} from "as-pect";

const performanceConfiguration: IPerformanceConfiguration = {
  // put performance configuration values here

// Create a TestContext
const runner = new TestContext({
  // reporter: new EmptyReporter(), // Use this to override default test reporting
  // testRegex: /.*/, // Use this to run only tests that match this regex
  // groupRegex: /.*/, // Use this to run only groups that match this regex
  fileName: "./test.spec.ts", // Always set the filename

// put your assemblyscript imports here
const imports = runner.createImports({});

// instantiate your test module here via the "assemblyscript/lib/loader" module
const wasm = instantiateStreaming<IAspectExports>(

runner.run(wasm); // run the tests synchronously

// loop over each group and test in that group
for (const group of runner.testGroups) {
  for (const test of group.tests) {
    console.log(test.name, test.pass ? "pass" : "fail");

If you want to compile each test suite manually, it's possible to use the asc compiler yourself by including the following file in your compilation.


By default, as-pect always shows the generated compiler flags.


To contribute please see CONTRIBUTING.md.

Thanks to @willemneal and @MaxGraey for all their support in making as-pect the best software it can be.

Other Contributors:

Special Thanks

Special thanks to the AssemblyScript team for creating AssemblyScript itself.