Really Stuck on this one. Getting an error when i add a different value for the unit

Right, this is what im doing....

function getArea(width = 45, length = 69, unit = 'square feet') { const area = width * length; return ${area} ${unit}; }

thats my code, and if i pass the "unit" parameter an argument that is a number it works, but not if i send it a text string. For instance if I call getArea(5,10,miles)

the console gives me this error:

VM131:1 Uncaught ReferenceError: miles is not defined at <anonymous>:1:13

but ONLY when i put in a text string instead of a number. It still works for the default input being 'square feet' if its left blank but surely i need to be able to put in an argument that is one made of words not numbers

In fact, now i think of it, i'm not even sure how you define what each parameter of a function's type will be??? Did i miss something? How do I tell the browser that "length" should expect a number for instance?

Even so, you'd think that if i set the "unit" parameter to have a default of a string, it would be able to accept another string???

I think im probably a bit tired but this is making no sense. HELP!!