Use data from variable in another function

I've got a function like this (value is from select):

function getSelectValuePower () {
 var power = document.getElementById("power").value;
  sessionStorage.setItem("power", power);
  console.log(power);
}

How can I use value from variable power in another function ?
For example :

I want to use it here :

function getValue () {
  var number = 10;
  var selectedValue = number + power;
  console.log(selectedValue2);
}

So, when my power is 7, the result in getValue() should be 17.

I tried to do it with global variables, but doesn't work.

Get power in second function:

var power = sessionStorage.getItem("power");

you can also use parseInt() as well

function getValue() {
  const number = 10;
  const power = parseInt(sessionStorage.getItem("power")) || 0;
  const selectedValue = number + power;
  console.log(selectedValue);
}

You can do it by declaring power outside the function or by creating a class. Here's how you can do it.

Using global declaration:

var power = document.getElementById("power").value;
function getSelectValuePower() {
    sessionStorage.setItem("power", power);
    console.log(power);
}

function getValue() {
    var number = 10;
    var selectedValue = number + power;
    console.log(selectedValue);
}

Using a class:

class PowerSelector {
  constructor() {
    this.powerElement = document.getElementById("power");
    this.number = 10;
  }

  getSelectValuePower() {
    const power = this.powerElement.value;
    sessionStorage.setItem("power", power);
    console.log(power);
  }

  getValue() {
    const selectedValue = Number(this.number) + Number(this.powerElement.value);
    console.log(selectedValue);
  }
}

const powerSelector = new PowerSelector();
powerSelector.getSelectValuePower();
powerSelector.getValue();