should be a number in chai mocha code example

Example: 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});