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Create a constructor function Accumulator(startingValue).

Object that it creates should:

Store the current value in the property value. The starting value is set to the argument of the constructor startingValue.
The read() method should use prompt to read a new number and add it to value.

In other words, the value property is the sum of all user-entered values with the initial value startingValue.

Heres the demo of the code:

          let accumulator = new Accumulator(1); // initial value 1

accumulator.read(); // adds the user-entered value
accumulator.read(); // adds the user-entered value

alert(accumulator.value); // shows the sum of these values

Run the demo

Open a sandbox with tests.

solution

function Accumulator(startingValue) {
  this.value = startingValue;

  this.read = function() {
    this.value += +prompt('How much to add?', 0);
  };

}

let accumulator = new Accumulator(1);
accumulator.read();
accumulator.read();
alert(accumulator.value);

Open the solution with tests in a sandbox.

Create new Accumulator