The answer
Track current input, previous value, and operation. Number buttons append to current. Operation buttons store the operation and previous value. Equals performs the calculation. Clear resets everything. Use a grid layout for buttons.
How to Build a Calculator in an Interview
How do I build a calculator in a machine coding interview?
More FAQs in How to Build a Calculator in an Interview
Check if the divisor is 0 before dividing. If so, display 'Error' or 'Cannot divide by zero' instead of performing the calculation. Do not let JavaScript produce Infinity; handle it explicitly.
When a new operation is set and there is already a pending operation, calculate the pending operation first (calculate()), then store the result as the new previous and set the new operation. This allows chaining like 5 + 3 * 2.
Add a keydown listener. Map number keys to inputChar, operator keys to setOp, Enter/= to calculate, Escape to clear, and decimal to inputChar('.'). Prevent default for Enter to avoid form submission.
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