Typescript assign null to a type don't trigger error

Question

Can anyone explain me, please, why this code dont trigger an error?

import { Injectable } from '@angular/core';

interface Animal{
  name: string;
}

@Injectable()
export class AnimalService {
  lion: Animal = null;
  constructor() {}
  get(){
   return this.lion;
 }
}

Answer 1

You have to set "strictNullChecks": true in your tsconfig.json for it to throw error when you do lion: Animal = null;.

See the documentation for strictNullChecks compiler flag:

In strict null checking mode, the null and undefined values are not in the domain of every type and are only assignable to themselves and any (the one exception being that undefined is also assignable to void).

Also see: https://ariya.io/2016/10/typescript-2-and-strict-null-checking