# route(root, defaultRoute, routes)

- [Description](#description)
- [Signature](#signature)
	- [Static members](#static-members)
		- [m.route.set](#mrouteset)
		- [m.route.get](#mrouteget)
		- [m.route.prefix](#mrouteprefix)
		- [m.route.link](#mroutelink)
	- [RouteResolver](#routeresolver)
		- [routeResolver.onmatch](#routeresolveronmatch)
		- [routeResolver.render](#routeresolverrender)
- [How it works](#how-it-works)
- [Typical usage](#typical-usage)
- [Navigating to different routes](#navigating-to-different-routes)
- [Routing parameters](#routing-parameters)
	- [Key parameter](#key-parameter)
	- [Variadic routes](#variadic-routes)
	- [History state](#history-state)
- [Changing router prefix](#changing-router-prefix)
- [Advanced component resolution](#advanced-component-resolution)
	- [Wrapping a layout component](#wrapping-a-layout-component)
	- [Authentication](#authentication)
	- [Preloading data](#preloading-data)
	- [Code splitting](#code-splitting)

---

### Description

Navigate between "pages" within an application

```javascript
var Home = {
	view: function() {
		return "Welcome"
	}
}

m.route(document.body, "/home", {
	"/home": Home, // defines `http://localhost/#!/home`
})
```

You can only have one `m.route` call per application.

---

### Signature

`m.route(root, defaultRoute, routes)`

Argument               | Type                                     | Required | Description
---------------------- | ---------------------------------------- | -------- | ---
`root`                 | `Element`                                | Yes      | A DOM element that will be the parent node to the subtree
`defaultRoute`         | `String`                                 | Yes      | The route to redirect to if the current URL does not match a route
`routes`               | `Object<String,Component|RouteResolver>` | Yes      | An object whose keys are route strings and values are either components or a [RouteResolver](#routeresolver)
**returns**            |                                          |          | Returns `undefined`

[How to read signatures](signatures.md)


#### Static members

##### m.route.set

Redirects to a matching route, or to the default route if no matching routes can be found.

`m.route.set(path, data, options)`

Argument          | Type      | Required | Description
----------------- | --------- | -------- | ---
`path`            | `String`  | Yes      | The path to route to, without a prefix. The path may include slots for routing parameters
`data`            | `Object`  | No       | Routing parameters. If `path` has routing parameter slots, the properties of this object are interpolated into the path string
`options.replace` | `Boolean` | No       | Whether to create a new history entry or to replace the current one. Defaults to false
`options.state`   | `Object`  | No       | The `state` object to pass to the underlying `history.pushState` / `history.replaceState` call. This state object becomes available in the `history.state` property, and is merged into the [routing parameters](#routing-parameters) object. Note that this option only works when using the pushState API, but is ignored if the router falls back to hashchange mode (i.e. if the pushState API is not available)
`options.title`   | `String`  | No       | The `title` string to pass to the underlying `history.pushState` / `history.replaceState` call.
**returns**       |           |          | Returns `undefined`

##### m.route.get

Returns the last fully resolved routing path, without the prefix. It may differ from the path displayed in the location bar while an asynchronous route is [pending resolution](#code-splitting).

`path = m.route.get()`

Argument          | Type      | Required | Description
----------------- | --------- | -------- | ---
**returns**       | String    |          | Returns the last fully resolved path

##### m.route.prefix

Defines a router prefix. The router prefix is a fragment of the URL that dictates the underlying [strategy](#routing-strategies) used by the router.

`m.route.prefix(prefix)`

Argument          | Type      | Required | Description
----------------- | --------- | -------- | ---
`prefix`          | `String`  | Yes      | The prefix that controls the underlying [routing strategy](#routing-strategies) used by Mithril.
**returns**       |           |          | Returns `undefined`

##### m.route.link

`eventHandler = m.route.link(vnode)`

Argument          | Type        | Required | Description
----------------- | ----------- | -------- | ---
`vnode`           | `Vnode`     | Yes      | This method is meant to be used in conjunction with an `<a>` [vnode](vnodes.md)'s [`oncreate` hook](lifecycle-methods.md)
**returns**       | Function(e) |          | Returns an event handler that calls `m.route.set` with the link's `href` as the `path`

#### RouteResolver

A RouterResolver is an object that contains an `onmatch` method and/or a `render` method. Both methods are optional, but at least one must be present. A RouteResolver is not a component, and therefore it does NOT have lifecycle methods. As a rule of thumb, RouteResolvers should be in the same file as the `m.route` call, whereas component definitions should be in their own modules.

