fetch item prices - throttle end points with promise queue

app/util/index.ts

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+export * from './promise-queue';

app/util/promise-queue.ts

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+/**
+ * Allow promises to be queued up and executed in order.
+ */
+export class PromiseQueue {
+  private queue: any[] = [];
+  private locked: boolean = false;
+
+  /** Add new promise to queue */
+  add(p: () => Promise<any>, delay?: number) {
+    this._add(p, false, delay);
+  }
+
+  /** Add new promise to the from of the queue */
+  addToBeginning(p: () => Promise<any>, delay?: number) {
+    this._add(p, true, delay);
+  }
+
+  /** Clear the queue and reset state. */
+  clearQueue() {
+    this.queue = [];
+  }
+
+  private delay = (ms: number) => () => new Promise(resolve => setTimeout(resolve, ms));
+
+  _add = (p: () => Promise<any>, beginning: boolean = false, delay?: number) => {
+    const newPromise = delay ? [p, this.delay(delay)] : [p];
+    beginning ? this.queue.unshift(newPromise) : this.queue.push(newPromise);
+    if (!this.locked) {
+      this.next();
+    }
+  };
+
+  private next() {
+    this.locked = true;
+    const p = this.queue.shift();
+    const finished = () => {
+      this.queue.length > 0 ? this.next() : (this.locked = false);
+    };
+    Promise.all(p.map((v: any) => v()))
+      .then(finished)
+      .catch(finished);
+  }
+}