TypeScript type assertion from mixed object list

Question

Is it possible to find an item in an array by it's custom type?

type A = {
  x: number;
};

type B = {
  y: number;
};

type Mixed = A | B;

const list: Mixed[] = [{x:5}, {y:3}];

const a = list.find(item => 'x' in item);  // <-- how to make something like this work?

Answer 1

Jan isn't wrong. Here's a better answer, showing how to filter the mixed array into two typed lists - without the potential for a "undefined" value to be mistyped as A or B.

type A = {
    x: number;
};
  
type B = {
   y: number;
};
  
type Mixed = A | B;
  
const list: Mixed[] = [{x:5}, {y:3}];
  
const isA = (candidate: Mixed): candidate is A => {
      return (candidate && candidate.hasOwnProperty('x'))
  }

const isB = (candidate: Mixed): candidate is B => {
    return (candidate && candidate.hasOwnProperty('y'))
}

const AItems = list.filter(item => isA(item)) as A[];
const BItems = list.filter(item => isB(item)) as B[];