`routeResolver = {onmatch, render}`

##### routeResolver.onmatch

The `onmatch` hook is called when the router needs to find a component to render. It is called once per router path changes, but not on subsequent redraws while on the same path. It can be used to run logic before a component initializes (for example authentication logic, data preloading, redirection analytics tracking, etc)

This method also allows you to asynchronously define what component will be rendered, making it suitable for code splitting and asynchronous module loading. To render a component asynchronously return a promise that resolves to a component.

For more information on `onmatch`, see the [advanced component resolution](#advanced-component-resolution) section

`routeResolver.onmatch(args, requestedPath)`

Argument        | Type                           | Description
--------------- | ------------------------------ | ---
`args`          | `Object`                       | The [routing parameters](#routing-parameters)
`requestedPath` | `String`                       | The router path requested by the last routing action, including interpolated routing parameter values, but without the prefix. When `onmatch` is called, the resolution for this path is not complete and `m.route.get()` still returns the previous path.
**returns**     | `Component|Promise<Component>` | Returns a component or a promise that resolves to a component

If `onmatch` returns a component or a promise that resolves to a component, this component is used as the `vnode.tag` for the first argument in the RouteResolver's `render` method. Otherwise, `vnode.tag` is set to `"div"`. Similarly, if the `onmatch` method is omitted, `vnode.tag` is also `"div"`.

If `onmatch` returns a promise that gets rejected, the router redirects back to `defaultRoute`. You may override this behavior by calling `.catch` on the promise chain before returning it.

##### routeResolver.render

The `render` method is called on every redraw for a matching route. It is similar to the `view` method in components and it exists to simplify [component composition](#wrapping-a-layout-component).

`vnode = routeResolve.render(vnode)`

Argument            | Type            | Description
------------------- | --------------- | -----------
`vnode`             | `Object`        | A [vnode](vnodes.md) whose attributes object contains routing parameters. If onmatch does not return a component or a promise that resolves to a component, the vnode's `tag` field defaults to `"div"`
`vnode.attrs`       | `Object`        | A map of URL parameter values
**returns**         | `Vnode`         | Returns a vnode

---

#### How it works

Routing is a system that allows creating Single-Page-Applications (SPA), i.e. applications that can go from a "page" to another without causing a full browser refresh.

It enables seamless navigability while preserving the ability to bookmark each page individually, and the ability to navigate the application via the browser's history mechanism.

Routing without page refreshes is made partially possible by the [`history.pushState`](https://developer.mozilla.org/en-US/docs/Web/API/History_API#The_pushState()_method) API. Using this API, it's possible to programmatically change the URL displayed by the browser after a page has loaded, but it's the application developer's responsibility to ensure that navigating to any given URL from a cold state (e.g. a new tab) will render the appropriate markup.

#### Routing strategies

The routing strategy dictates how a library might actually implement routing. There are three general strategies that can be used to implement a SPA routing system, and each has different caveats:

- Using the [fragment identifier](https://en.wikipedia.org/wiki/Fragment_identifier) (aka the hash) portion of the URL. A URL using this strategy typically looks like `http://localhost/#!/page1`
- Using the querystring. A URL using this strategy typically looks like `http://localhost/?/page1`
- Using the pathname. A URL using this strategy typically looks like `http://localhost/page1`

Using the hash strategy is guaranteed to work in browsers that don't support `history.pushState` (namely, Internet Explorer 9), because it can fall back to using `onhashchange`. Use this strategy if you want to support IE9.

The querystring strategy also technically works in IE9, but it falls back to reloading the page. Use this strategy if you want to support anchored links and you are not able to make the server-side necessary to support the pathname strategy.

The pathname strategy produces the cleanest looking URLs, but does not work in IE9 *and* requires setting up the server to serve the single page application code from every URL that the application can route to. Use this strategy if you want cleaner-looking URLs and do not need to support IE9.

Single page applications that use the hash strategy often use the convention of having an exclamation mark after the hash to indicate that they're using the hash as a routing mechanism and not for the purposes of linking to anchors. The `#!` string is known as a *hashbang*.

The default strategy uses the hashbang.

