mount(root, component)

Description

Activates a component, enabling it to autoredraw on user events

var state = {
  count: 0,
  inc: function () {
    state.count++;
  },
};

var Counter = {
  view: function () {
    return m('div', { onclick: state.inc }, state.count);
  },
};

m.mount(document.body, Counter);

To pass arguments when mounting a component use:

m.mount(element, {
  view: function () {
    return m(Component, attrs);
  },
});

Signature

m.mount(element, Component)

Argument	Type	Required	Description
element	Element	Yes	A DOM element that will be the parent node to the subtree
Component	Component|null	Yes	The component to be rendered. null unmounts the tree and cleans up internal state.
returns			Returns nothing

How to read signatures

How it works

m.mount(element, Component), when called renders the component into the element and subscribe the (element, Component) pair to the redraw subsystem. That tree will be re-rendered when manual or automatic redraws are triggered.

On redraw, the new vDOM tree is compared (or "diffed") with the old one, and the existing DOM tree is modified only where needed to reflect the changes. Unchanged DOM nodes are not touched at all.

Replace a component

Running mount(element, OtherComponent) where element is a current mount point replaces the component previously mounted with OtherComponent.

Unmount

Using m.mount(element, null) on an element with a previously mounted component unmounts it and cleans up Mithril.js internal state. This can be useful to prevent memory leaks when removing the root node manually from the DOM.

Headless mounts

In certain more advanced situations, you may want to subscribe and listen for redraws without rendering anything to the screen. This can be done using a headless mount, created by simply invoking m.mount with an element that's not added to the live DOM tree and putting all your useful logic in the component you're mounting with. You still need a view in your component, just it doesn't have to return anything useful and it can just return a junk value like null or undefined.

var elem = document.createElement('div');

// Subscribe
m.mount(elem, {
  oncreate: function () {
    // once added
  },
  onupdate: function () {
    // on each redraw
  },
  onremove: function () {
    // clean up whatever you need
  },

  // Necessary boilerplate
  view: function () {},
});

// Unsubscribe
m.mount(elem, null);

There's no need to worry about other mount roots. Multiple roots are supported and they won't step on each other. You can even do the above in a component when integrating with another framework, and it won't be a problem.

Performance considerations

It may seem wasteful to generate a vnode tree on every redraw, but as it turns out, creating and comparing JavaScript data structures is surprisingly cheap compared to reading and modifying the DOM.

Touching the DOM can be extremely expensive for a couple of reasons. Alternating reads and writes can adversely affect performance by causing several browser repaints to occur in quick succession, whereas comparing virtual dom trees allows writes to be batched into a single repaint. Also, the performance characteristics of various DOM operations vary between implementations and can be difficult to learn and optimize for all browsers. For example, in some implementations, reading childNodes.length has a complexity of O(n); in some, reading parentNode causes a repaint, etc.

In contrast, traversing a JavaScript data structure has a much more predictable and sane performance profile, and in addition, a vnode tree is implemented in such a way that enables modern JavaScript engines to apply aggressive optimizations such as hidden classes for even better performance.

Differences from m.render

A component rendered via m.mount automatically redraws in response to view events, m.redraw() calls or m.request() calls. Vnodes rendered via m.render() do not.

m.mount() is suitable for application developers integrating Mithril.js widgets into existing codebases where routing is handled by another library or framework, while still enjoying Mithril.js' auto-redrawing facilities.

m.render() is suitable for library authors who wish to manually control rendering (e.g. when hooking to a third party router, or using third party data-layer libraries like Redux).