Wiki ▸ [[API Reference]] ▸ [[Core]] ▸ Internals

Various utilities for implementing reusable components.


# d3.functor(value)

If the specified value is a function, returns the specified value. Otherwise, returns a function that returns the specified value. This method is used internally as a lazy way of upcasting constant values to functions, in cases where a property may be specified either as a function or a constant. For example, many D3 layouts allow properties to be specified this way, and it simplifies the implementation if we automatically convert constant values to functions.

# d3.rebind(target, source, names…)

Copies the methods with the specified names from source to target, and returns target. Calling one of the named methods on the target object invokes the same-named method on the source object, passing any arguments passed to the target method, and using the source object as the this context. If the source method returns the source object, the target method returns the target object (“setter” method); otherwise, the target method returns the return value of the source method (“getter” mode). The rebind operator allows inherited methods (mix-ins) to be rebound to a subclass on a different object.


D3’s behaviors and higher level components, such as the brush, use d3.dispatch to broadcast custom events.

dispatching events

For visualizations with coordinated views, d3.dispatch provides a convenient lightweight mechanism for loosely-coupled components. Organizing your code with d3.dispatch can assist with separation of concerns and make your code easier to maintain.

# d3.dispatch(types…)

Creates a new dispatcher object for the specified types. Each argument is a string representing the name of the event type, such as "zoom" or "change". The returned object is an associative array; each type name is associated with a dispatch object. For example, if you wanted to create an event dispatcher for "start" and "end" events, you can say:

var dispatch = d3.dispatch("start", "end");

Then, you can access the dispatchers for the different event types as dispatch.start and dispatch.end. For example, you might add an event listener:

dispatch.on("start", listener);

And then later dispatch an event to all registered listeners:


For details on how to pass arguments to listeners, see dispatch.

# dispatch.on(type[, listener])

Adds or removes an event listener for the specified type. The type is a string event type name, such as "start" or "end". The specified listener is invoked with the context and arguments determined by the caller; see dispatch.

If an event listener was already registered for the same type, the existing listener is removed before the new listener is added. To register multiple listeners for the same event type, the type may be followed by an optional namespace, such as "" and "". Likewise, you can remove all registered listeners for a given namespace by saying dispatch.on(".foo", null).

If listener is not specified, returns the currently-assigned listener for the specified type, if any.

# dispatch.type(arguments…)

The type method (such as dispatch.start in the above example) notifies each registered listener, passing the listener the specified arguments. The this context will be used as the context of the registered listeners. For example, to invoke all registered listeners with the context foo and the argument bar, say foo, bar ). Thus, you can pass whatever arguments you want to the listeners; most commonly, you might create an object that represents the event, or pass along the current datum ( d ) and index ( i ). You can also control the "this" context of the listeners using call or apply.

For example, if you wanted a native "click" event to your "custom" event, while preserving the context and arguments, you could say:

selection.on("click", function(d, i) {
  dispatch.custom.apply(this, arguments);