D3.json() fetch data help

I am trying to fetch data from a provided JSON file and then output data from the JSON object into a separate array, for example:

I want to push the first entry in each array within the JSON object to a separate variable after calling d3.json() to fetch the data, but when I try to do this with a forEach I am just getting a blank array returned.

let dataset = [];

  .then(data => data.data.forEach(ea => dataset.push(ea[0])));


Where am I going wrong?