[Solved]-How to make curve links and add tooltip on links at the same time?-d3.js

You can simply push the original object itself inside bilinks...

bilinks.push([s, i, t, link]);

...which you can get as the fourth object in the datum array:

link.on("mouseover", function(d){
    console.log(d[3])
});

Here is Bostock's code with that change: http://blockbuilder.org/GerardoFurtado/6e849f33c828a9cac7037a4786f83766