Зачем в коде if? [закрыт]

function Calculator() { 
 
  var methods = { 
    "-": function(a, b) { 
      return a - b; 
    }, 
    "+": function(a, b) { 
      return a + b; 
    } 
  }; 
 
  this.calculate = function(str) { 
 
    var split = str.split(' '), 
      a = +split[0], 
      op = split[1], 
      b = +split[2] 
 
    if (!methods[op] || isNaN(a) || isNaN(b)) { 
      return NaN; 
    } 
 
    return methods[op](a, b); 
  } 
 
  this.addMethod = function(name, func) { 
    methods[name] = func; 
  }; 
} 
 
var calc = new Calculator;