---

### Typical usage

Normally, you need to create a few [components](components.md) to map routes to:

```javascript
var Home = {
	view: function() {
		return [
			m(Menu),
			m("h1", "Home")
		]
	}
}

var Page1 = {
	view: function() {
		return [
			m(Menu),
			m("h1", "Page 1")
		]
	}
}
```

In the example above, there are two components: `Home` and `Page1`. Each contains a menu and some text. The menu is itself being defined as a component to avoid repetition:

```javascript
var Menu = {
	view: function() {
		return m("nav", [
			m("a[href=/]", {oncreate: m.route.link}, "Home"),
			m("a[href=/page1]", {oncreate: m.route.link}, "Page 1"),
		])
	}
}
```

Now we can define routes and map our components to them:

```javascript
m.route(document.body, "/", {
	"/": Home,
	"/page1": Page1,
})
```

Here we specify two routes: `/` and `/page1`, which render their respective components when the user navigates to each URL. By default, the SPA router prefix is `#!`

---

### Navigating to different routes

In the example above, the `Menu` component has two links. You can specify that their `href` attribute is a route URL (rather than being a regular link that navigates away from the current page), by adding the hook `{oncreate: m.route.link}`

You can also navigate programmatically, via `m.route.set(route)`. For example, `m.route.set("/page1")`.

When navigating to routes, there's no need to explicitly specify the router prefix. In other words, don't add the hashbang `#!` in front of the route path when linking via `m.route.link` or redirecting.

---

### Routing parameters

Sometimes we want to have a variable id or similar data appear in a route, but we don't want to explicitly specify a separate route for every possible id. In order to achieve that, Mithril supports parameterized routes:

```javascript
var Edit = {
	view: function(vnode) {
		return [
			m(Menu),
			m("h1", "Editing " + vnode.attrs.id)
		]
	}
}
m.route(document.body, "/edit/1", {
	"/edit/:id": Edit,
})
```

In the example above, we defined a route `/edit/:id`. This creates a dynamic route that matches any URL that starts with `/edit/` and is followed by some data (e.g. `/edit/1`, `edit/234`, etc). The `id` value is then mapped as an attribute of the component's [vnode](vnodes.md) (`vnode.attrs.id`)

It's possible to have multiple arguments in a route, for example `/edit/:projectID/:userID` would yield the properties `projectID` and `userID` on the component's vnode attributes object.

In addition to routing parameters, the `attrs` object also includes a `path` property that contains the current route path, and a `route` property that contains the matched routed.

#### Key parameter

When a user navigates from a parameterized route to the same route with a different parameter (e.g. going from `/page/1` to `/page/2` given a route `/page/:id`, the component would not be recreated from scratch since both routes resolve to the same component, and thus result in a virtual dom in-place diff. This has the side-effect of triggering the `onupdate` hook, rather than `oninit`/`oncreate`. However, it's relatively common for a developer to want to synchronize the recreation of the component to the route change event.

To achieve that, it's possible to combine route parameterization with the virtual dom [key reconciliation](keys.md) feature:

```javascript
m.route(document.body, "/edit/1", {
	"/edit/:key": Edit,
})
```

This means that the [vnode](vnodes.md) that is created for the root component of the route has a route parameter object `key`. Route parameters become `attrs` in the vnode. Thus, when jumping from one page to another, the `key` changes and causes the component to be recreated from scratch (since the key tells the virtual dom engine that old and new components are different entities).

You can take that idea further to create components that recreate themselves when reloaded:

`m.route.set(m.route.get(), {key: Date.now()})`

Or even use the [`history state`](#history-state) feature to achieve reloadable components without polluting the URL:

`m.route.set(m.route.get(), null, {state: {key: Date.now()}})`

#### Variadic routes

It's also possible to have variadic routes, i.e. a route with an argument that contains URL pathnames that contain slashes:

```javascript
m.route(document.body, "/edit/pictures/image.jpg", {
	"/files/:file...": Edit,
})
```

#### History state

It's possible to take full advantage of the underlying `history.pushState` API to improve user's navigation experience. For example, an application could "remember" the state of a large form when the user leaves a page by navigating away, such that if the user pressed the back button in the browser, they'd have the form filled rather than a blank form.

