According to http://chaijs.com/api/bdd/#a, a/an can be used to check for the type of a variable.

.a(type)

@param{ String } type

@param{ String } message _optional_

The a and an assertions are aliases that can be used
either as language chains or to assert a value's type.

However, I'm not able to check for the variable beeing an integer. The given examples, e.g. expect('1337').to.be.a('string'); work for me, but the following does not:

expect(42).to.be.an('integer');

expect(42).to.be.an('Integer');

expect(42).to.be.an('int');

expect(42).to.be.an('Int');

All of them are giving me the following error when running mocha:

Uncaught AssertionError: expected 42 to be an integer

How do I test with chai for a variable beeing an integer?