WARNING: This documentation is for an old version of mithril! Please see the current docs for more accurate info.

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m.request

This is a high-level utility for working with web services, which allows writing asynchronous code relatively procedurally.

By default, it assumes server responses are in JSON format and optionally instantiates a class with the response data.

It provides a number of useful features out of the box:

  • The ability to get an early reference to a container that will hold the asynchronous response
  • The ability to queue operations to be performed after the asynchronous request completes
  • The ability to "cast" the response to a class of your choice
  • The ability to unwrap data in a response that includes metadata properties

Basic usage

The basic usage pattern for m.request returns an m.prop getter-setter, which is populated when the AJAX request completes.

The returned getter-setter can be thought of as a box: you can pass this reference around cheaply, and you can "unwrap" its value when needed.

var users = m.request({method: "GET", url: "/user"});

//assuming the response contains the following data: `[{name: "John"}, {name: "Mary"}]`
//then when resolved (e.g. in a view), the `users` getter-setter will contain a list of users
//i.e. users() //[{name: "John"}, {name: "Mary"}]

Note that this getter-setter holds an undefined value until the AJAX request completes. Attempting to unwrap its value early will likely result in errors.

The returned getter-setter also implements the promise interface (also known as a thennable): this is the mechanism you should always use to queue operations to be performed on the data from the web service.

The simplest use case of this feature is to implement functional value assignment via m.prop (i.e. the same thing as above). You can bind a pre-existing getter-setter by passing it in as a parameter to a .then method:

var users = m.prop([]); //default value

m.request({method: "GET", url: "/user"}).then(users)
//assuming the response contains the following data: `[{name: "John"}, {name: "Mary"}]`
//then when resolved (e.g. in a view), the `users` getter-setter will contain a list of users
//i.e. users() //[{name: "John"}, {name: "Mary"}]

This syntax allows you to bind intermediate results before piping them down for further processing, for example:

var users = m.prop([]); //default value
var doSomething = function() { /*...*/ }

m.request({method: "GET", url: "/user"}).then(users).then(doSomething)

While both basic assignment syntax and thennable syntax can be used to the same effect, typically it's recommended that you use the assignment syntax whenever possible, as it's easier to read.

The thennable mechanism is intended to be used in three ways:

  • in the model layer: to process web service data in transformative ways (e.g. filtering a list based on a parameter that the web service doesn't support)
  • in the controller layer: to bind redirection code upon a condition
  • in the controller layer: to bind error messages

Processing web service data

This step is meant to be done in the model layer. Doing it in the controller level is also possible, but philosophically not recommended, because by tying logic to a controller, the code becomes harder to reuse due to unrelated controller dependencies.

In the example below, the listEven method returns a getter-setter that resolves to a list of users containing only users whose id is even.

//model
var User = {}

User.listEven = function() {
    return m.request({method: "GET", url: "/user"}).then(function(list) {
        return list.filter(function(user) {return user.id % 2 == 0});
    });
}

//controller
var controller = function() {
    this.users = User.listEven()
}

Bind redirection code

This step is meant to be done in the controller layer. Doing it in the model level is also possible, but philosophically not recommended, because by tying redirection to the model, the code becomes harder to reuse due to overly tight coupling.

In the example below, we use the previously defined listEven model method and queue a controller-level function that redirects to another page if the user list is empty.

//controller
var controller = function() {
    this.users = User.listEven().then(function(users) {
        if (users.length == 0) m.route("/add");
    })
}

Binding errors

Mithril thennables take two functions as optional parameters: the first parameter is called if the web service request completes successfully. The second one is called if it completes with an error.

Error binding is meant to be done in the controller layer. Doing it in the model level is also possible, but generally leads to more code in order to connect all the dots.

In the example below, we bind an error getter-setter to our previous controller so that the error variable gets populated if the server throws an error.

