require is not a function

[dylan-chong]

iex(2)> Execjs.eval("const aaa = require('aaa')")
** (Execjs.RuntimeError) TypeError: require is not a function
    lib/execjs.ex:90: Execjs.extract_result/1
    lib/execjs.ex:55: Execjs.exec/1

[dylan-chong]

I see that require was intentionally set to undefined as a function argument. Why? I want to be able to require libraries/ JavaScript files I've created