For example, you could create a form like this:

```javascript
var state = {
	term: "",
	search: function() {
		// save the state for this route
		// this is equivalent to `history.replaceState({term: state.term}, null, location.href)`
		m.route.set(m.route.get(), null, {replace: true, state: {term: state.term}})

		// navigate away
		location.href = "https://google.com/?q=" + state.term
	}
}

var Form = {
	oninit: function(vnode) {
		state.term = vnode.attrs.term || "" // populated from the `history.state` property if the user presses the back button
	},
	view: function() {
		return m("form", [
			m("input[placeholder='Search']", {oninput: m.withAttr("value", function(v) {state.term = v}), value: state.term}),
			m("button", {onclick: state.search}, "Search")
		])
	}
}

m.route(document.body, "/", {
	"/": Form,
})
```

This way, if the user searches and presses the back button to return to the application, the input will still be populated with the search term. This technique can improve the user experience of large forms and other apps where non-persisted state is laborious for a user to produce.

---

### Changing router prefix

The router prefix is a fragment of the URL that dictates the underlying [strategy](#routing-strategies) used by the router.

```javascript
// set to pathname strategy
m.route.prefix("")

// set to querystring strategy
m.route.prefix("?")

// set to hash without bang
m.route.prefix("#")

// set to pathname strategy on a non-root URL
// e.g. if the app lives under `http://localhost/my-app` and something else lives under `http://localhost`
m.route.prefix("/my-app")
```

---

### Advanced component resolution

Instead of mapping a component to a route, you can specify a RouteResolver object. A RouteResolver object contains a `onmatch()` and/or a `render()` method. Both methods are optional but at least one of them must be present.

```javascript
m.route(document.body, "/", {
	"/": {
		onmatch: function(args, requestedPath) {
			return Home
		},
		render: function(vnode) {
			return vnode // equivalent to m(Home)
		},
	}
})
```

RouteResolvers are useful for implementing a variety of advanced routing use cases.

---

#### Wrapping a layout component

It's often desirable to wrap all or most of the routed components in a reusable shell (often called a "layout"). In order to do that, you first need to create a component that contains the common markup that will wrap around the various different components:

```javascript
var Layout = {
	view: function(vnode) {
		return m(".layout", vnode.children)
	}
}
```

In the example above, the layout merely consists of a `<div class="layout">` that contains the children passed to the component, but in a real life scenario it could be as complex as neeeded.

One way to wrap the layout is to define an anonymous component in the routes map:

```javascript
// example 1
m.route(document.body, "/", {
	"/": {
		view: function() {
			return m(Layout, m(Home))
		},
	},
	"/form": {
		view: function() {
			return m(Layout, m(Form))
		},
	}
})
```

However, note that because the top level component is an anonymous component, jumping from the `/` route to the `/form` route (or vice-versa) will tear down the anonymous component and recreate the DOM from scratch. If the Layout component had [lifecycle methods](lifecycle-methods.md) defined, the `oninit` and `oncreate` hooks would fire on every route change. Depending on the application, this may or may not be desirable.

If you would prefer to have the Layout component be diffed and maintained intact rather than recreated from scratch, you should instead use a RouteResolver as the root object:

```javascript
// example 2
m.route(document.body, "/", {
	"/": {
		render: function() {
			return m(Layout, m(Home))
		},
	},
	"/form": {
		render: function() {
			return m(Layout, m(Form))
		},
	}
})
```

Note that in this case, if the Layout component the `oninit` and `oncreate` lifecycle methods would only fire on the Layout component on the first route change (assuming all routes use the same layout).

To clarify the difference between the two examples, example 1 is equivalent to this code:

```javascript
// functionally equivalent to example 1
var Anon1 = {
	view: function() {
		return m(Layout, m(Home))
	},
}
var Anon2 = {
	view: function() {
		return m(Layout, m(Form))
	},
}

m.route(document.body, "/", {
	"/": {
		render: function() {
			return m(Anon1)
		}
	},
	"/form": {
		render: function() {
			return m(Anon2)
		}
	},
})
```

Since `Anon1` and `Anon2` are different components, their subtrees (including `Layout`) are recreated from scratch. This is also what happens when components are used directly without a RouteResolver.

In example 2, since `Layout` is the top-level component in both routes, the DOM for the `Layout` component is diffed (i.e. left intact if it has no changes), and only the change from `Home` to `Form` triggers a recreation of that subsection of the DOM.

