A Json.Decoder for grabbing event.keyCode. This helps you define keyboard listeners like this:

import Json.Decode as Json


onKeyUp : (Int -> msg) -> Attribute msg
onKeyUp tagger =
  on "keyup" (Json.map tagger keyCode)

Note: It looks like the spec is moving away from event.keyCode and towards event.key. Once this is supported in more browsers, we may add helpers here for onKeyUp, onKeyDown, onKeyPress, etc.

A Json.Decoder for grabbing event.target.checked. We use this to define onCheck as follows:

import Json.Decode as Json


onCheck : (Bool -> msg) -> Attribute msg
onCheck tagger =
  on "input" (Json.map tagger targetChecked)

A Json.Decoder for grabbing event.target.value. We use this to define onInput as follows:

import Json.Decode as Json


onInput : (String -> msg) -> Attribute msg
onInput tagger =
  stopPropagationOn "input" <|
    Json.map alwaysStop (Json.map tagger targetValue)


alwaysStop : a -> (a, Bool)
alwaysStop x =
  (x, True)

You probably will never need this, but hopefully it gives some insights into how to make custom event handlers.

Create an event listener that may stopPropagation or preventDefault.

Note: If you need something even more custom (like capture phase) check out the lower-level event API in elm/virtual-dom.

Create an event listener that may preventDefault. Your decoder must produce a message and a Bool that decides if preventDefault should be called.

For example, the onSubmit function in this library always prevents the default behavior:

onSubmit : msg -> Attribute msg onSubmit msg = preventDefaultOn "submit" (Json.map alwaysPreventDefault (Json.succeed msg))

alwaysPreventDefault : msg -> ( msg, Bool )
alwaysPreventDefault msg =
  ( msg, True )

Create an event listener that may stopPropagation. Your decoder must produce a message and a Bool that decides if stopPropagation should be called.

Note: This creates a passive event listener, enabling optimizations for touch, scroll, and wheel events in some browsers.

Create a custom event listener. Normally this will not be necessary, but you have the power! Here is how onClick is defined for example:

import Json.Decode as Decode


onClick : msg -> Attribute msg
onClick message =
  on "click" (Decode.succeed message)

The first argument is the event name in the same format as with JavaScript's addEventListener function.

The second argument is a JSON decoder. Read more about these here. When an event occurs, the decoder tries to turn the event object into an Elm value. If successful, the value is routed to your update function. In the case of onClick we always just succeed with the given message.

If this is confusing, work through the Elm Architecture Tutorial. It really helps!

Note: This creates a passive event listener, enabling optimizations for touch, scroll, and wheel events in some browsers.

Detect a submit event with preventDefault in order to prevent the form from changing the page’s location. If you need different behavior, create a custom event handler.

Detect change events on checkboxes. It will grab the boolean value from event.target.checked on any input event.

Check out targetChecked for more details on how this works.

Detect input events for things like text fields or text areas.

For more details on how onInput works, check out targetValue.

Note 1: It grabs the string value at event.target.value, so it will not work if you need some other information. For example, if you want to track inputs on a range slider, make a custom handler with on.

Note 2: It uses stopPropagationOn internally to always stop propagation of the event. This is important for complicated reasons explained here and here.