My solutions with template literals and unary plus

// declare program variables
let num1;
let num2;
let message;

// announce the program
alert("Let's do some math!");

// collect numeric input and convert into numbers
num1 = prompt("Please type a number");
num1 = +num1;
num2 = prompt("Please type another number");
num2 = +num2;

if (num2 === 0) {
  alert("The second number is 0. You can't divide by zero. Reload and try again.");
} else if (Number.isNaN(num1) || Number.isNaN(num2)) {
  alert("At least one of the values you typed is not a number. Reload and try again.");
} else {

  // build an HTML message
  message = `
    <h1>Math with the numbers ${num1} and ${num2}</h1>
    ${num1} + ${num2} = ${num1 + num2} <br>
    ${num1} * ${num2} = ${num1 * num2} <br>
    ${num1} / ${num2} = ${num1 / num2} <br>
    ${num1} - ${num2} = ${num1 - num2}
  `
  // write message to web page
  document.write(message);
}