meekg33k

const isPresentSomeObject = (arg: SomeObjectType | undefined): boolean => {
  if (arg && Object.keys(arg).length > 0) { 
    return true;
  }
  return false;
}

meekg33k

if (someObject && Object.keys(someObject).length > 0) { 
  //Our updated test to check if an object is really not empty
}

meekg33k

const identityStringFn = (arg: string): string => arg;
const identityNumberFn = (arg: number): number => arg;
const identityBooleanFn = (arg: boolean): boolean => arg;

meekg33k

interface Cat {
  meow();
  walk();
}

interface Dog {
  bark();
  walk();
}

meekg33k

const isCat = (animal: Animal):boolean => {
  return (animal as Cat).meow
}

const isDog = (animal: Animal): boolean => {
  return (animal as Dog).bark
}

meekg33k

const isCat = (animal: Animal): animal is Cat => {
  //Hey compiler, this animal is a Cat
  return true;
}

meekg33k

type SomeObjectType = {
  id: number;
  text: string;
}
let someObject: SomeObjectType | undefined;
if (someObject && Object.keys(someObject).length > 0) {
  //This evaluates as truthy because it's not undefined 
  someObject.id = 12;  //TS compiler doesn't complain here because the object exists
}

meekg33k

if (isPresentSomeObject(someObject)) {
  //Back off compiler, I got this!
  someObject!.id = 12; 
}

meekg33k

//Keep adding more types to the union, good luck with that
type ObjectType = SomeObjectType | AnotherObjectType 
  | ... | AnyOtherObjectYouWantToAddType | undefined;

//OR

//Just make the argument of type any 😱
const isPresentAny = (arg: any): boolean => {
  if (arg && Object.keys(arg).length > 0) { 
    return true;

meekg33k

type SomeObjectType;
type AnotherObjectType;
type AnyOtherObjectYouWantToAddType;

const isPresentObjectGeneric = <T>(arg: T): boolean => {
  if (arg && Object.keys(arg).length > 0) { 
    return true;
  }
  return false;
}