The get method is used to target a property of the current assertion's value (i.e.,
it "actual" value) with a new assertion.
For example:
let o = { a: 4 };
expect(o).get('a').to.be(4);
The above is roughly equivalent to:
expect(o.a).to.be(4);
The difference is that get produces undefined as the actual value if the current
value cannot be indexed. For example:
expect(undefined).get('a').to.be(undefined);