JS vs TS

javascript.js

var things = [
  new Thing1(),
  new Thing1(),
  new Thing2()
];

var OnlyThingOnes = things.filter(function(t){return t.thingOneOnlyProperty !== undefined});

typescript.ts

things: (Thing1|Thing2)[];

//constructor code
this.things = [
  new Thing1(),
  new Thing1(),
  new Thing2()
];
var OnlyThingOne = <Thing1[]>this.things.filter(m => (<thing1>m).thingOneOnlyProperty !== undefined);