test-mock-example.md

Test mock example

This example shows how a REST API client can implement mocking for simple testing.

Let’s say the client has methods to get/list/create/update/delete todos.

client.getTodo(id);
client.listTodos();
client.createTodo(options);
client.updateTodo(options);
client.deleteTodo(id);

The exposed hook is called request.

All methods would use an internal request method to which they pass options such as the http verb, the URL, query parameters and request body.

The implementation of the hook could look like this:

function request (options) {
  return hook('request', options, (options) => (
    const {url, ...fetchOptions} = options
    return fetch(url, fetchOptions)
  ))
}

Now when writing a test for "getTodo", the wrap hook allows us to prevent from an HTTP request to be even sent, instead we test if the options passed fetch are what we expect.

test("client.getTodo(123)", () => {
  const client = getClient();
  client.hook.wrap("request", (fetch, options) => {
    assert.equal(options.method, "GET");
    assert.equal(options.url, "https://api.acme-inc.com/todos/123");
  });

  client.getTodo(123);
});