let height = 40 let isCheck = true let rowHeight = height + (isCheck ? 50 : 20)

Question

why would the answer be 90?

Answer 1

That is ternary Operator or conditional Operator you can say.

If the condition provided is true than first part is executed otherwise second

Your code is equivalent to :
if (isCheck){
rowHeight = height + 50;
} else {
rowHeight = height + 50;
}

Answer 2

let height = 40
let isCheck = true
let rowHeight = height + (isCheck ? 50 : 20)