mithril-vndb/docs/request.md

request(options)

• API
• How it works
• Typical usage
• Loading icons and error messages
• Dynamic URLs
• Aborting requests
• File uploads
• Monitoring progress
• Casting response to a type
• Non-JSON responses
• Why JSON instead of HTML
• Why XMLHttpRequest instead of fetch

---

API

promise = m.request([url,] options)

Argument	Type	Required	Description
url	String	No	If present, it's equivalent to having the options {method: "GET", url: url}. Values passed to the options argument override options set via this shorthand.
options.method	String	Yes	The HTTP method to use. This value should be one of the following: GET, POST, PUT, PATCH, DELETE, HEAD or OPTIONS.
options.url	String	Yes	The URL to send the request to. The URL may be either absolute or relative, and it may contain interpolations.
options.data	any	No	The data to be interpolated into the URL and serialized into the querystring (for GET requests) or body (for other types of requests).
options.async	Boolean	No	Whether the request should be asynchronous. Defaults to true.
options.user	String	No	A username for HTTP authorization. Defaults to undefined.
options.password	String	No	A password for HTTP authorization. Defaults to undefined. This option is provided for XMLHttpRequest compatibility, but you should avoid using it because it sends the password in plain text over the network.
options.config	xhr = Function(xhr)	No	Exposes the underlying XMLHttpRequest object for low-level configuration. Defaults to the identity function.
options.type	any = Function(any)	No	A constructor to be applied to each object in the response. Defaults to the identity function.
options.serialize	string = Function(any)	No	A serialization method to be applied to data. Defaults to JSON.stringify, or if options.data is an instance of FormData, defaults to the identity function (i.e. function(value) {return value}).
options.deserialize	any = Function(string)	No	A deserialization method to be applied to the response. Defaults to a small wrapper around JSON.parse that returns null for empty responses.
options.extract	string = Function(xhr, options)	No	A hook to specify how the XMLHttpRequest response should be read. Useful for reading response headers and cookies. Defaults to a function that returns xhr.responseText. If defined, the xhr parameter is the XMLHttpRequest instance used for the request, and options is the object that was passed to the m.request call. If a custom extract callback is set, options.deserialize is ignored.
options.useBody	Boolean	No	Force the use of the HTTP body section for data in GET requests when set to true, or the use of querystring for other HTTP methods when set to false. Defaults to false for GET requests and true for other methods.
returns	Promise		A promise that resolves to the response data, after it has been piped through the extract, deserialize and type methods

How to read signatures

---

How it works

The m.request utility is a thin wrapper around XMLHttpRequest, and allows making HTTP requests to remote servers in order to save and/or retrieve data from a database.

m.request({
	method: "GET",
	url: "/api/v1/users",
})
.then(function(users) {
	console.log(users)
})

Calls to m.request return a promise.

By default, m.request assumes the response is in JSON format and parses it into a Javascript object (or array).

---

Typical usage

Here's an illustrative example of a component that uses m.request to retrieve some data from a server.

var Data = {
	todos: {
		list: [],
		fetch: function() {
			m.request({
				method: "GET",
				url: "/api/todos",
			})
			.then(function(items) {
				Data.todos.items = items
			})
		}
	}
}

var Todos = {
	oninit: Data.todos.fetch,
	view: function(vnode) {
		return Data.todos.list.map(function(item) {
			return m("div", item.title)
		})
	}
}

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

Let's assume making a request to the server URL /api/items returns an array of objects in JSON format.

When m.route is called at the bottom, the Todos component is initialized. oninit is called, which calls m.request. This retrieves an array of objects from the server asynchronously. "Asynchronously" means that Javascript continues running other code while it waits for the response from server. In this case, it means fetch returns, and the component is rendered using the original empty array as Data.todos.list. Once the request to the server completes, the array of objects items is assigned to Data.todos.items and the component is rendered again, yielding a list of <div>s containing the titles of each todo.

---

Loading icons and error messages

Here's an expanded version of the example above that implements a loading indicator and an error message:

var Data = {
	todos: {
		list: null,
		error: "",
		fetch: function() {
			m.request({
				method: "GET",
				url: "/api/todos",
			})
			.then(function(items) {
				Data.todos.items = items
			})
			.catch(function(e) {
				Data.todos.error = e.message
			})
		}
	}
}

var Todos = {
	oninit: Data.todos.fetch,
	view: function(vnode) {
		return Data.todos.error ? [
			m(".error", Data.todos.error)
		] : Data.todos.list ? [
			Data.todos.list.map(function(item) {
				return m("div", item.title)
			})
		] : m(".loading-icon")
	}
}

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

There are a few difference between this example and the one before. Here, Data.todos.list is null at the beginning. Also, there's an extra field error for holding an error message, and the view of the Todos component was modified to displays an error message if one exists, or display a loading icon if Data.todos.list is not an array.

