util: implement debounce

doc/api/util.md

@@ -387,6 +387,86 @@ The `--throw-deprecation` command-line flag and `process.throwDeprecation`
 property take precedence over `--trace-deprecation` and
 `process.traceDeprecation`.
 
+## `util.debounce(fn, wait[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `fn` {Function} The function to debounce.
+* `wait` {integer} The number of milliseconds to delay `fn`.
+* `options` {Object}
+  * `rejectOnCancel` {boolean} When `true`, a call superseded by a later call
+    rejects with an `AbortError`. **Default:** `false`.
+  * `signal` {AbortSignal} An `AbortSignal` that cancels a pending call when
+    aborted.
+* Returns: {Function} The debounced function.
+
+Creates a function that delays calling `fn` until `wait` milliseconds have
+elapsed since the most recent invocation. The debounced function returns a
+{Promise} for the value returned by `fn`. If `fn` throws or returns a rejected
+promise, the returned promise is rejected with the same reason.
+
+When the debounced function is called more than once before the delay expires,
+`fn` receives the arguments from the most recent call. By default, the promises
+from all calls resolve or reject with the result of that invocation. If
+`options.rejectOnCancel` is `true`, the promises from superseded calls reject
+with an `AbortError` instead.
+
+The returned function has the following properties:
+
+* `cancel([reason])` cancels the pending invocation. Its pending promises reject
+  with an `AbortError`. If provided, `reason` is set as the error's `cause`.
+* `flush()` cancels the delay and invokes `fn` immediately. It has no effect if
+  no invocation is pending.
+* `pending` {Promise|null} is the promise returned by the most recent call in
+  the current debounce window, or `null` if no invocation is pending.
+* `pendingCount` {integer} is the number of calls awaiting the invocation in
+  the current debounce window.
+* `ref()` makes the pending and future timeout keep the Node.js event loop
+  active. Returns the debounced function.
+* `unref()` allows the event loop to exit while a timeout is pending. This also
+  applies to future timeouts. Returns the debounced function.
+
+When invoked, `fn` has the debounced function as its `this` value. Once `fn` is
+invoked, a new debounce window can begin even if a promise returned by `fn` is
+still pending. The debounced function preserves the `name` and `length` of `fn`.
+
+```mjs
+import { setTimeout as wait } from 'node:timers/promises';
+import { debounce } from 'node:util';
+
+const fn = debounce(async (value) => {
+  await wait(100);
+  return value;
+}, 50);
+
+const first = fn(1);
+const second = fn(2);
+
+console.log(await first);  // 2
+console.log(await second); // 2
+```
+
+A debounced function can be used to trigger an action after a period of
+inactivity. Each call resets the timeout:
+
+```cjs
+const { debounce } = require('node:util');
+
+const onInactivity = debounce(() => {
+  console.log('No activity for 5 seconds');
+}, 5_000).unref();
+
+process.stdin.on('data', (data) => {
+  console.log(`Received ${data.length} bytes`);
+  onInactivity();
+});
+
+// Start the initial inactivity timeout.
+onInactivity();
+```
+
 ## `util.diff(actual, expected)`
 
