This is a demonstration of d3-queue, a tiny library for running asynchronous tasks with configurable concurrency. Here three external JSON files are loaded concurrently using d3-request, and then a
ready function renders the sum of their contents:
d3_queue.queue(2) .defer(d3_request.json, "1.json") .defer(d3_request.json, "2.json") .defer(d3_request.json, "3.json") .await(ready);
The concurrency of the queue is set to two, such that the first two files (1.json and 2.json) are loaded in parallel. When either of those files has loaded, a third request is made for 3.json. See the API reference for details.
If you’d rather not load these modules separately, they are available as part of D3 4.0.