Call original debounced function with proper "this" context

lib/throttle.js

@@ -33,21 +33,27 @@ const throttle = (timeout, fn, ...args) => {
 };
 
 // Debounce function, delayed execution
-// Signature: timeout, fn, ...args
+// Signature: timeout, fn, ...presetArgs
 //   timeout - <number>, msec
 //   fn - <Function>, to be debounced
-//   args - <Array>, arguments for fn, optional
-const debounce = (timeout, fn, ...args) => {
+//   presetArgs - <Array>, pre-populated arguments for fn, optional
+const debounce = (timeout, fn, ...presetArgs) => {
   let timer;
-
-  const debounced = () => (args ? fn(...args) : fn());
-
-  const wrapped = () => {
-    if (timer) clearTimeout(timer);
-    timer = setTimeout(debounced, timeout);
+  return function (...newArgs) {
+    const args = newArgs.length !== 0 ? newArgs : presetArgs;
+    if (
+      newArgs.length === 0 &&
+      timer !== undefined &&
+      timer.refresh !== undefined
+    ) {
+      // if possible, refresh the timer without
+      // allocating a new JavaScript object
+      timer.refresh();
+    } else {
+      clearTimeout(timer);
+      timer = setTimeout(() => fn.apply(this, args), timeout);
+    }
   };
-
-  return wrapped;
 };
 
 const FN_TIMEOUT = 'Metasync: asynchronous function timed out';

test/examples.js

@@ -399,43 +399,25 @@ metatests.test('trottle', (test) => {
 metatests.test('debounce', (test) => {
   const expectedResult = ['E', 'I'];
   const result = [];
-  let state;
 
-  const fn = () => {
-    result.push(state);
+  const fn = (arg) => {
+    result.push(arg);
   };
 
-  const f1 = metasync.debounce(500, fn, ['I']);
+  const f1 = metasync.debounce(500, fn);
 
   // to be called one time (E)
-  state = 'A';
-  f1();
-  state = 'B';
-  f1();
-  state = 'C';
-  f1();
-  state = 'D';
-  f1();
-  state = 'E';
-  f1();
+  f1('A');
+  f1('B');
+  f1('C');
+  f1('D');
+  f1('E');
 
   // to be called one time (I)
-  setTimeout(() => {
-    state = 'F';
-    f1();
-  }, 600);
-  setTimeout(() => {
-    state = 'G';
-    f1();
-  }, 700);
-  setTimeout(() => {
-    state = 'H';
-    f1();
-  }, 1000);
-  setTimeout(() => {
-    state = 'I';
-    f1();
-  }, 1100);
+  setTimeout(() => f1('F'), 600);
+  setTimeout(() => f1('G'), 700);
+  setTimeout(() => f1('H'), 1000);
+  setTimeout(() => f1('I'), 1100);
 
   setTimeout(() => {
     test.strictSame(result, expectedResult);

test/throttle.js

@@ -67,7 +67,7 @@ metatests.test('throttle without arguments for function', (test) => {
   test.strictSame(callCount, 1);
 });
 
-metatests.test('debounce', (test) => {
+metatests.test('debounce with pre-populated arguments', (test) => {
   let count = 0;
 
   const fn = (arg1, arg2, ...otherArgs) => {
@@ -85,6 +85,22 @@ metatests.test('debounce', (test) => {
   test.strictSame(count, 0);
 });
 
+metatests.test('debounce with arguments', (test) => {
+  let count = 0;
+
+  const fn = (...args) => {
+    test.strictSame(args, [1, 2, 3]);
+    count++;
+    test.end();
+  };
+
+  const debouncedFn = metasync.debounce(1, fn);
+
+  debouncedFn(0, 1, 2);
+  debouncedFn(1, 2, 3);
+  test.strictSame(count, 0);
+});
+
 metatests.test('debounce without arguments for function', (test) => {
   let count = 0;
 
@@ -101,6 +117,30 @@ metatests.test('debounce without arguments for function', (test) => {
   test.strictSame(count, 0);
 });
 
+metatests.test('debounce with proper "this" binding', (test) => {
+  let count = 0;
+  let originalThis;
+
+  class DebounceTest {
+    constructor() {
+      this.debouncedMethod = metasync.debounce(1, this.originalMethod);
+      originalThis = this;
+    }
+
+    originalMethod(...args) {
+      test.strictSame(args, []);
+      test.strictSame(this, originalThis);
+      count++;
+      test.end();
+    }
+  }
+
+  const debounceTestInstance = new DebounceTest();
+  debounceTestInstance.debouncedMethod();
+  debounceTestInstance.debouncedMethod();
+  test.strictSame(count, 0);
+});
+
 metatests.test('timeout with sync function', (test) => {
   const syncFn = (callback) => callback(null, 'someVal');
   metasync.timeout(1, syncFn, (err, res, ...args) => {