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WebAssembly – Runtime API

  1. WebAssembly – Static Methods
  2. WebAssembly – Constructor Property

Note: Azion Functions Runtime provides the WebAssembly module interface as defined by MDN. You can review the documentation directly in MDN website here.

1. WebAssembly – Static methods

1.1.WebAssembly.compile()

The WebAssembly.compile() function compiles WebAssembly binary code into a WebAssembly.Module object. This function is useful if it is necessary to a compile a module before it can be instantiated (otherwise, the WebAssembly.instantiate() function should be used).

Syntax

WebAssembly.compile(bufferSource)

Parameters

bufferSource

A typed array or ArrayBuffer containing the binary code of the .wasm module you want to compile.

Example

fetch('http://url/simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.compile(bytes)
).then(mod =>
  //your code here
);

1.2. WebAssembly.compileStreaming()

The WebAssembly.compileStreaming() function compiles a WebAssembly.Module from a streamed underlying source. This function can be necessary to compile a module before it can be instantiated (otherwise, the WebAssembly.instantiateStreaming() function should be used).

Syntax

WebAssembly.compileStreaming (source)

Parameters

source

A Response object, representing the underlying source of a .wasm module you want to stream and compile.

Example

var importObject = { imports: { imported_func: arg => console.log(arg) } };

WebAssembly.compileStreaming(fetch('http://url/simple.wasm'))
.then(module => WebAssembly.instantiate(module, importObject))
.then(instance => instance.exports.exported_func());

1.3. WebAssembly.instantiate()

The WebAssembly.instantiate() function allows you to compile and instantiate WebAssembly code. This function has two overloads:

  • The primary overload takes the WebAssembly binary code, in the form of a typed array or ArrayBuffer, and performs both compilation and instantiation in one step;
  • The secondary overload takes an already-compiled WebAssembly.Module and returns a Promise that resolves to an Instance of that Module.

Syntax (primary overload)

WebAssembly.instantiate(bufferSource, importObject);

Parameters

bufferSource

A typed array or ArrayBuffer containing the binary code of the .wasm module you want to compile.

importObject (optional)

An object containing the values to be imported into the newly-created Instance, such as functions or WebAssembly.Memory objects. There must be one matching property for each declared import of the compiled module or else a WebAssembly.LinkError is thrown.

Example

var importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

fetch('http://url/simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes, importObject)
).then(result =>
  result.instance.exports.exported_func()
);

Syntax (secondary overload)

WebAssembly.instantiate(module, importObject);

Parameters

module

The WebAssembly.Module object to be in stantiated.

importObject (optional)

An object containing the values to be imported into the newly-created Instance, such as functions or WebAssembly.Memory objects. There must be one matching property for each declared import of module or else a WebAssembly.LinkError is thrown.

Example

WebAssembly.compileStreaming(fetch('http://url/simple.wasm'))
.then(module => WebAssembly.instantiate(module))
.then(instance => instance.exports.exported_func());

1.4. WebAssembly.instantiateStreaming()

The WebAssembly.instantiateStreaming() function compiles and instantiates a WebAssembly module directly from a streamed underlying source. This is the most efficient, optimized way to load wasm code.

Syntax

WebAssembly.instantiateStreaming(source, importObject)

Parameters

source

A Response object or a promise that will fulfill with one, representing the underlying source of a .wasm module you want to stream, compile, and instantiate.

importObject (Optional)

An object containing the values to be imported into the newly-created Instance, such as functions or WebAssembly.Memory objects. There must be one matching property for each declared import of the compiled module or else a WebAssembly.LinkError is thrown.

Example

var importObject = { imports: { imported_func: arg => console.log(arg) } };

WebAssembly.instantiateStreaming(fetch('http://url/simple.wasm'), importObject)
.then(obj => obj.instance.exports.exported_func());

1.5. WebAssembly.validate()

The WebAssembly.validate() function validates a given typed array of WebAssembly binary code, returning whether the bytes form a valid wasm module (true) or not (false).

Syntax

WebAssembly.validate(bufferSource)

Parameters

bufferSource

A typed array or ArrayBuffer containing WebAssembly binary code to be validated.

