关于 JavaScript 函数格式的问题

Question

I'm pretty new to JavaScript, and I'm working with JavaScript's datePicker library. I'm pretty much using the code out of the box, and the function that reads in the current date selected has me a bit confused. Here's the code (which I've modified slightly to format the date - in order to string-match it to the dates of my dataset).

const picker = datepicker(document.querySelector('#inlineDatepicker'), {

  // Event callbacks.
  onSelect: function(instance) {
  console.log(instance)
  var instanceSplit = instance.dateSelected.toString().split(" " ,4)
  var instanceClean = instanceSplit.toString().replace(/,/g, ' ')
    if (instanceClean === 'Mon Feb 11 2019') {
            console.log('datematch!')
            return 'blue'
        }       
      },
   });

I'm not familiar with the format of the onSelect: function(instance) {} function. All the functions I've ever seen are written in this format: function someFunction(parameter) {} .

Which leads me to my question: what is the actual function name of the onSelect: function(instance) {} function? For example, if I want to change the appearance of my data based on the date selected - ie return a stroke of 'blue' on my data points - what function name do I call? For example, the code here attempts to pass in the returned stroke value in the above function, but it doesn't recognize onSelect as a function (says it's undefined):

var events = mapG.selectAll("circle")
    .data(allSFEvents)
    .enter().append("circle")
    .style("stroke", onSelect)

Answer 1

Depending on what you mean by "name", that function either doesn't have a name (it's anonymous ), which is the strict meaning according to the definition of the language; or else the "name" in colloquial terms is the name of the property to which it's assigned.

The problem with the colloquial meaning, that the name is the property name, is that it can lead to false assumptions about the relationship between the anonymous function and the object where that property lives. There's no permanent relationship, in fact; the anonymous function is free of any implicit association with any "parent" object.

In your case, it's unlikely that you'd ever want to directly call that function. It's set up as an event handler, which means that the runtime or some framework will invoke your function at the appropriate time. In general, if an object property refers to a function, you can call that function via the property:

var someObject = {
  someFunction: function() { alert("Hi!"); }
};

someObject.someFunction(); // alerts "Hi!"