Unable to concatenate multiple JSON files' data

Question

I'm trying to add the elements of several JSON files together into one array, however when printing data, it results in an empty array.

let dataUrls = ["file1.json", "file2.json", "file3.json"];
let data = getData(dataUrls);
function getData(dataUrls) {
    let data = [];
    for (let i = 0; i < dataUrls.length; i++) {
        d3.json(dataUrls[i]).then(d => {
            data = data.concat(d);
        });
    }
    return data;
}
console.log(data)