First off, you shouldn’t be storing numbers as strings unless they are for presentation only. Usually, you want to store numbers as numbers, then convert or coerce to a string only at the point you display them in the UI. Storing numbers as strings can lead to some weird bugs, especially if you coerce them implicitly (for example: Number(Boolean('0')) === 1).
To answer your question, though, you can define the variable before you use it within the constructor function, then set both items to the variable. Trying to set a property to a value of another property of the same object at the same time as setting that other property on the object will never work.