 <!-- YAML

lib/internal/util/debounce.js

@@ -0,0 +1,185 @@
+'use strict';
+
+const {
+  ArrayPrototypePush,
+  ObjectDefineProperties,
+  PromiseWithResolvers,
+  ReflectApply,
+} = primordials;
+
+const {
+  AbortError,
+} = require('internal/errors');
+const {
+  validateAbortSignal,
+  validateBoolean,
+  validateFunction,
+  validateInteger,
+  validateObject,
+} = require('internal/validators');
+const { addAbortListener } = require('internal/events/abort_listener');
+const { kEmptyObject } = require('internal/util');
+const { TIMEOUT_MAX } = require('internal/timers');
+const { clearTimeout, setTimeout } = require('timers');
+const { markPromiseAsHandled } = internalBinding('util');
+
+/**
+ * @typedef {object} DebounceOptions
+ * @property {AbortSignal} [signal] An AbortSignal that cancels a pending call.
+ * @property {boolean} [rejectOnCancel] Whether to reject superseded calls.
+ */
+
+/**
+ * Creates a function that delays calling `fn` until `wait` milliseconds have
+ * elapsed since its most recent invocation.
+ * @param {Function} fn
+ * @param {number} wait
+ * @param {DebounceOptions} [options]
+ * @returns {Function}
+ */
+function debounce(fn, wait, options = kEmptyObject) {
+  validateFunction(fn, 'fn');
+  validateInteger(wait, 'wait', 0, TIMEOUT_MAX);
+  validateObject(options, 'options');
+
+  const {
+    rejectOnCancel = false,
+    signal,
+  } = options;
+
+  validateBoolean(rejectOnCancel, 'options.rejectOnCancel');
+  validateAbortSignal(signal, 'options.signal');
+
+  if (signal?.aborted) {
+    throw new AbortError(undefined, { __proto__: null, cause: signal.reason });
+  }
+
+  let args;
+  let pendingCalls = [];
+  let refed = true;
+  let timeout;
+
+  function rejectPending(error) {
+    const calls = pendingCalls;
+    pendingCalls = [];
+    for (let i = 0; i < calls.length; i++) {
+      calls[i].reject(error);
+    }
+  }
+
+  function cancel(reason) {
+    if (timeout === undefined) return;
+    clearTimeout(timeout);
+    timeout = undefined;
+    args = undefined;
+    const error = reason === undefined ?
+      new AbortError() :
+      new AbortError(undefined, { __proto__: null, cause: reason });
+    rejectPending(error);
+  }
+
+  function ref() {
+    refed = true;
+    timeout?.ref();
+    return debounced;
+  }
+
+  function unref() {
+    refed = false;
+    timeout?.unref();
+    return debounced;
+  }
+
+  function flush() {
+    if (timeout === undefined) return;
+
+    clearTimeout(timeout);
+    timeout = undefined;
+    const callArgs = args;
+    args = undefined;
+    const calls = pendingCalls;
+    pendingCalls = [];
+
+    let result;
+    try {
+      result = ReflectApply(fn, debounced, callArgs);
+    } catch (error) {
+      for (let i = 0; i < calls.length; i++) {
+        calls[i].reject(error);
+      }
+      return;
+    }
+
+    for (let i = 0; i < calls.length; i++) {
+      calls[i].resolve(result);
+    }
+  }
+
+  function debounced(...callArgs) {
+    if (timeout !== undefined) {
+      markPromiseAsHandled(pendingCalls[pendingCalls.length - 1].promise);
+      if (rejectOnCancel) {
+        rejectPending(new AbortError('The debounced call was superseded'));
+      }
+      timeout.refresh();
+    } else {
+      timeout = setTimeout(flush, wait);
+      if (!refed) timeout.unref();
+    }
+
+    const call = PromiseWithResolvers();
+    ArrayPrototypePush(pendingCalls, call);
+    args = callArgs;
+    return call.promise;
+  }
+
+  function createFn(value) {
+    return {
+      __proto__: null,
+      configurable: true,
+      enumerable: true,
+      writable: true,
+      value,
+    };
+  }
+
+  ObjectDefineProperties(debounced, {
+    __proto__: null,
+    cancel: createFn(cancel),
+    flush: createFn(flush),
+    ref: createFn(ref),
+    unref: createFn(unref),
+    pending: {
+      __proto__: null,
+      enumerable: false,
+      get() {
+        return timeout === undefined ? null : pendingCalls[pendingCalls.length - 1].promise;
+      },
+    },
+    pendingCount: {
+      __proto__: null,
+      enumerable: false,
+      get() { return pendingCalls.length; },
+    },
+    length: {
+      __proto__: null,
+      configurable: true,
+      value: fn.length,
+    },
+    name: {
+      __proto__: null,
+      configurable: true,
+      value: fn.name,
+    },
+  });
+
+  if (signal !== undefined) {
+    addAbortListener(signal, () => {
+      cancel(signal.reason);
+    });
+  }
+
+  return debounced;
+}
+
+module.exports = debounce;

lib/util.js

@@ -668,6 +668,15 @@ defineLazyProperties(
   ['diff'],
 );
 
+ObjectDefineProperties(module.exports, {
+  debounce: {
+    __proto__: null,
+    configurable: true,
+    enumerable: true,
+    get() { return require('internal/util/debounce'); },
+  },
+});
+
 defineLazyProperties(
   module.exports,
   'internal/util/trace_sigint',