d3.queue()

d3.queue([concurrency])

Constructs a new queue with the specified concurrency. If concurrency is not specified, the queue has infinite concurrency. Otherwise, concurrency is a positive integer. For example, if concurrency is 1, then all tasks will be run in series. If concurrency is 3, then at most three tasks will be allowed to proceed concurrently; this is useful, for example, when loading resources in a web browser.