foo.ts

interface Foo {
  value?: string;
}

const item1: Foo = { value: 'foo' }

// ERROR: Object is possibly 'undefined'.
console.log(`Message ${item1.value.substring(4)}`);

const item2 = { value: 'foo' }

// all good in the hood
console.log(`Message ${item2.value.substring(4)}`);