Test debounce function.

tests/test-coreutils/src/debounce.spec.ts

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+// Copyright (c) Jupyter Development Team.
+// Distributed under the terms of the Modified BSD License.
+
+import { expect } from 'chai';
+
+import { debounce } from '@jupyterlab/coreutils';
+
+describe('debounce()', () => {
+  it('should debounce a function within an interval', async () => {
+    let called = 0;
+    const interval = 500;
+    const debounced = debounce(async () => {
+      called += 1;
+    }, interval);
+    const one = debounced();
+    const two = debounced();
+    const three = debounced();
+    const four = debounced();
+    const five = debounced();
+    await five;
+    expect(called).to.equal(1);
+    await four;
+    expect(called).to.equal(1);
+    await three;
+    expect(called).to.equal(1);
+    await two;
+    expect(called).to.equal(1);
+    await one;
+    expect(called).to.equal(1);
+    await debounced();
+    expect(called).to.equal(2);
+    await debounced();
+    expect(called).to.equal(3);
+  });
+});