withfunction.js

class Operation {
  constructor(a, b) {
    this.a = a
    this.b = b
  }

  getSum() {
    return this.a + this.b
  }
}

let o = new Operation(2, 3);
o.getSum(); // 5

o.getDiff(); // Output - Uncaught TypeError: o.getDiff is not a function
 
// Check if the function exists
typeof o.getDiff != "undefined" && o.getDiff(); 

// With the use of optional chaining
o.getDiff?.() // Output - undefined