I'm just trying to fix some issues on my simple calculator.  I only have 3 issues right now that I am trying to fix.

1. When I'm trying to click the +,/,* and - twice the result will be NaN, how can i fix it like even if I click it twice it will be the same in clicking it once.

2. How can I create a default value of 0 that when i click a number the default value will automatically remove.

3. How can I add a maxlenght on my textbox?

I also want to add 2 buttons.

a backspace button and a decimal point.

Here's the link for my calculator  https://jsfiddle.net/harehareboy/u0n6dxn3/3

Thanks