function keysIn( obj ) {
	var out;
	var key;

	out = [];
	for ( key in obj ) { // eslint-disable-line guard-for-in
		out.push( key );
	}
	return out;
}
Returns an array of an object's own and inherited enumerable property names.

@param {ObjectLike} obj - input object
@returns {Array} key array

@example
function Foo() {
    this.beep = 'boop';
    return this;
}

Foo.prototype.foo = 'bar';

var obj = new Foo();

var keys = keysIn( obj );
// e.g., returns [ 'beep', 'foo' ]