error TS2339: Property 'x' does not exist on type 'Y'

Question

I don\'t understand why this code generates TypeScript error. (It\'s not the original code and is a bit derived, so please ignore the non-sense in the example):

Answer 1

The correct fix is to add the property in the type definition as explained by @Nitzan Tomer. But also you can just define property as any, if you want to write code almost as in JavaScript:

arr.filter((item:any) => {
    return item.isSelected == true;
}