d3.csv Load Data Shows Undefined

Question

I am trying to load a csv file using below code.

Method 1:

d3.csv("data/aaa.csv", function(error, data) {
        console.log(data[1]);
    })

The console shows undefined instead of the data row. Nonetheless, this is the most common method you can find on all forums.

However, if I use below method 2 loading the data, it correctly shows the data row.

Method 2:

d3.csv("data/aaa.csv").then(function(data) {
      console.log(data[1]);
    });

I am a bit confused why this is happening.