Also you must add
const utils = require("./utils");

to wasm/tester.js and c/tester.js

Thanks for the proposal, but I don't like to add the dependency to utiils.js and add there isDefined. I prefer to add it at the end of wasm/tester.js and remove utils. before its use.
Do you want to change your PR? If so I'll accept it.

@alrubio I made the changes as you suggested.