request(options)
- Core
- Optional
- Tooling
Description
Makes XHR (aka AJAX) requests, and returns a promise
m.request({
method: "PUT",
url: "/api/v1/users/:id",
data: {id: 1, name: "test"}
})
.then(function(result) {
console.log(result)
})
Signature
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 |
No | The HTTP method to use. This value should be one of the following: GET , POST , PUT , PATCH , DELETE , HEAD or OPTIONS . Defaults to GET . |
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.withCredentials |
Boolean |
No | Whether to send cookies to 3rd party domains. Defaults to false |
options.timeout |
Number |
No | The amount of milliseconds a request can take before automatically being terminated. Defaults to undefined . |
options.config |
xhr = Function(xhr) |
No | Exposes the underlying XMLHttpRequest object for low-level configuration. Defaults to the identity function. |
options.headers |
Object |
No | Headers to append to the request before sending it (applied right before options.config ). |
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 xhr.responseText . Defaults to a small wrapper around JSON.parse that returns null for empty responses. If extract is defined, deserialize will be skipped. |
options.extract |
any = Function(xhr, options) |
No | A hook to specify how the XMLHttpRequest response should be read. Useful for processing response data, reading headers and cookies. By default this is a function that returns xhr.responseText , which is in turn passed to deserialize . If a custom extract callback is provided, the xhr parameter is the XMLHttpRequest instance used for the request, and options is the object that was passed to the m.request call. Additionally, deserialize will be skipped and the value returned from the extract callback will be left as-is when the promise resolves. Furthermore, when an extract callback is provided, exceptions are not thrown when the server response status code indicates an error. |
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. |
options.background |
Boolean |
No | If false , redraws mounted components upon completion of the request. If true , it does not. Defaults to false . |
returns | Promise |
A promise that resolves to the response data, after it has been piped through the extract , deserialize and type methods |
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)
})
A call to m.request
returns a promise and triggers a redraw upon completion of its promise chain.
By default, m.request
assumes the response is in JSON format and parses it into a Javascript object (or array).
If the HTTP response status code indicates an error, the returned Promise will be rejected. Supplying an extract callback will prevent the promise rejection.
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/v1/todos",
})
.then(function(items) {
Data.todos.list = 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.list
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/v1/todos",
})
.then(function(items) {
Data.todos.list = 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 differences 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, files[i])
}
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.upload.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.
Custom headers may also be helpful in this regard. For example, if you're requesting an SVG, you probably want to set the content type accordingly. To override the default JSON request type, set options.headers
to an object of key-value pairs corresponding to request header names and values.
m.request({
method: "GET",
url: "/files/image.svg",
headers: {
"Content-Type": "image/svg+xml; charset=utf-8",
"Accept": "image/svg, text/*"
},
deserialize: function(value) {return value}
})
Retrieving response details
By default Mithril attempts to parse xhr.responseText
as JSON and returns the parsed object. It may be useful to inspect a server response in more detail and process it manually. 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 XHR 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, whereasfetch()
is not supported by Internet Explorer, Safari and Android (non-Chromium).
Currently, due to lack of browser support, fetch()
typically requires a polyfill, which is over 11kb uncompressed - nearly three times larger than Mithril's XHR module.
Despite being much smaller, Mithril's XHR module supports many important and not-so-trivial-to-implement features like URL interpolation, querystring serialization and JSON-P requests, in addition to its ability to integrate seamlessly to Mithril's autoredrawing subsystem. The fetch
polyfill does not support any of those, and requires extra libraries and boilerplates to achieve the same level of functionality.
In addition, Mithril's XHR module is optimized for JSON-based endpoints and makes that most common case appropriately terse - i.e. m.request(url)
- whereas fetch
requires an additional explicit step to parse the response data as JSON: fetch(url).then(function(response) {return response.json()})
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.
Avoid anti-patterns
Promises are not the response data
The m.request
method returns a Promise, not the response data itself. It cannot return that data directly because an HTTP request may take a long time to complete (due to network latency), and if Javascript waited for it, it would freeze the application until the data was available.
// AVOID
var users = m.request("/api/v1/users")
console.log("list of users:", users)
// `users` is NOT a list of users, it's a promise
// PREFER
m.request("/api/v1/users").then(function(users) {
console.log("list of users:", users)
})
License: MIT. © Leo Horie.