calculator.js

const getCalculateRegex = operator => new RegExp(
  '([0-9]+) ?\\' + operator + ' ?([0-9]+)', 'ig'
);

const operations = [
  { operator: '*', fn: (n1, n2) => n1 * n2 },
  { operator: '/', fn: (n1, n2) => n1 / n2 },
  { operator: '+', fn: (n1, n2) => n1 + n2 },
  { operator: '-', fn: (n1, n2) => n1 - n2 }
];

function calculate(inputString) {
  let result = inputString;

  operations.forEach(({ operator, fn }) => {
    const regex = getCalculateRegex(operator);
    result = result.replace(regex, (_, n1, n2) => fn(
      ...[n1, n2].map(parseFloat)
    ));
  });

  return parseFloat(result);
}

console.assert(calculate('2 + 2') === 4, 'Add');
console.assert(calculate('10 / 5') === 2, 'Divide');
console.assert(calculate('3 * 2') === 6, 'Multiply');