Accept constructor in "an" for matching custom types

Instead of just accepting a string, it’d be nice if “a” and “an” would accept a constructor to test if an object inherits from another.

// es5
function Bar() {}
Bar.prototype = Object.create(Foo.prototype);
// es6
class Bar extends Foo {}
// assertion
expect(Bar).to.be.a(Foo);

Bar.prototype instanceof Object; // true
Bar.prototype instanceof Foo; // true
Bar.prototype instanceof Baz; // false

[keithamus]

Hey @thejameskyle that sounds like a great idea! Of course, for now, you can use instanceof (e.g. expect(Bar).to.be.an.instanceof(Foo): http://chaijs.com/api/bdd/#instanceof-section). But yes, a and an could definitely support this too. If you’d like to formulate a PR, with tests and documentation, then I’d look to merging it.

[jamiebuilds]

Clearing out old issues, closing for inactivity