var x = 20;

var A = function () {
  this.list = [1, 2, 3, 4, 5];
  this.x = 2;
}

A.prototype.test = function() {
  var self = this; // или var that = this;e
  this.list.forEach(function (element) {
    console.log(Math.pow(element, self.x));
  });
}

var a = new A();
a.test();