//controller
var controller = function() {
    this.error = m.prop("")

    this.users = User.listEven().then(function(users) {
        if (users.length == 0) m.route("/add");
    }, this.error)
}

If the controller doesn't already have a success callback to run after a request resolves, you can still bind errors like this:

//controller
var controller = function() {
    this.error = m.prop("")

    this.users = User.listEven().then(null, this.error)
}

Queuing Operations

As you saw, you can chain operations that act on the response data. Typically this is required in three situations:

  • in model-level methods if client-side processing is needed to make the data useful for a controller or view.
  • in the controller, to redirect after a model service resolves.
  • in the controller, to bind error messages

In the example below, we take advantage of queuing to debug the AJAX response data prior to doing further processing on the user list

//a FP-friendly console.log
var log = function(value) {
    console.log(value)
}

var users = m.request({method: "GET", url: "/user"})
    .then(log);
    .then(function(users) {
        //add one more user to the response
        return users.concat({name: "Jane"})
    })

//assuming the response contains the following data: `[{name: "John"}, {name: "Mary"}]`
//then when resolved (e.g. in a view), the `users` getter-setter will contain a list of users
//i.e. users() //[{name: "John"}, {name: "Mary"}, {name: "Jane"}]

Casting the Response Data to a Class

It's possible to auto-cast a JSON response to a class. This is useful when we want to control access to certain properties in an object, as opposed to exposing all the fields in POJOs (plain old Javascript objects) for arbitrary processing.

In the example below, User.list returns a list of User instances.

var User = function(data) {
    this.name = m.prop(data.name);
}

User.list = function() {
    return m.request({method: "GET", url: "/user", type: User});
}

var users = User.list();
//assuming the response contains the following data: `[{name: "John"}, {name: "Mary"}]`
//then when resolved (e.g. in a view), `users` will contain a list of User instances
//i.e. users()[0].name() == "John"

Unwrapping Response Data

Often, web services return the relevant data wrapped in objects that contain metadata.

Mithril allows you to unwrap the relevant data, by providing two callback hooks: unwrapSuccess and unwrapError.

These hooks allow you to unwrap different parts of the response data depending on whether it succeed or failed.

var users = m.request({
    method: "GET",
    url: "/user",
    unwrapSuccess: function(response) {
        return response.data;
    },
    unwrapError: function(response) {
        return response.error;
    }
});

//assuming the response is: `{data: [{name: "John"}, {name: "Mary"}], count: 2}`
//then when resolved (e.g. in a view), the `users` getter-setter will contain a list of users
//i.e. users() //[{name: "John"}, {name: "Mary"}]

Using Different Data Transfer Formats

By default, m.request uses JSON to send and receive data to web services. You can override this by providing serialize and deserialize options:

var users = m.request({
    method: "GET",
    url: "/user",
    serialize: mySerializer,
    deserialize: myDeserializer
});

One typical way to override this is to receive as-is responses. The example below shows how to receive a plain string from a txt file.

var file = m.request({
    method: "GET",
    url: "myfile.txt",
    deserialize: function(value) {return value;}
});

File uploads with FormData

To use the HTML5 FormData object as the payload for a request, you need to override the serialize option. By default, serialize converts an object to JSON, but in the case of a FormData payload, you want to pass the object intact.

//assume the file comes from an HTML5 drag-n-drop event
var file = e.dataTransfer.files[0]

var data = new FormData();
data.append("file", file)

m.request({
    method: "POST",
    url: "/upload",
    serialize: function(data) {return data}
})

Using variable data formats

By default, Mithril assumes both success and error responses are in JSON format, but some servers may not return JSON responses when returning HTTP error codes (e.g. 404)

You can get around this issue by using extract

var nonJsonErrors = function(xhr) {
  return xhr.status > 200 ? JSON.stringify(xhr.responseText) : xhr.responseText
}

m.request({method: "GET", url: "/foo/bar.x", extract: nonJsonErrors})
  .then(function(data) {}, function(error) {console.log(error)})

Extracting Metadata from the Response

The extract method can be used to read metadata from HTTP response headers or the status field of an XMLHttpRequest.

var extract = function(xhr, xhrOptions) {
    if (xhrOptions.method == "HEAD") return xhr.getResponseHeader("x-item-count")
    else return xhr.responseText
}

m.request({method: "POST", url: "/foo", extract: extract});

Custom request rejections

If you want to be able to handle a condition as an error in a promise rejection handler, you can throw an Error from extract to reject the promise.

