如何检查eval名称是否未定义

I have a form that is basically a calculator. you can type an equation in and it will evaluate it. I also have 2 memory fields(text boxes named m1 and m2) where you can type something in and it will hold that value and then when you are typing an expression in the first box, you can reference m1 or m2 in your equation and it will evaluate using the numbers you entered in the memory fields.I have a form that is basically a calculator. y