add promise limiter in javascript

Author: luckyb13

javascript/pLimit.js

@@ -0,0 +1,61 @@
+// pLimit.js
+// Minimal promise concurrency limiter (like p-limit). Queue tasks, run at most N at once.
+
+/**
+ * @typedef {() => Promise<any>} Task
+ */
+
+/**
+ * Create a concurrency limiter.
+ * @param {number} concurrency - number of tasks to run simultaneously (>=1)
+ * @returns {{
+ *  (fn: (...args:any[])=>Promise<any>|any, ...args:any[]): Promise<any>,
+ *  activeCount: () => number,
+ *  pendingCount: () => number,
+ *  clearQueue: () => void
+ * }}
+ */
+export function pLimit(concurrency = 4) {
+  if (!Number.isInteger(concurrency) || concurrency < 1) {
+    throw new Error("concurrency must be an integer >= 1");
+  }
+
+  /** @type {Task[]} */
+  const queue = [];
+  let active = 0;
+
+  const next = () => {
+    if (active >= concurrency) return;
+    const task = queue.shift();
+    if (!task) return;
+    active++;
+    task().finally(() => {
+      active--;
+      next();
+    });
+  };
+
+  const run = (fn, ...args) => new Promise((resolve, reject) => {
+    const task = async () => {
+      try {
+        resolve(await fn(...args));
+      } catch (e) {
+        reject(e);
+      }
+    };
+    queue.push(task);
+    // Try to start tasks ASAP (microtask ensures consistent ordering)
+    queue.length && queue.length <= concurrency ? Promise.resolve().then(next) : next();
+  });
+
+  run.activeCount = () => active;
+  run.pendingCount = () => queue.length;
+  run.clearQueue = () => { queue.length = 0; };
+
+  return run;
+}
+
+// // Example usage:
+// const limit = pLimit(2);
+// const wait = ms => new Promise(r => setTimeout(r, ms));
+// await Promise.all([1,2,3,4,5].map(n => limit(async () => { await wait(100*n); return n; })));