en.javascript.info/1-js/3-object-basics/7-array/5-array-input-sum/solution.md

Please note the subtle, but important detail of the solution. We don't convert value to number instantly after prompt, because after value = +value we would not be able to tell an empty string (stop sign) from the zero (valid number). We do it later instead.

function sumInput() {
 
  let numbers = [];

  while (true) {

    let value = prompt("A number please?", 0);

    // should we cancel?
    if (value === "" || value === null || !isFinite(value)) break;

    numbers.push(+value);
  }

  let sum = 0;
  for (let number of numbers) {
    sum += number;
  }
}

alert( sumInput() );