calc_with_json.js

calc = (a, b, op) => operations[op](a,b)

operations = {
  '+': (a, b) => a + b,
  '-': (a, b) => a - b,
  '*': (a, b) => a * b,
  '/': (a, b) => a / b
}

calc_with_switch.js

calc = (a, b, op) => {
  
  switch (op) {
      
    case '+': 
      return a + b 
      break
      
    case '-': 
      return a - b
     break
      
    case '*': 
      return a * b
      break
      
    case '/': 
      return a / b
      break
  }
}

jsbin_with_tests.js

/* my fancy test suite */

const expect = (fn, input, description, expected) => {
  const result = fn(...input);
                    
  (result === expected)
   ? console.log(`passou   :: ${description} :: esperava ${expected}, recebeu ${result}`)
   : console.error(`deu ruim :: ${description} :: esperava ${expected}, recebeu ${result}`)
}


/* so-called tests */
rodaTestes = () => {
  console.clear()
  
  expect(calc, [1,1, '+'], 'tenta somar dois numeros', 2)
  expect(calc, [1,1, '-'], 'tenta subtrair dois numeros', 0)
  expect(calc, [2,2, '*'], 'tenta multiplicar dois numeros', 4)
  expect(calc, [3,3, '/'], 'tenta dividir um numero pelo outro', 1)
}



/* behold the codigo */
calc = (a, b, op) => {
  
  switch (op) {
    case '+': 
      return a + b 
      break
      
    case '-': 
      return a - b
     break
      
    case '*': 
      return a * b
      break
      
    case '/': 
      return a / b
      break
  }  
}

calc_json = (a, b, op) => operations[op](a,b)

operations = {
  '+': (a, b) => a + b,
  '-': (a, b) => a - b,
  '*': (a, b) => a * b,
  '/': (a, b) => a / b
}

/* roda testes */
rodaTestes()