Example

fetch('http://url/simple.wasm').then(response =>
  response.arrayBuffer()
).then(function(bytes) {
  const valid = WebAssembly.validate(bytes);
  console.log("The given bytes are "
    + (valid ? "" : "not ") + "a valid wasm module");
});

2. WebAssembly - Constructor properties

2.1. WebAssembly.CompileError() constructor

The WebAssembly.CompileError() constructor creates a new WebAssembly CompileError object, which indicates an error during WebAssembly decoding or validation.

Syntax

new WebAssembly.CompileError()

new WebAssembly.CompileError(message)

new WebAssembly.CompileError(message, fileName)

new WebAssembly.CompileError(message, fileName, lineNumber)

Parameters

message (optional)

Human-readable description of the error.

fileName (optional)

The name of the file containing the code that caused the exception.

lineNumber (optional)

The line number of the code that caused the exception.

Example

try {
  throw new WebAssembly.CompileError('Hello', 'someFile', 10);
} catch (e) {
  console.log(e instanceof CompileError); // true
  console.log(e.message);                 // "Hello"
  console.log(e.name);                    // "CompileError"
  console.log(e.fileName);                // "someFile"
  console.log(e.lineNumber);              // 10
  console.log(e.columnNumber);            // 0
  console.log(e.stack);                   // returns the location where the code was run
}

2.2. WebAssembly.Instance() constructor

The WebAssembly.Instance() constructor creates a new Instance object which is a stateful, executable instance of a WebAssembly.Module.

Syntax

new WebAssembly.Instance(module, importObject)

Parameters

module

The WebAssembly.Module object to be instantiated.

importObject (optional)

An object containing the values to be imported into the newly-created Instance, such as functions or WebAssembly.Memory objects.

Example

const importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

fetch('http://url/simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes => {
  let mod = new WebAssembly.Module(bytes);
  let instance = new WebAssembly.Instance(mod, importObject);
  instance.exports.exported_func();
})

2.3. WebAssembly.LinkError() constructor

The WebAssembly.LinkError() constructor creates a new WebAssembly LinkError object, which indicates an error during module instantiation (besides traps from the start function). Syntax

new WebAssembly.LinkError()

new WebAssembly.LinkError(message)

new WebAssembly.LinkError(message, fileName)

new WebAssembly.LinkError(message, fileName, lineNumber)

Parameters

message (optional)

Human-readable description of the error.

fileName (optional)

The name of the file containing the code that caused the exception.

lineNumber (optional)

The line number of the code that caused the exception.

Examples

try {
  throw new WebAssembly.LinkError('Hello', 'someFile', 10);
} catch (e) {
  console.log(e instanceof LinkError); // true
  console.log(e.message);                 // "Hello"
  console.log(e.name);                    // "LinkError"
  console.log(e.fileName);                // "someFile"
  console.log(e.lineNumber);              // 10
  console.log(e.columnNumber);            // 0
  console.log(e.stack);                   // returns the location where the code was run
}

2.4. WebAssembly.Memory() constructor

The WebAssembly.Memory() constructor creates a new Memory object whose buffer property is a resizable ArrayBuffer or SharedArrayBuffer that holds the raw bytes of memory accessed by a WebAssembly Instance.

A memory created by JavaScript or in WebAssembly code will be accessible and mutable from both JavaScript and WebAssembly.

Syntax

new WebAssembly.Memory(memoryDescriptor)

Parameters

memoryDescriptor

An object that can contain the following members:

  • initial

    The initial size of the WebAssembly Memory, in units of WebAssembly pages.

  • maximum Optional

    The maximum size the WebAssembly Memory is allowed to grow to, in units of WebAssembly pages. When present, the maximum parameter acts as a hint to the engine to reserve memory up front. However, the engine may ignore or clamp this reservation request. Unshared WebAssembly memories don’t need to set a maximum, but shared memories do.

  • shared Optional

    A boolean value that defines whether the memory is a shared memory or not. If set to true, it is a shared memory. The default is false.

Examples

Creating a new Memory instance

var memory = new WebAssembly.Memory({initial:10, maximum:100});

Creating a shared memory

let memory = new WebAssembly.Memory({initial:10, maximum:100, shared:true});

2.5. WebAssembly.Module() constructor

A WebAssembly.Module() constructor creates a new Module object containing stateless WebAssembly code that has already been compiled by the browser and can be efficiently shared with Workers, and instantiated multiple times.

The WebAssembly.Module() constructor function can be called to synchronously compile given WebAssembly binary code. However, the primary way to get a Module is through an asynchronous compilation function like WebAssembly.compile().

