mocha chai not to be undefined code example

Example 1: what does chai expect return

Object.prototype.b = 2;

expect({a: 1}).to.have.own.property('a');
expect({a: 1}).to.have.property('b');
expect({a: 1}).to.not.have.own.property('b');

expect({a: 1}).to.own.include({a: 1});
expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});

Example 2: chai length greater than

expect('foo').to.have.lengthOf(3); // Recommended
expect('foo').to.have.lengthOf.above(2); // Not recommended

expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
expect([1, 2, 3]).to.have.lengthOf.above(2); // Not recommended