module

The WebAssembly.Module object whose custom sections are being considered.

sectionName

The string name of the desired custom section.

A (possibly empty) array containing ArrayBuffer copies of the contents of all custom sections matching sectionName.

If module is not a WebAssembly.Module object instance, a TypeError is thrown.

A wasm module consists of a sequence of sections. Most of these sections are fully specified and validated by the wasm spec, but modules can also contain custom sections that are ignored and skipped over during validation. (Read High level structure for information on section structures, and how normal sections ("known sections") and custom sections are distinguished.)

This provides developers with a way to include custom data inside wasm modules for other purposes, for example the name custom section, which allows developers to provide names for all the functions and locals in the module (like "symbols" in a native build).

Note that the WebAssembly text format currently doesn't have a syntax specified for adding new custom sections; you can however add a name section to your wasm during conversion from text format over to .wasm. The wast2wasm command available as part of the wabt tool has a --debug-names option — specify this during conversion to get a .wasm with a names custom section, for example:

wast2wasm simple-name-section.was -o simple-name-section.wasm --debug-names

The following example uses WebAssembly.Module.customSections to check if a loaded module instance contains a "name" custom section. A module contains a "name" custom section if WebAssembly.Module.customSections returns an ArrayBuffer with a length greater than 0.

See custom-section.html source code and live example.

WebAssembly.compileStreaming(fetch('simple-name-section.wasm'))
.then(function(mod) {
  const nameSections = WebAssembly.Module.customSections(mod, "name");
  if (nameSections.length !== 0) {
    console.log("Module contains a name section");
    console.log(nameSections[0]);
  };
});

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