class Sample {
  constructor() {
    this.prefix = 'this is';
    this.list = [1, 2, 3, 4, 5];
  }
  mapList() {
    return this.list.map(function(x) {
      return this.prefix + x;
    });
  }
}
const obj = new Sample();
obj.mapList();             // Cannot read property 'prefix' of undefined

function Sample() { ... }
Sample.prototype.mapList = function() {
  return this.list.map(function(x) {
    return this.prefix + x
  });
}
// 'this' is undefined when mapList() is called