Indexed DB cursor ranges on multiple properties

The index you created is a composite index. It is query like this:

index = objectStore.index('treelocation');
index.get([123456, 654321]); 

Alternatively, you can use two indexes for each coorrd. In my opinion it is better.

x_index = objectStore.index('xcoord'); 
y_index = objectStore.index('ycoord');
x_species = x_index.get(IDBKeyRange.only(123456)) 
y_species = y_index.get(IDBKeyRange.only(654321)) 
species = x_species.intersect(y_species); // this is result

I have been told the solution is indeed IDBKeyRange.bound([lowX,lowY],[highX,highY]).

The range suggestion is part of the answer, but even with array keys it really is just a 1-dimensional range, not a 2- (or N-) dimensional selection of the data. With your current schema you'll need to do something like this:

index.openCursor([lowX, lowY], [highX, highY]).onsuccess = function(e) {
    var cursor = e.target.result;
    if (!cursor) return; // done!

    var x = cursor.key[0], y = cursor.key[1];
    // assert(lowX <= x && x <= highX);
    if (y < lowY) {
        cursor.continue([x, lowY]);
    } else if (y > highY) {
        cursor.continue([x + 1, lowY]);
    } else {
        processRecord(cursor.value); // we got one!
        cursor.continue();
    }
};

(if coordinates are not integral, replace + 1 with an appropriate epsilon)

I've posted an example of a generalized solution to:

https://gist.github.com/inexorabletash/704e9688f99ac12dd336