transition.duration()

transition.duration([value])

For each selected element, sets the transition duration to the specified value in milliseconds. The value may be specified either as a constant or a function. If a function, it is immediately evaluated for each selected element, in order, being passed the current datum d and index i, with the this context as the current DOM element. The function’s return value is then used to set each element’s transition duration. If a duration is not specified, it defaults to 250ms.

If a value is not specified, returns the current value of the duration for the first (non-null) element in the transition. This is generally useful only if you know that the transition contains exactly one element.