Syntax

new WebAssembly.Module(bufferSource)

Parameters

bufferSource

A typed array or ArrayBuffer containing the binary code of the .wasm module you want to compile.

Example

const importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

function createWasmModule(bytes) {
  return new WebAssembly.Module(bytes);
}

fetch('http://url/simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes => {
  let mod = createWasmModule(bytes);
  WebAssembly.instantiate(mod, importObject)
  .then(result =>
     result.exports.exported_func()
  );
})

2.6. WebAssembly.RuntimeError() constructor

The WebAssembly.RuntimeError() constructor creates a new WebAssembly RuntimeError object — the type that is thrown whenever WebAssembly specifies a trap.

Syntax

new WebAssembly.RuntimeError()

new WebAssembly.RuntimeError(message)

new WebAssembly.RuntimeError(message, fileName)

new WebAssembly.RuntimeError(message, fileName, lineNumber)

Parameters

message (Optional)

Human-readable description of the error.

fileName (Optional)

The name of the file containing the code that caused the exception.

lineNumber (Optional)

The line number of the code that caused the exception.

Example

try {
  throw new WebAssembly.RuntimeError('Hello', 'someFile', 10);
} catch (e) {
  console.log(e instanceof WebAssembly.RuntimeError); // true
  console.log(e.message);                             // "Hello"
  console.log(e.name);                                // "RuntimeError"
  console.log(e.fileName);                            // "someFile"
  console.log(e.lineNumber);                          // 10
  console.log(e.columnNumber);                        // 0
  console.log(e.stack);                               // returns the location where the code was run
}

2.7. WebAssembly.Table() constructor

The WebAssembly.Table() constructor creates a new Table object of the given size and element type.

Syntax

new WebAssembly.Table(tableDescriptor)

Parameters

tableDescriptor

An object that can contain the following members:

  • element

    A string representing the type of value to be stored in the table. This can have a value of “anyfunc” (functions) or “externref” (host references).

  • initial

    The initial number of elements of the WebAssembly Table.

  • maximum Optional

    The maximum number of elements the WebAssembly Table is allowed to grow to.

Example

var tbl = new WebAssembly.Table({initial:2, element:"anyfunc"});
var len = tbl.length;  // "2"
var v1 = tbl.get(0);  // "null"
var v2 = tbl.get(1);  // "null"

// Creating an import object that contains the table
var importObj = {
  js: {
    tbl:tbl
  }
};

2.8. WebAssembly.Tag() constructor

The WebAssembly.Tag() constructor creates a new WebAssembly.Tag object.

Syntax

new WebAssembly.Tag(type)

Parameters

type

An object that can contain the following members:

  • parameters

    An array of data types (“i32”, “i64”, “f32”, “f64”, “v128”, “externref”, “anyfunc”).

Exceptions

TypeError

The type.parameters argument is not supplied, does not contain at least one value, or contains an unsupported tag descriptor.

Example

This creates a tag with two values:

const tag = new WebAssembly.Tag({ parameters: ["i32", "i64"] });

2.9. WebAssembly.Exception constructor

The WebAssembly.Exception() constructor is used to create a new WebAssembly.Exception.

The constructor accepts a Tag argument and a payload array of data fields. The data types of each of the payload elements must match the corresponding data type specified in the Tag.

The constructor may also take an options object. The options.traceStack property can be set true (by default it is false) to indicate that WebAssembly code that throws the exception may populate the exception’s stack property with a stack track.

Syntax

new Exception(tag, payload, options)

Parameters

tag

An WebAssembly.Tag defining the data types expected for each of the values in the payload.

payload

An array of one or more data fields comprising the payload of the exception. The elements must match the data types of the corresponding elements in the tag. If the number of data fields in the payload and their types don’t match, a TypeError exception is thrown.

options (optional)

An object with the following optional fields:

traceStack (optional)

true if the Exception may have a stack trace attached to its stack property, otherwise false. This is false by default (if options or options.traceStack are not provided).

Exceptions

TypeError

The payload and tag sequences do not have the same number of elements and/or the elements are not of matching types.

Example

// Create tag and use it to create an exception
const tag = new WebAssembly.Tag({ parameters: ["i32", "f32"] });
const exception = new WebAssembly.Exception(tag, [42, 42.3]);

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