Support of isof function

[girlpunk]

OData version 4 introducses support for the isof function. Documentation is available at https://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part2-url-conventions.html#_Toc31361017, however implementation should be fairly straightforward.

[bodia-uz]

From the doc:

Example 95: orders that are also BigOrders
http://host/service/Orders?$filter=isof(NorthwindModel.BigOrder)
http://host/service/Orders?$filter=isof($it,NorthwindModel.BigOrder)

Example 96: orders of a customer that is a VIPCustomer
http://host/service/Orders?$filter=isof(Customer,NorthwindModel.VIPCustomer)

If I understand correctly, .fn should solve it:

// isof(Customer,NorthwindModel.VIPCustomer)
const filter = f()
  .fn('isof', 'Customer', 'NorthwindModel.VIPCustomer', false);

// isof(NorthwindModel.BigOrder)
const filter = f()
  .fn('isof', 'NorthwindModel.BigOrder', undefined, false);

odata-filter-builder/test/ODataFilterBuilder_spec.js

Lines 464 to 480 in 876f2a5

	describe('fn', () => {
	it('substringof', () => {
	const filter = f()
	.fn('substringof', 'Name', 'John', true, true);
	expect(filter.toString())
	.toBe("substringof('John', Name)");
	});
	it('substringof + toLower', () => {
	const filter = f()
	.fn('substringof', x => x.toLower('Name'), 'john', true, true);
	expect(filter.toString())
	.toBe("substringof('john', tolower(Name))");
	});
	});