The WebAssembly.customSections() function returns a copy of the contents of all custom sections in the given module with the given string name.


WebAssembly.Module.customSections(module, sectionName);


The WebAssembly.Module object whose custom sections are being considered.
The string name of the desired custom section.

Return value

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 is comprised 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


Using customSections

The following example (see the custom-section.html source and live example) compiles the loaded simple-name-section.wasm byte code.

We then do a check using WebAssembly.Module.customSections, looking to see whether the module instance contains a "name" custom section by checking that its length is more than 0. Since there is a "name" section in the example, an ArrayBuffer object is returned.

.then(function(mod) {
  var nameSections = WebAssembly.Module.customSections(mod, "name");
  if (nameSections.length != 0) {
    console.log("Module contains a name section");


WebAssembly JavaScript Interface
The definition of 'customSections()' in that specification.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
customSectionsChrome Full support 57Edge Full support 16Firefox Full support 52
Full support 52
Notes Disabled in the Firefox 52 Extended Support Release (ESR).
IE No support NoOpera Full support 44Safari Full support 11WebView Android Full support 57Chrome Android Full support 57Firefox Android Full support 52
Full support 52
Notes Disabled in the Firefox 52 Extended Support Release (ESR).
Opera Android Full support 43Safari iOS Full support 11Samsung Internet Android Full support 7.0nodejs Full support 8.0.0


Full support  
Full support
No support  
No support
See implementation notes.
See implementation notes.

See also