// Function to perform subtraction function subtract(a, b) { return a - b; } // Function to perform multiplication function multiply(a, b) { return a * b ...

... unless the user attempts to divide by zero, in which case you should reset the state and display an error message using a JavaScript alert.

Insert a custom JavaScript function from Excel's formula database ... functions for many reasons," Microsoft says, "such as: (1) Calculate math operations, like whether a number is prime.