TypeScript -> Inline Type Guard

InlineTypeGuard.ts

// Source: https://www.benmvp.com/blog/filtering-undefined-elements-from-array-typescript/

/* 
 * Let's say I have a list of MyObjects and I want to filter down to all of the contained MySubProperties.
 * 
 * I need to filter out the objects with undefined property values since they cannot have a subproperty, and
 * I need TypeScript to understand that the result type of that filter is MyProperty[] rather than 
 * (MyProperty | undefined)[] - that way, ts won't flag the access of p.subProperty for possibly undefined values. 
 */

interface MyProperty {
  subProperty: string;
}

interface MyObject {
 property?: MyProperty;
}

const myObjects:MyObject[] = [...someObjects];

const mySubProperties = myObjects
        .map(o => o.property)
        .filter((property): property is MyProperty => !!property)
        .map(p => p.subProperty);

/* Some things to note, per the site referenced above:
 - `!!property` returns true if property is not falsey (false, 0, null, undefined)
 - The `property is MyProperty` return type guarantees a boolean return value for the filter
 - The above factors allow TypeScript to infer the return type of this filter as MyProperty[] so that 
   the following call to `.map` does not show any "possibly-undefined" ts errors when accessing `p.subProperty`
*/