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Create a constructor function Calculator that creates objects with 3 methods:

read() prompts for two values and saves them as object properties with names a and b respectively.
sum() returns the sum of these properties.
mul() returns the multiplication product of these properties.

For instance:

          let calculator = new Calculator();
calculator.read();

alert( "Sum=" + calculator.sum() );
alert( "Mul=" + calculator.mul() );
        

Run the demo

Open a sandbox with tests.

solution

function Calculator() {

  this.read = function() {
    this.a = +prompt('a?', 0);
    this.b = +prompt('b?', 0);
  };

  this.sum = function() {
    return this.a + this.b;
  };

  this.mul = function() {
    return this.a * this.b;
  };
}

let calculator = new Calculator();
calculator.read();

alert( "Sum=" + calculator.sum() );
alert( "Mul=" + calculator.mul() );

Open the solution with tests in a sandbox.

Create new Calculator