I'm not up-to-date regarding JS programming, so: why is this solved as it is, i.e. as anonymous function that is immediately called? First idea would be to prevent leaking anything into a global context but this shouldn't be an issue here. Can anyone explain?

    var supported = (() => {
      try {
        if (
          typeof WebAssembly === "object" &&
          typeof WebAssembly.instantiate === "function"
        ) {
          const module = new WebAssembly.Module(
            Uint8Array.of(0x0, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00)
          );
          if (module instanceof WebAssembly.Module)
            return (
              new WebAssembly.Instance(module) instanceof WebAssembly.Instance
            );
        }
      } catch (e) { }
      return false;
    })();

Edit: is it maybe to save one from having to think about leaking anything, i.e. wrap everything in (() => { ... })(); and call it a day?