---

#### Authentication

The RouterResolver's `onmatch` hook can be used to run logic before the top level component in a route is initializated. The example below shows how to implement a login wall that prevents users from seeing the `/secret` page unless they login.

```javascript
var isLoggedIn = false

var Login = {
	view: function() {
		return m("form", [
			m("button[type=button]", {
				onclick: function() {
					isLoggedIn = true
					m.route.set("/secret")
				}
			}, "Login")
		])
	}
}

m.route(document.body, "/secret", {
	"/secret": {
		onmatch: function() {
			if (!isLoggedIn) m.route.set("/login")
			else return Home
		}
	},
	"/login": Login
})
```

When the application loads, `onmatch` is called and since `isLoggedIn` is false, the application redirects to `/login`. Once the user pressed the login button, `isLoggedIn` would be set to true, and the application would redirect to `/secret`. The `onmatch` hook would run once again, and since `isLoggedIn` is true this time, the application would render the `Home` component.

For the sake of simplicity, in the example above, the user's logged in status is kept in a global variable, and that flag is merely toggled when the user clicks the login button. In a real life application, a user would obviously have to supply proper login credentials, and clicking the login button would trigger a request to a server to authenticate the user:

```javascript
var Auth = {
	username: "",
	password: "",
	
	setUsername: function(value) {
		Auth.username = value
	},
	setPassword: function(value) {
		Auth.password = value
	},
	login: function() {
		m.request({
			url: "/api/v1/auth",
			data: {username: Auth.username, password: Auth.password}
		}).then(function(data) {
			localStorage.setItem("auth-token": data.token)
			m.route.set("/secret")
		})
	}
}

var Login = {
	view: function() {
		return m("form", [
			m("input[type=text]", {oninput: m.withAttr("value", Auth.setUsername), value: Auth.username}),
			m("input[type=password]", {oninput: m.withAttr("value", Auth.setPassword), value: Auth.password}),
			m("button[type=button]", {onclick: Auth.login, "Login")
		])
	}
}

m.route(document.body, "/secret", {
	"/secret": {
		onmatch: function() {
			if (!localStorage.getItem("auth-token")) m.route.set("/login")
			else return Home
		}
	},
	"/login": Login
})
```

---

#### Preloading data

Typically, a component can load data upon initialization. Loading data this way renders the component twice (once upon routing, and once after the request completes).

```javascript
var state = {
	users: [],
	loadUsers: function() {
		return m.request("/api/v1/users").then(function(users) {
			state.users = users
		})
	}
}

m.route(document.body, "/user/list", {
	"/user/list": {
		oninit: state.loadUsers,
		view: function() {
			return state.users.length > 0 ? state.users.map(function(user) {
				return m("div", user.id)
			}) : "loading"
		}
	},
})
```

In the example above, on the first render, the UI displays `"loading"` since `state.users` is an empty array before the request completes. Then, once data is available, the UI redraws and a list of user ids is shown.

RouteResolvers can be used as a mechanism to preload data before rendering a component in order to avoid UI flickering and thus bypassing the need for a loading indicator:

```javascript
var state = {
	users: [],
	loadUsers: function() {
		return m.request("/api/v1/users").then(function(users) {
			state.users = users
		})
	}
}

m.route(document.body, "/user/list", {
	"/user/list": {
		onmatch: state.loadUsers,
		render: function() {
			return state.users.map(function(user) {
				return m("div", user.id)
			})
		}
	},
})
```

Above, `render` only runs after the request completes, making the ternary operator redundant.

---

#### Code splitting

In a large application, it may be desirable to download the code for each route on demand, rather than upfront. Dividing the codebase this way is known as code splitting or lazy loading. In Mithril, this can be accomplished by returning a promise from the `onmatch` hook:

At its most basic form, one could do the following:

```javascript
// Home.js
module.export = {
	view: function() {
		return [
			m(Menu),
			m("h1", "Home")
		]
	}
}
```

```javascript
// index.js
function load(file) {
	return m.request({
		method: "GET",
		url: file,
		extract: function(xhr) {
			return new Function("var module = {};" + xhr.responseText + ";return module.exports;")
		}
	})
}

m.route(document.body, "/", {
	"/": {
		onmatch: function() {
			return load("Home.js")
		},
	},
})
```

However, realistically, in order for that to work on a production scale, it would be necessary to bundle all of the dependencies for the `Home.js` module into the file that is ultimately served by the server.

Fortunately, there are a number of tools that facilitate the task of bundling modules for lazy loading. Here's an example using [webpack's code splitting system](https://webpack.github.io/docs/code-splitting.html):

```javascript
m.route(document.body, "/", {
	"/": {
		onmatch: function() {
			// using Webpack async code splitting
			return new Promise(function(resolve) {
				require(['./Home.js'], resolve)
			})
		},
	},
})
```