app.update is not a function

Hi Guys
When I go to the second step, and basic update function and run the test,
```
/**
* `update` transforms the `model` based on the `action`.
* @param {String} action - the desired action to perform on the model.
* @param {Object} model - the App's (current) model (or "state").
* @return {Object} new_model - the transformed model.
*/
function update(action, model) {
switch (action) { // action (String) determines which case
default: // if action unrecognised or undefined,
return model; // return model unmodified
} // default? https://softwareengineering.stackexchange.com/a/201786/211301
}
```
it supposed to pass the test, but there is the following error message, anyone know why?

> TAP version 13
> # `model` (Object) has desired keys
> ok 1 `todos` and `hash` keys are present.
> ok 2 model.todos is an Array
> # `update` default case should return model unmodified
> /Users/rushlin/Downloads/mystuff/javascript-todo-list/test/todo-app1.test.js:19
> const unmodified_model = app.update('UNKNOWN_ACTION', model);
> ^
>
> TypeError: app.update is not a function
> at Test. (/Users/rushlin/Downloads/mystuff/javascript-todo-list/test/todo-app1.test.js:19:32)
> at Test.bound [as _cb] (/Users/rushlin/node_modules/tape/lib/test.js:84:32)
> at Test.run (/Users/rushlin/node_modules/tape/lib/test.js:101:31)
> at Test.bound [as run] (/Users/rushlin/node_modules/tape/lib/test.js:84:32)
> at Immediate.next (/Users/rushlin/node_modules/tape/lib/results.js:83:19)
> at processImmediate (internal/timers.js:456:21)
> shanshandeMacBook-Pro:javascript-todo-list rushlin$
>