WebAssembly.Table()

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

This is a JavaScript wrapper object — an array-like structure representing a WebAssembly Table, which stores function references. A table created by JavaScript or in WebAssembly code will be accessible and mutable from both JavaScript and WebAssembly.

Syntax

var myTable = 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. At the moment this can only have a value of "anyfunc" (functions).

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.

Exceptions

• If tableDescriptor is not of type object, a TypeError is thrown.
• If maximum is specified and is smaller than initial, a RangeError is thrown.

Table instances

All Table instances inherit from the Table() constructor's prototype object — this can be modified to affect all Table instances.

Instance properties

Table.prototype.constructor

Returns the function that created this object's instance. By default this is the WebAssembly.Table() constructor.

Table.prototype.length

Returns the length of the table, i.e. the number of elements.

Instance methods

Table.prototype.get()

Accessor function — gets the element stored at a given index.

Table.prototype.grow()

Increases the size of the Table instance by a specified number of elements.

Table.prototype.set()

Sets an element stored at a given index to a given value.

Examples

The following example (see table2.html source code and live version) creates a new WebAssembly Table instance with an initial size of 2 elements. We then print out the table length and contents of the two indexes (retrieved via Table.prototype.get() to show that the length is two and both elements are null.

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

We then create an import object that contains the table:

var importObj = {
  js: {
    tbl:tbl
  }
};

Finally, we load and instantiate a wasm module (table2.wasm) using our fetchAndInstantiate() utility function.  The table2.wasm module contains two functions (one that returns 42 and another that returns 83) and stores both into elements 0 and 1 of the imported table (see text representation).  So after instantiation, the table still has length 2, but the elements now contain callable Exported WebAssembly Functions which we can call from JS.

fetchAndInstantiate('table2.wasm', importObject).then(function(instance) {
  console.log(tbl.length);    // "2"
  console.log(tbl.get(0)());  // "42"
  console.log(tbl.get(1)());  // "83"
});

Note how you've got to include a second function invocation operator at the end of the accessor to actually invoke the referenced function and log the value stored inside it (e.g. get(0)() rather than get(0)) .

This example shows that we're creating and accessing the table from JavaScript, but the same table is visible and callable inside the wasm instance too.

Specifications

Browser compatibility

[1] WebAssembly is enabled in Firefox 52+, although disabled in the Firefox 52 Extended Support Release (ESR.)

[2] Currently supported behind the “Experimental JavaScript Features” flag. See this blog post for more details.

See also

• WebAssembly overview page
• WebAssembly concepts
• Using the WebAssembly JavaScript API