Seeking the seeds for writing tidy event handlers in JS
The simple way of avoiding needing to define a named function like double_increment is just to use a lambda. You could simply write

d3.select('#inc').on('click',
    () =&gt; {
        incrementer1.increment();
        incrementer2.increment();
    });


You could also get fancy and loop through the incrementers too

var incrementors = [incrementer1, incrementer2]

d3.select('#inc').on('click',
    () =&gt; {
        for (inc of incrementors) {
            inc.increment();
        } 
     };


This lets you easily add new incrementors later if needed.