---

Dynamic URLs

Request URLs may contain interpolations:

m.request({
	method: "GET",
	url: "/api/v1/users/:id",
	data: {id: 123},
}).then(function(user) {
	console.log(user.id) // logs 123
})

In the code above, :id is populated with the data from the {id: 123} object, and the request becomes GET /api/v1/users/123.

Interpolations are ignored if no matching data exists in the data property.

m.request({
	method: "GET",
	url: "/api/v1/users/foo:bar",
	data: {id: 123},
})

In the code above, the request becomes GET /api/v1/users/foo:bar

---

Aborting requests

Sometimes, it is desirable to abort a request. For example, in an autocompleter/typeahead widget, you want to ensure that only the last request completes, because typically autocompleters fire several requests as the user types and HTTP requests may complete out of order due to the unpredictable nature of networks. If another request finishes after the last fired request, the widget would display less relevant (or potentially wrong) data than if the last fired request finished last.

m.request() exposes its underlying XMLHttpRequest object via the options.config parameter, which allows you to save a reference to that object and call its abort method when required:

var searchXHR = null
function search() {
	abortPreviousSearch()
	
	m.request({
		method: "GET",
		url: "/api/v1/users",
		data: {search: query},
		config: function(xhr) {searchXHR = xhr}
	})
}
function abortPreviousSearch() {
	if (searchXHR !== null) searchXHR.abort()
	searchXHR = null
}

---

File uploads

To upload files, first you need to get a reference to a File object. The easiest way to do that is from a <input type="file">.

m.render(document.body, [
	m("input[type=file]", {onchange: upload})
])

function upload(e) {
	var file = e.target.files[0]
}

The snippet above renders a file input. If a user picks a file, the onchange event is triggered, which calls the upload function. e.target.files is a list of File objects.

Next, you need to create a FormData object to create a multipart request, which is a specially formatted HTTP request that is able to send file data in the request body.

function upload(e) {
	var file = e.target.files[0]
	
	var data = new FormData()
	data.append("myfile", file)
}

Next, you need to call m.request and set options.method to an HTTP method that uses body (e.g. POST, PUT, PATCH) and use the FormData object as options.data.

function upload(e) {
	var file = e.target.files[0]
	
	var data = new FormData()
	data.append("myfile", file)
	
	m.request({
		method: "POST",
		url: "/api/v1/upload",
		data: data,
	})
}

Assuming the server is configured to accept multipart requests, the file information will be associated with the myfile key.

Multiple file uploads

It's possible to upload multiple files in one request. Doing so will make the batch upload atomic, i.e. no files will be processed if there's an error during the upload, so it's not possible to have only part of the files saved. If you want to save as many files as possible in the event of a network failure, you should consider uploading each file in a separate request instead.

To upload multiple files, simply append them all to the FormData object. When using a file input, you can get a list of files by adding the multiple attribute to the input:

m.render(document.body, [
	m("input[type=file][multiple]", {onchange: upload})
])

function upload(e) {
	var files = e.target.files
	
	var data = new FormData()
	for (var i = 0; i < files.length; i++) {
		data.append("file" + i, file)
	}
	
	m.request({
		method: "POST",
		url: "/api/v1/upload",
		data: data,
	})
}

---

Monitoring progress

Sometimes, if a request is inherently slow (e.g. a large file upload), it's desirable to display a progress indicator to the user to signal that the application is still working.

m.request() exposes its underlying XMLHttpRequest object via the options.config parameter, which allows you to attach event listeners to the XMLHttpRequest object:

var progress = 0

m.mount(document.body, {
	view: function() {
		return [
			m("input[type=file]", {onchange: upload}),
			progress + "% completed"
		]
	}
})

function upload(e) {
	var file = e.target.files[0]
	
	var data = new FormData()
	data.append("myfile", file)
	
	m.request({
		method: "POST",
		url: "/api/v1/upload",
		data: data,
		config: function(xhr) {
			xhr.addEventListener("progress", function(e) {
				progress = e.loaded / e.total
				
				m.redraw() // tell Mithril that data changed and a re-render is needed
			})
		}
	})
}

In the example above, a file input is rendered. If the user picks a file, an upload is initiated, and in the config callback, a progress event handler is registered. This event handler is fired whenever there's a progress update in the XMLHttpRequest. Because the XMLHttpRequest's progress event is not directly handled by Mithril's virtual DOM engine, m.redraw() must be called to signal to Mithril that data has changed and a redraw is required.

---

Casting response to a type

Depending on the overall application architecture, it may be desirable to transform the response data of a request to a specific class or type (for example, to uniformly parse date fields or to have helper methods).

