More thorough testing

[apttx]

Add

• [x] negative numbers
• [x] function values
• [x] empty function calls
• [x] undefined values
• [x] values passed via prop

[hexagon6]

Maybe try to use a fuzzer? e.g. https://www.npmjs.com/package/test-each#fuzz-testing

[apttx]

Not sure how applicable this is to this, but I'll keep it in mind. Thanks!