Add .isPending property to the debounced function (#44)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -7,6 +7,7 @@ Set the `immediate` option to `true` to execute the function immediately at the
 
 The returned function has the following methods:
 
+- `.isPending` indicates whether the debounce delay is currently active.
 - `.clear()` cancels any scheduled executions.
 - `.flush()` if an execution is scheduled then it will be immediately executed and the timer will be cleared.
 - `.trigger()` executes the function immediately and clears the timer if it was previously set.
@@ -20,6 +21,7 @@ declare function debounce<F extends AnyFunction>(
 declare namespace debounce {
 	type DebouncedFunction<F extends AnyFunction> = {
 		(...arguments_: Parameters<F>): ReturnType<F> | undefined;
+		readonly isPending: boolean;
 		clear(): void;
 		flush(): void;
 		trigger(): void;

index.js

@@ -65,6 +65,12 @@ function debounce(function_, wait = 100, options = {}) {
 		return result;
 	};
 
+	Object.defineProperty(debounced, 'isPending', {
+		get() {
+			return timeoutId !== undefined;
+		},
+	});
+
 	debounced.clear = () => {
 		if (!timeoutId) {
 			return;

readme.md

@@ -23,6 +23,12 @@ window.onresize = debounce(resize, 200);
 
 *(You can also use `const debounce = require('debounce')`)*
 
+To check if the debounce delay is currently active:
+
+```js
+window.onresize.isPending;
+```
+
 To later clear the timer and cancel currently scheduled executions:
 
 ```js
@@ -51,6 +57,7 @@ Set the `immediate` option to `true` to execute the function immediately at the
 
 The returned function has the following methods:
 
+- `.isPending` indicates whether the debounce delay is currently active.
 - `.clear()` cancels any scheduled executions.
 - `.flush()` if an execution is scheduled then it will be immediately executed and the timer will be cleared.
 - `.trigger()` executes the function immediately and clears the timer if it was previously set.

test.js

@@ -449,3 +449,15 @@ test('calling the trigger should not affect future function calls', async () =>
 
 	clock.restore();
 });
+
+test('should correctly handle debounce state transitions', async () => {
+	const callback = sinon.spy();
+	const fn = debounce(callback, 100);
+	assert.strictEqual(fn.isPending, false, 'isPending should be false for first execution');
+
+	fn();
+	assert.strictEqual(fn.isPending, true, 'isPending should be true within debounce delay');
+
+	fn.trigger();
+	assert.strictEqual(fn.isPending, false, 'isPending should be false after debounce triggered');
+});