You can pass a constructor as the options.type parameter and Mithril will instantiate it for each object in the HTTP response.

function User(data) {
	this.name = data.firstName + " " + data.lastName
}

m.request({
	method: "GET",
	url: "/api/v1/users",
	type: User
})
.then(function(users) {
	console.log(users[0].name) // logs a name
})

In the example above, assuming /api/v1/users returns an array of objects, the User constructor will be instantiated (i.e. called as new User(data)) for each object in the array. If the response returned a single object, that object would be used as the data argument.

---

Non-JSON responses

Sometimes a server endpoint does not return a JSON response: for example, you may be requesting an HTML file, an SVG file, or a CSV file. By default Mithril attempts to parse a response as if it was JSON. To override that behavior, define a custom options.deserialize function:

m.request({
	method: "GET",
	url: "/files/icon.svg",
	deserialize: function(value) {return value}
})
.then(function(svg) {
	m.render(document.body, m.trust(svg))
})

In the example above, the request retrieves an SVG file, does nothing to parse it (because deserialize merely returns the value as-is), and then renders the SVG string as trusted HTML.

Of course, a deserialize function may be more elaborate:

m.request({
	method: "GET",
	url: "/files/data.csv",
	deserialize: parseCSV
})
.then(function(data) {
	console.log(data)
})

function parseCSV(data) {
	// naive implementation for the sake of keeping example simple
	return data.split("\n").map(function(row) {
		return row.split(",")
	})
}

Ignoring the fact that the parseCSV function above doesn't handle a lot of cases that a proper CSV parser would, the code above logs an array of arrays

---

Retrieving response details

By default Mithril attempts to parse a response as JSON and returns xhr.responseText. It may be useful to inspect a server response in more detail, this can be accomplished by passing a custom options.extract function:

m.request({
	method: "GET",
	url: "/api/v1/users",
	extract: function(xhr) {return {status: xhr.status, body: xhr.responseText}}
})
.then(function(response) {
	console.log(response.status, response.body)
})

The parameter to options.extract is the XMLHttpRequest object once its operation is completed, but before it has been passed to the returned promise chain, so the promise may still end up in an rejected state if processing throws an exception.

---

Why JSON instead of HTML

Many server-side frameworks provide a view engine that interpolates database data into a template before serving HTML (on page load or via AJAX) and then employ jQuery to handle user interactions.

By contrast, Mithril is framework designed for thick client applications, which typically download templates and data separately and combine them in the browser via Javascript. Doing the templating heavy-lifting in the browser can bring benefits like reducing operational costs by freeing server resources. Separating templates from data also allow template code to be cached more effectively and enables better code reusability across different types of clients (e.g. desktop, mobile). Another benefit is that Mithril enables a retained mode UI development paradigm, which greatly simplifies development and maintenance of complex user interactions.

By default, m.request expects response data to be in JSON format. In a typical Mithril application, that JSON data is then usually consumed by a view.

You should avoid trying to render server-generated dynamic HTML with Mithril. If you have an existing application that does use a server-side templating system, and you wish to re-architecture it, first decide whether the effort is feasible at all to begin with. Migrating from a thick server architecture to a thick client architecture is typically a somewhat large effort, and involves refactoring logic out of templates into logical data services (and the testing that goes with it).

Data services may be organized in many different ways depending on the nature of the application. RESTful architectures are popular with API providers, and service oriented architectures are often required where there are lots of highly transactional workflows.

---

Why XMLHttpRequest instead of fetch

fetch() is a newer Web API for fetching resources from servers, similar to XMLHttpRequest.

Mithril's m.request uses XMLHttpRequest instead of fetch() for a number of reasons:

• fetch is not fully standardized yet, and may be subject to specification changes.
• XMLHttpRequest calls can be aborted before they resolve (e.g. to avoid race conditions in for instant search UIs).
• XMLHttpRequest provides hooks for progress listeners for long running requests (e.g. file uploads).
• XMLHttpRequest is supported by all browsers, whereas fetch() is not supported by Internet Explorer and Safari.

Currently, due to lack of browser support, fetch() typically requires a polyfill, which is over 11kb uncompressed - nearly three times larger than Mithril's m.request.

Despite being much smaller, m.request supports many important and not-so-trivial-to-implement features like URL interpolation, querystring serialization and JSON-P requests. The fetch polyfill does not support any of those.

The fetch() API does have a few technical advantages over XMLHttpRequest in a few uncommon cases:

• it provides a streaming API (in the "video streaming" sense, not in the reactive programming sense), which enables better latency and memory consumption for very large responses (at the cost of code complexity).
• it integrates to the Service Worker API, which provides an extra layer of control over how and when network requests happen. This API also allows access to push notifications and background synchronization features.

In typical scenarios, streaming won't provide noticeable performance benefits because it's generally not advisable to download megabytes of data to begin with. Also, the memory gains from repeatedly reusing small buffers may be offset or nullified if they result in excessive browser repaints. For those reasons, choosing fetch() streaming instead of m.request is only recommended for extremely resource intensive applications.