This is a getter-setter factory utility. It returns a function that stores information.

Note that modifying the values of m.prop getter-setters does not trigger redrawing. Instead, Mithril's redrawing system relies on m.startComputation and m.endComputation. These functions are internally called by Mithril when you initialize a component via m.mount or m.route, and when you trigger event handlers that were created within templates with m().


//define a getter-setter with initial value `John`
var name = m.prop("John");

//read the value
var a = name(); //a == "John"

//set the value to `Mary`
name("Mary"); //Mary

//read the value
var b = name(); //b == "Mary"

It can be used in conjunction with m.withAttr to implement data binding in the view-to-model direction and to provide uniform data access for model entity properties.

//a contrived example of bi-directional data binding
var User = {
    model: function(name) {
        this.name = m.prop(name);
    controller: function() {
        this.user = new User.model("John Doe");
    view: function(controller) {
        m.render("body", [
            m("input", {onchange: m.withAttr("value", controller.user.name), value: controller.user.name()})

In the example above, the usage of m.prop allows the developer to change the implementation of the user name getter/setter without the need for code changes in the controller and view.

m.prop can also be used in conjunction with m.request and m.deferred to bind data on completion of an asynchronous operation.

var users = m.prop([]);
var error = m.prop("");

m.request({method: "GET", url: "/users"})
    .then(users, error); //on success, `users` will be populated, otherwise `error` will be populated
//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 User instances
//i.e. users()[0].name() == "John"

Third-party promise library support

If a promise is passed into m.prop(), a Mithril promise is returned. Mithril promises are also getter-setter functions, which are populated with the resolved value if the promise is fulfilled successfully.

Until the promise is resolved, the value of the prop will resolve to undefined

Here's an example using the Q promise library:

var deferred = Q.defer()
var users = m.prop(deferred.promise)

users() // undefined


//wait for next tick for Q's A+ compliant promise to actually resolve
setTimeout(function() {

    users() // Hello
    users.then(function(value) {
        console.log(value) //Hello

}, 1000)

Serializing getter-setters

Getter-setters are JSON-serializable:

var data = {foo: m.prop("bar")};
JSON.stringify(data); // '{"foo": "bar"}'

This allows getter-setters to be passed directly as parameters to m.request, for example.


How to read signatures

GetterSetter prop([any initialValue])

    GetterSetter :: any getterSetter([any value])
  • any initialValue (optional)

    An initialization value. If not provided, the value of the getter-setter's internal store defaults to undefined.

  • returns any getterSetter([any value])

    A getter-setter method.

    • any value (optional)

      If provided, it updates the getter-setter's internal store to the provided value.

      If not provided, return the current internally stored value.

    • returns any value

      This method always returns the value of the internal store, regardless of whether it was updated or not.