test/binaryen.js/custom-section.js

function assert(x) {
  if (!x) throw 'error!';
}

Binaryen.setAPITracing(true);
var module = new Binaryen.Module();

module.addCustomSection("hello", [119, 111, 114, 108, 100]);

assert(module.validate());
console.log(module.emitText());