title: The operand of a 'delete' operator must be optional. in index.test.tsx
body: <p>I am new at using TS, and now I am  running a test using jest on react/typescript and I have got this error from type script saying <code> The operand of a 'delete' operator must be optional.</code> and I can't find a way to stop TS to stop yelling
  I have tried</p>
  <pre><code>interface Thing {
    prop: string | undefined;
  }
  </code></pre>
  <p>but it didn't work with jest and the test still failing because of this,
  any suggestion on this please,
  this is my code snippet</p>
  <pre><code>  it('mergeFirestoreData', () =&gt; {
      const returnValue = mergeFirestoreData(`/${collectionNames.PERMITS}/id`, new PermitCore(), new PermitSwarm());
      const finalReturn = new Permit();
      delete finalReturn['endPermitReason']; //The operand of a 'delete' operator must be optional.ts(2790)
      expect(returnValue).toEqual(finalReturn);
    });
  </code></pre>