Cancellable callback wrapper

cancellable.js

/**
 * Returns a function with a .cancel() function which can be used to prevent the
 * given function from being called.
 *
 * Use case: triggering an asyncronous function with new data while an existing
 * function for the same task but with old data is still pending a callback, so
 * the callback only gets called for the last one to run.
 */
function cancellable(func) {
  var cancelled = false

  var cancellabled = function() {
    if (!cancelled) {
      func.apply(null, arguments)
    }
  }

  cancellabled.cancel = function() {
    cancelled = true
    if (typeof func.onCancel == 'function') {
      func.onCancel()
    }
  }

  return cancellabled
}