This web app follows Project: Calculator from The Odin Project, including all extension criteria (except making it look nice, but maybe I'll do that later!).

All core features from assignment specification:

• Operations are strung together, evaluating from left to right. E.g. 12 + 7 - 5 * 3 = eventually evaluates to 42.
• Numbers are rounded to fit on screen. User cannot input more than the screen size. Very large/small numbers are represented using scientific notation. The negative sign is also taken into account.
• Pressing the = button twice in a row does nothing.
• NaN output (such as dividing by zero) puts the calculator into an error state that can be cleared using the AC button.
• The decimal point button can only be used once per number.
• Keybinds implemented: 0-9, ,, ., +, -, *, x, /, \, =, enter, escape, ctrl + z, and backspace.

• Pressing - before a number is entered will make the number negative.
• Pressing +, ×, or ÷ before a number is entered will reuse the existing "current number". (This behaviour may be a bit confusing to use though since there's a delay in seeing the number.)
• Keyboard presses will also highlight the key pressed in the UI.

• I'm relying on the Javascript Number implementation for numerical precision, which is represented using IEEE 754 double-precision floating point. This may have undesirable behaviour compared to BCD (binary-coded decimal), particularly around digits to the right of the decimal point. I haven't looked into how to mitigate these issues, and I don't know of any examples where these precision issues show up in this calculator. I'm not going to bother since this is meant to be a simple web development exercise.
• I didn't really try too hard to make this app render well on all platforms. This app was developed using Google Chrome on a minimally-customized Windows 10 system.
• The code is hard to read. I'll need to figure out how to untangle it.