fix incorrect return types

[trusktr]

This is what I see at runtime:

const formulaResult = Parser.evaluate(this.durationFormula, formulaInputs)
console.log('typeof result:', typeof formulaResult) // string

The return of evaluate was number, but I see strings being returned.

Why was Expression#evaluate's return type any? Does it return things other than strings?

[trusktr]

Oh wait, I see my formula is passing in a single input that is a string. Maybe that's why...

[trusktr]

Yeah, that was it, my formula only had one input (equivalent to 1 * x, but looks like it simply return the value back). 😆