# Migrating to Aurelia 2

Aurelia 2 is a complete rewrite of Aurelia that shares many of the same loved and familiar features of Aurelia 1. Understandably, in the spirit of progress, not everything is the same. In this section, we are going to guide you through what has changed and how you can migrate over your Aurelia 1 applications to Aurelia 2.

## COMPAT PACKAGE

An quickest way to get an application in v1 up an running in v2 is to include the compat package. It can be done via 2 steps:

1. installing the compat package via
  ```
  npm install @aurelia/compat-v1
  ```
2. include the compat package into your app:
  ```ts
  import { compatRegistration } from '@aurelia/compat-v1';

  ...
  Aurelia
    .register(compatRegistration, ...)
    .app(...)
    .start()
  ```

## BREAKING CHANGES

### Event

In v2, `preventDefault` is no longer called by default. This breaking change could show up in unexpected places:
- click events: in v1, clicking on a button inside a form will not submit the form, while it will in v2, as the click event default behavior is no longer prevented
    {% hint style="info" %}
    Even though clicking default behavior is not prevented, form submission without an action will not reload the page as this default behavior is still prevented in v2, so you don't need to add `:prevent` to every button `click`, or form `submit` listener.
    {% endhint %}

- drag events: in v1, implementing drag/drop will have `preventDefault` called automatically, but in v2, they will need to be explicitly called by the application

Sometimes, if it's desirable to call `preventDefault` in an event binding, use `prevent` modifier, like the following example:

```html
<button click.trigger:prevent="doWork()">Submit manually</button>

<div dragstart.trigger="prepareDragdrop()" drop.trigger:prevent="onDrop()">
```

Read more about modifiers in [event modifier doc here](../../templates/template-syntax/event-binding.md#event-modifiers)

### Scope selection

In v2, when trying to bind with a non-existent property, the closest boundary scope (scope of the owning custom element) will be selected, instead of the immediate scope of the binding (v1 behavior).

### Internal binding property `observeProperty` has been renamed to `observe`

In v1, if you happen to use `.observeProperty` method from bindings in your application/library, then change it to `observe` instead. The parameters of the signature remain the same.

### Internal binding property `sourceExpression` has been renamed to `ast`

In v1, if you happen to use `.sourceExpression` property from bindings in your application/library, then change it to `ast` instead. The type of the property remains the same.

### Enhance API changes:

In v1, `enhance` method on an `Aurelia` instance has the signature:

```typescript
class Aurelia {
  ...

  enhance(elementOrConfig: Element | IEnhancementConfig): View;
}
```

In v2, `enhance` method on an `Aurelia` instance has the signature:

```typescript
interface IAurelia {
  ...

  enhance(enhancementConfig: IEnhancementConfig): IEnhancedView;
}
```

Parent container and resources can be specified through this config.

### View model ref binding (view-model.ref="...")

In v2, in order to get a reference to the underlying component view model, use `component.ref` instead of view-model.ref
This is to make terminologies consistent as we are moving towards component oriented terms.

### If attribute (if.bind="...")

* The primary property of `If` has been renamed from `condition` to `value`. If you are using `if.bind`, you are not affected. If you are using the multi prop binding syntax, the template looks like this:

```html
<div if="condition.bind: yes">
```

Change it to:

```html
<div if="value.bind: yes">
```

## Binding Engine

* `BindingEngine` has been removed in v2, but can still be imported from `@aurelia/compat-v1` package for ease of migration. The `collectionObserver` method on the compat package of `BindingEngine` is not the same with v1, per the follow comparison:
  v2
  ```ts
  collectionObserver(collection): { subscribe: (callback: (collection, indexMap)) => { dispose(): void } }
  ```
  v1
  ```ts
  collectionObserver(collection): { subscribe: (callback: (collection, splices)) => { dispose(): void } }
  ```

## Binding commands

- `.delegate` command has been removed, use `.trigger` instead. With shadow DOM, even though `.delegate` works, it doesn't feel as natural as `.trigger`, and the performance benefits `.delegate` command used to give when browsers were slow adding many event listeners is no longer as big.
- `.call` command has been removed, use lambda functions instead to create function that preserves the `this` context. Refer to [lambda expression](../../templates/lambda-expressions.md)

## Compose

- `<compose>` has been renamed to `<au-compose>`. The bindable properties of this component have also been changed:
  - viewModel -> component
  - view -> template
  - model remains the same

- Examples migration fix:
  ```html
  v1:
  <compose view.bind="...">
  <compose view-model.bind="...">

  v2:
  <au-compose template.bind="...">
  <au-compose component.bind="...">
  ```

- In Aurelia 2, all bindings are passed through to the underlying custom element
composition, so `component.ref` (`view-model.ref` in v1) no longer means getting a reference to the composer, but the composed view model instead.

Read more about dynamic composition in v2 in this [dynamic composition doc](../../getting-to-know-aurelia/dynamic-composition.md) and [dynamic ui composition doc](../../app-basics/dynamic-ui-composition.md).

## Replaceable & replaceable part

If you are using `replaceable`/`part`/`repaceable-part` combo in your v1 applications, you'll need to replace them with `<au-slot>` elements and `au-slot` attributes.
Refer to the [au slot doc](../../components/shadow-dom-and-slots.md#au-slot) for more information.

## General changes

* Custom attributes are no longer considered to have a binding to the primary bindable when their template usage is with an empty string, like the following examples:
    ```html
    <div my-attr>
    <div my-attr="">
    ```
    Both of the above usages will be considered as "plain" usage, to avoid overriding the defaul value in the custom attribute component instance.
* Templates no longer need to have `<template>` tags as the start and ending tags. Templates can be pure HTML with enhanced Aurelia markup but `<template>` doesn't need to be explicitly defined.
* `PLATFORM.moduleName` is gone. This was to address a limitation in Aurelia 1. Aurelia 2 now works well with all bundlers and does not require the addition of this code to use code splitting or tell the bundler where template code is.
* Better intellisense support for TypeScript applications. Using the new injection interfaces, you can now inject strongly typed Aurelia packages such as Fetch Client, Router or Internationalization. These packages are prefixed with an "I" such as `IHttpClient`, `IRouter` and so on.
* empty binding expressions are automatically inferred based on the target of the binding, like the following example:
    ```html
    <input value.bind>
    <input value.bind="">
    ```
    means:
    ```html
    <input value.bind="value">
    ```
    This is automatically applied to `.bind`/`.one-time`/`.to-view`/`.from-view`/`.two-way`/`.attr` binding commands.

## Plugins:

### Web-Components plugin

* Remove automatic `au-` prefix
* Remove auto-conversion of Aurelia element -> WC element. Applications need to explicitly define this. This should make mix-matching & controlling things easier.