This is useful, for example, if you received invalid JSON from the server in production and you want to display a message to the user saying that the server is offline.

var extract = function(xhr, xhrOptions) {
    try {
        return JSON.stringify(xhr.responseText)
    }
    catch (e) {
        //e instanceof SyntaxError == true
        //by default `e` would be caught by Mithril's promise exception monitor and rethrown to the console
        //this new error follows Promises/A+ specifications and triggers a rejection in the downstream promises without hitting the console.
        throw new Error("Server is offline")
    }
}

m.request({method: "POST", url: "/foo", extract: extract});

You can read more about the promise exception monitor here.

Configuring the underlying XMLHttpRequest

The config option can be used to arbitrarily configure the native XMLHttpRequest instance and to access properties that would not be accessible otherwise.

The example below show how to configure a request where the server expects requests to have a Content-Type: application/json header

var xhrConfig = function(xhr) {
    xhr.setRequestHeader("Content-Type", "application/json");
}

m.request({method: "POST", url: "/foo", config: xhrConfig});

Aborting a request

The config option can also be used to retrieve the XMLHttpRequest instance for aborting the request. This idiom can also be used to attach onprogress event handlers.

var transport = m.prop();

m.request({method: "POST", url: "/foo", config: transport});

//the `transport` getter-setter contains an instance of XMLHttpRequest
transport().abort();

Using JSON-P

To make JSON-P requests, add the dataType option instead of method. You should not add the callback querystring parameter; Mithril already does that internally.

m.request({dataType: "jsonp", url: "/api/User"});

Some services (e.g. Flickr) don't follow the convention of calling the callback parameter callback. In order to specify the name of the querystring parameter that indicates the callback function, use the callbackKey option:

m.request({
    dataType: "jsonp",
    callbackKey: "jsoncallback",
    url: "http://api.flickr.com/services/feeds/photos_public.gne?tags=monkey&tagmode=any&format=json"
});

Rendering before web service requests finish

By default, Mithril waits for web service requests to complete before attempting a redraw. This ensures that data being accessed in the view isn't nullable as a result of asynchronous data not being available yet.

However, sometimes we do want to be able to redraw before a web service request completes, either because one web service out of many is slow, or because we don't need its response in order to redraw.

Setting the background option to true prevents a request from affecting redrawing. This means it's possible for a view to attempt to use data before it is available. You can specify an initial value for the m.request getter-setter in order to avoid having to write defensive code against potential null reference exceptions:

var demo = {}

demo.controller = function() {
    return {
        users: m.request({method: "GET", url: "/api/user", background: true, initialValue: []})
    }
}

//in the view
demo.view = function(ctrl) {
    //This view gets rendered before the request above completes
    //Calling .map doesn't throw an error because we defined the initial value to be an empty array, instead of undefined
    return ctrl.users().map(function() {
        return m("div", user.name)
    })
}

Signature

How to read signatures

Promise request(Options options)

where:
    Promise :: GetterSetter { Promise then(any successCallback(any value), any errorCallback(any value)) }
    GetterSetter :: any getterSetter([any value])
    Options :: XHROptions | JSONPOptions
    XHROptions :: Object {
        String method,
        String url,
        [String user,]
        [String password,]
        [Object<any> data,]
        [Boolean background,]
        [any initialValue,]
        [any unwrapSuccess(any data),]
        [any unwrapError(any data),]
        [String serialize(any dataToSerialize),]
        [any deserialize(String dataToDeserialize),]
        [any extract(XMLHttpRequest xhr, XHROptions options),]
        [void type(Object<any> data),]
        [XMLHttpRequest? config(XMLHttpRequest xhr, XHROptions options)]
    }
    JSONPOptions :: Object {
        String dataType,
        String url,
        String callbackKey,
        Object<any> data
    }

  • XHROptions options

    A map of options for the XMLHttpRequest

    • String method

      The HTTP method. Must be either "GET", "POST", "PUT", "DELETE", "HEAD" or "OPTIONS"

    • String url

      The URL to request. If the URL is not in the same domain as the application, the target server must be configured to accept cross-domain requests from the application's domain, i.e. its responses must include the header Access-Control-Allow-Origin: *.

    • String user (optional)

      A user for HTTP authentication. Defaults to undefined

    • String password (optional)

      A password for HTTP authentication. Defaults to undefined

    • Object data (optional)

      Data to be sent. It's automatically placed in the appropriate section of the request with the appropriate serialization based on method

    • Boolean background (optional)

      Determines whether the m.request can affect template rendering. Defaults to false.

