selection.join() improvements

[pavelschon]

First of all, I'd like to thank for the selection.join(), it's a great improvement over traditional enter-update-exit pattern.
However, I feel there's some space for improvements.

Using arrow functions are good only for the simple case like this:

selection.join(
    enter => enter.append('div').classed('enter', true),
    update => update.classed('enter', false).classed('update', true),
    exit => exit.classed('update', false).classed('exit', true).transition().remove()
)

When you need to do something more complex, you will probably have to use regular functions:

selection.join(
    function(enter) {

    },
    function(update) {

    },
    function(exit) {
    
    }
)

or even factor-out functions for readability:

function doEnter(enter) {
   
}

function doUpdate(update) {

}

function doExit(exit) {
    
}

selection.join(doEnter, doUpdate, doExit)

One may not need doUpdate() and will write:

selection.join(doEnter, doExit)  // bug!

I propose the following syntax: pass functions in the object:

selection.join({
    enter: doEnter,
    update: doUpdate,
    exit: doExit
})

Passing functions in the object will prevent bugs and improve the readability.

[mbostock]

Thanks for the feedback, but I don’t want to change the API.

[pavelschon]

So you're happy with the current API? Could you elaborate more?

[curran]

@pavelschon The motivations for the proposal are not clear.

When you need to do something more complex, you will probably have to use regular functions:

Why? The only reason to change would be if you need to access this.

Passing functions in the object will prevent bugs and improve the readability.

How so? What is the benefit?

Also keep in mind that changing this API would be a breaking change, and keeping the old API as well would introduce more surface area to the library. This proposal does not seem to hold water IMO.

[pavelschon]

How so? What is the benefit?

It's already mentioned in the first post - if you pass doExit as 2nd positional argument, it will most likely cause a bug:

selection.join(doEnter, doExit)  // bug!

SInce JS does not support named arguments, then the only way to do it is to pass an object.

[pavelschon]

And it is not a breaking change: you can call join() in 2 ways already:

selection.join(element) // as a shorthand
selection.join(doEnter, doUpdate, doExit)

What's the problem to add another API like:

selection.join(obj)

?