d3.js v5 - Promise.all replaced d3.queue

Here's an approach with ES6 async functions and ES6 array destructuring:

async function chart() {
  const [first, second] = await Promise.all([
    d3.json('data1.json'),
    d3.json('data2.json'),
  ])
  console.log('data2.json :', second)
}

chart()

You can simplify that code a lot: you don't net to use new Promise with d3.json, since d3.json will itself create the promise.

So, you can just do:

var files = ["data1.json", "data2.json", "data3.json"];
var promises = [];

files.forEach(function(url) {
    promises.push(d3.json(url))
});

Promise.all(promises).then(function(values) {
    console.log(values)
});

Or, if you're into the code golf, even shorter:

var files = ["data1.json", "data2.json", "data3.json"];

Promise.all(files.map(url => d3.json(url))).then(function(values) {
    console.log(values)
});

Since I cannot use JSON files in the S.O. snippet, check the console in this bl.ocks: https://bl.ocks.org/GerardoFurtado/f08993c9c729b0b3452ef1803ad9dcbf/c4b45c5acce6033085a667cbb7d34203d15de0f0