# 🦉 Props 🦉

## Content

- [Overview](#overview)
- [Definition](#definition)
- [Binding function props](#binding-function-props)
- [Dynamic Props](#dynamic-props)
- [Default Props](#default-props)
- [Props validation](#props-validation)
- [Good Practices](#good-practices)

## Overview

In Owl, `props` (short for _properties_) is an object which contains every piece
of data given to a component by its parent.

```js
class Child extends Component {
  static template = xml`<div><t t-esc="props.a"/><t t-esc="props.b"/></div>`;
}

class Parent extends Component {
  static template = xml`<div><Child a="state.a" b="'string'"/></div>`;
  static components = { Child };
  state = useState({ a: "fromparent" });
}
```

In this example, the `Child` component receives two props from its parent: `a`
and `b`. They are collected into a `props` object by Owl, with each value being
evaluated in the context of the parent. So, `props.a` is equal to `'fromparent'` and
`props.b` is equal to `'string'`.

Note that `props` is an object that only makes sense from the perspective of the
child component.

## Definition

The `props` object is made of every attributes defined on the template, with the
following exceptions:

- every attribute starting with `t-` are not props (they are QWeb directives),

In the following example:

```xml
<div>
    <ComponentA a="state.a" b="'string'"/>
    <ComponentB t-if="state.flag" model="model"/>
</div>
```

the `props` object contains the following keys:

- for `ComponentA`: `a` and `b`,
- for `ComponentB`: `model`,

## Binding function props

It is common to have the need to pass a callback as a prop. Since Owl components
are class based, the callback frequently needs to be bound to its owner component.
So, one can do this:

```js
class SomeComponent extends Component {
  static template = xml`
    <div>
      <Child callback="doSomething"/>
    </div>`;

  setup() {
    this.doSomething = this.doSomething.bind(this);
  }

  doSomething() {
    // ...
  }
}
```

However, this is such a common use case that Owl provides a special suffix to do
just that: `.bind`. This looks like this:

```js
class SomeComponent extends Component {
  static template = xml`
    <div>
      <Child callback.bind="doSomething"/>
    </div>`;

  doSomething() {
    // ...
  }
}
```

## Dynamic Props

The `t-props` directive can be used to specify totally dynamic props:

```xml
<div t-name="ParentComponent">
    <Child t-props="some.obj"/>
</div>
```

```js
class ParentComponent {
  static components = { Child };
  some = { obj: { a: 1, b: 2 } };
}
```

## Default Props

If the static `defaultProps` property is defined, it will be used to complete
props received by the parent, if missing.

```js
class Counter extends owl.Component {
  static defaultProps = {
    initialValue: 0,
  };
  ...
}
```

In the example above, the `initialValue` props is now by default set to 0.

## Props Validation

As an application becomes complex, it may be quite unsafe to define props in an informal way. This leads to two issues:

- hard to tell how a component should be used, by looking at its code.
- unsafe, it is easy to send wrong props into a component, either by refactoring a component, or one of its parents.

A props type system solves both issues, by describing the types and shapes
of the props. Here is how it works in Owl:

- `props` key is a static key (so, different from `this.props` in a component instance)
- it is optional: it is ok for a component to not define a `props` key.
- props are validated whenever a component is created/updated
- props are only validated in `dev` mode (see [how to configure an app](app.md#configuration))
- if a key does not match the description, an error is thrown
- it validates keys defined in (static) `props`. Additional keys given by the
  parent will cause an error.

For example:

```js
class ComponentA extends owl.Component {
    static props = ['id', 'url'];

    ...
}

class ComponentB extends owl.Component {
  static props = {
    count: {type: Number},
    messages: {
      type: Array,
      element: {type: Object, shape: {id: Boolean, text: String }
    },
   date: Date,
   combinedVal: [Number, Boolean]
  };

  ...
}
```

- it is an object or a list of strings
- a list of strings is a simplified props definition, which only lists the name
  of the props. Also, if the name ends with `?`, it is considered optional.
- all props are by default required, unless they are defined with `optional: true`
  (in that case, validation is only done if there is a value)
- valid types are: `Number, String, Boolean, Object, Array, Date, Function`, and all
  constructor functions (so, if you have a `Person` class, it can be used as a type)
- arrays are homogeneous (all elements have the same type/shape)

For each key, a `prop` definition is either a boolean, a constructor, a list of constructors, or an object:

- a boolean: indicate that the props exists, and is mandatory.
- a constructor: this should describe the type, for example: `id: Number` describe
  the props `id` as a number
- a list of constructors. In that case, this means that we allow more than one
  type. For example, `id: [Number, String]` means that `id` can be either a string
  or a number.
- an object. This makes it possible to have more expressive definition. The following sub keys are then allowed (but not mandatory):
  - `type`: the main type of the prop being validated
  - `element`: if the type was `Array`, then the `element` key describes the type of each element in the array. If it is not set, then we only validate the array, not its elements,
  - `shape`: if the type was `Object`, then the `shape` key describes the interface of the object. If it is not set, then we only validate the object, not its elements,
  - `validate`: this is a function which should return a boolean to determine if
    the value is valid or not. Useful for custom validation logic.

Examples:

```js
  // only the existence of those 3 keys is documented
  static props = ['message', 'id', 'date'];
```

```js
  // size is optional
  static props = ['message', 'size?'];
```

```js
  static props = {
    messageIds: {type: Array, element: Number},  // list of number
    otherArr: {type: Array},   // just array. no validation is made on sub elements
    otherArr2: Array,   // same as otherArr
    someObj: {type: Object},  // just an object, no internal validation
    someObj2: {
      type: Object,
      shape: {
        id: Number,
        name: {type: String, optional: true},
        url: String
      ]},    // object, with keys id (number), name (string, optional) and url (string)
    someFlag: Boolean,     // a boolean, mandatory (even if `false`)
    someVal: [Boolean, Date],   // either a boolean or a date
    otherValue: true,     // indicates that it is a prop
    kindofsmallnumber: {
      type: Number,
      validate: n => (0 <= n && n <= 10)
    },
    size: {
      validate:  e => ["small", "medium", "large"].includes(e)
    },
  };
```

## Good Practices

A `props` object is a collection of values that come from the parent. As such,
they are owned by the parent, and should never be modified by the child:

```js
class MyComponent extends Component {
  constructor(parent, props) {
    super(parent, props);
    props.a.b = 43; // Never do that!!!
  }
}
```

Props should be considered readonly, from the perspective of the child component.
If there is a need to modify them, then the request to update them should be
sent to the parent (for example, with an event).

Any value can go in a props. Strings, objects, classes, or even callbacks could
be given to a child component (but then, in the case of callbacks, communicating
with events seems more appropriate).