Show me how the concurrency limiter actually queues tasks.

Perfect! Now I have the implementation details. Let me explain how the concurrency limiter queues tasks.

Task Queueing Implementation

The p-limit library uses a Queue data structure to manage pending tasks. Here's how it works [index.js:L3-L116]:

Core Queueing Logic

1. Queue Creation: A Queue object stores pending functions [index.js:L14]

   const queue = new Queue();
   let activeCount = 0;
   

2. Enqueue Function: When you call limit(fn, ...args), the task is added to the queue via the enqueue function [index.js:L47-L56]:

   const enqueue = (function_, resolve, reject, arguments_) => {
       const queueItem = {reject};
       
       new Promise(internalResolve => {
           queueItem.run = internalResolve;
           queue.enqueue(queueItem);
       }).then(run.bind(undefined, function_, resolve, arguments_));
       
       if (activeCount < concurrency) {
           resumeNext();
       }
   };
   

3. Resume Next: The resumeNext function checks if capacity is available and dequeues the next task [index.js:L18-L23]:

   const resumeNext = () => {
       if (activeCount < concurrency && queue.size > 0) {
           activeCount++;
           queue.dequeue().run();
       }
   };
   

4. Task Execution: The run function executes a dequeued task and calls next() when complete [index.js:L25-L41]:

   const run = async (function_, resolve, arguments_) => {
       const result = (async () => function_(...arguments_))();
       resolve(result);
       try {
           await result;
       } catch {}
       next();  // Decrement activeCount and process next
   };
   

Flow Summary

• Tasks are enqueued immediately but only executed when activeCount < concurrency
• Each completed task triggers next(), which decrements activeCount and processes the next queued item
• The pendingCount property reflects queue.size [index.js:L65-L67]

FOLLOW_UPS:

• How does the Queue data structure work internally?
• Can I adjust concurrency while tasks are queued?
• What happens when clearQueue() is called?