What is the eventData Parameter in Mapbox GL JS? - mapbox-gl-js

Question

While looking at the documentation for the Map object's fitBounds() method, I noticed that one of the parameters is eventData. The description of the parameter reads, "Data to propagate to any event listeners."
This parameter is accepted by several other methods, all of which involve map movement of some kind (i.e. panTo, jumpTo, setPitch, and many others).
How would I use this parameter? What is it for?

Answer 1

When you call Map#fitBounds, Map fires events such as Map#move. If you provide eventData to Map#fitBounds, that data will be passed to the event's listeners.
map.on('move', function(event) {
// event.foo is 'bar'
});
map.fitBounds(bounds, {}, {foo: 'bar'});

Answer 2

And the credit goes to...
Lucas! Thank you, Lucas, for your answer. Here is ultimately I how I ended up using this.
Problem:
When using an event listener for 'moveend', one can see this event is fired several times during the animation after map.fitBounds is called. However, we want to do something only when we are truly finished fitting the bounds.
Solution:
Use the eventData parameter to propagate a custom event property to the event listener which we can use as a trigger. Here, we don't call map.setMaxBounds until the animation is completely finished:
// Get the bounds of the bss, fit the map to the bounds
let bssBounds = getBSSBounds(bss, 1); // Custom function using turf.buffer()
// Fit the map to the above boundaries using no options, propagate event data
map.fitBounds(bssBounds, {}, {newBounds: true});
// Set max bounds
map.on('moveend', (event) => {
// If the 'moveend' event has "newBounds" != undefined
if (event.newBounds) {
// Set the maxBounds slighlty wider than the fitBounds
let maxBounds = getBSSBounds(bss, 3); // Custom function using turf.buffer()
map.setMaxBounds(maxBounds)
}
});