Strict Mode

[MartinLoeper]

As explained in #18, several variable declarations are missing.
The strict mode behaviour is defined here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode.

Any assignment that silently fails in normal code (assignment to a non-writable global or property, assignment to a getter-only property, assignment to a new property on a non-extensible object) will throw in strict mode [...]

I also included #17 by @arkraft.
The changes provided are running in our environment without issues.

closes #18

[MartinLoeper]

Do not forget to squash the commits during merge. Unfortunately, I did not have time to do this today.