      If this option is set to true, then the request does NOT call m.startComputation / m.endComputation, and therefore the completion of the request does not trigger an update of the view, even if data has been changed. This option is useful for running operations in the background (i.e. without user intervention).

      In order to force a redraw after a background request, use m.redraw

      var demo = {}

demo.controller = function() {
  return {
      users: m.request({method: "GET", url: "/api/user", background: true, initialValue: []}).then(function(value) {
          //force redraw
          m.redraw()
          return value
      })
  }
}

demo.view = function(ctrl) {
  //this view renders twice (once immediately, and once after the request above completes)
  return ctrl.users.map(function(user) {
      return m("div", user.name)
  })
}

      It's recommended that you always set an initialValue when setting the background option to true.

    • any initialValue (optional)

      The value that populates the returned getter-setter before the request completes. This is useful when using the background option, in order to avoid the need for null checks in views that may be attempting to access the returned getter-setter before the asynchronous request resolves.

    • any unwrapSuccess(any data) (optional)

      A preprocessor function to unwrap the data from a success response in case the response contains metadata wrapping the data.

      The default value (if this parameter is falsy) is the identity function function(value) {return value}

      For example, if the response is {data: [{name: "John"}, {name: "Mary"}]} and the unwrap function is function(response) {return response.data}, then the response will be considered to be [{name: "John"}, {name: "Mary"}] when processing the type parameter

      • Object | Array data

        The data to unwrap

      • returns Object | Array unwrappedData

        The unwrapped data

    • any unwrapError(any data) (optional)

      A preprocessor function to unwrap the data from an error response in case the response contains metadata wrapping the data.

      The default value (if this parameter is falsy) is the identity function function(value) {return value}

      • Object | Array data

        The data to unwrap

      • returns Object | Array unwrappedData

        The unwrapped data

    • String serialize(any dataToSerialize) (optional)

      Method to use to serialize the request data

      The default value (if this parameter is falsy) is JSON.stringify

      • any dataToSerialize

        Data to be serialized

      • returns String serializedData

    • any deserialize(String dataToDeserialize) (optional)

      Method to use to deserialize the response data

      The default value (if this parameter is falsy) is JSON.parse

      • String dataToDeserialize

        Data to be deserialized

      • returns any deserializedData

    • any extract(XMLHttpRequest xhr, XHROptions options) (optional)

      Method to use to extract the data from the raw XMLHttpRequest. This is useful when the relevant data is either in a response header or the status field.

      If this parameter is falsy, the default value is a function that returns xhr.responseText.

    • void type(Object data) (optional)

      The response object (or the child items if this object is an Array) will be passed as a parameter to the class constructor defined by type

      If this parameter is falsy, the deserialized data will not be wrapped.

      For example, if type is the following class:

      var User = function(data) {
  this.name = m.prop(data.name);
}

      And the data is [{name: "John"}, {name: "Mary"}], then the response will contain an array of two User instances.

    • XMLHttpRequest? config(XMLHttpRequest xhr, XHROptions options) (optional)

      An initialization function that runs after open and before send. Useful for adding request headers and when using XHR2 features, such as the XMLHttpRequest's upload property.

      • XMLHttpRequest xhr

        The XMLHttpRequest instance.

      • XHROptions options

        The options parameter that was passed into m.request call

      • returns XMLHttpRequest? xhr

        You may return an XHR-like object (e.g. a XDomainRequest instance) to override the provided XHR instance altogether.

  • returns Promise promise

    returns a promise that can bind callbacks which get called on completion of the AJAX request.

  • JSONPOptions options

    A map of options for JSONP requests

    • String dataType

      Must be the string "jsonp"

    • String url

      The URL to request. If the URL is not in the same domain as the application, the target server must be configured to accept cross-domain requests from the application's domain, i.e. its responses must include the header Access-Control-Allow-Origin: *.

    • String callbackKey

      The name of the querystring key that defines the name of the callback function to be called by the response. Defaults to "callback"

      This option is useful for web services that use uncommon conventions for defining jsonp callbacks (e.g. foo.com/?jsonpCallback=doSomething)

    • Object data (optional)

      Data to be sent. It's automatically placed in the appropriate section of the request with the appropriate serialization based on method