Fixes #437 -- add an 'immediate' parameter to _.debounce.

test/functions.js

@@ -146,6 +146,20 @@ $(document).ready(function() {
     _.delay(function(){ ok(counter == 1, "incr was debounced"); start(); }, 220);
   });
 
+  asyncTest("functions: debounce asap", 2, function() {
+    var counter = 0;
+    var incr = function(){ counter++; };
+    var debouncedIncr = _.debounce(incr, 50, true);
+    debouncedIncr(); debouncedIncr(); debouncedIncr();
+    equal(counter, 1, 'incr was called immediately');
+    setTimeout(debouncedIncr, 30);
+    setTimeout(debouncedIncr, 60);
+    setTimeout(debouncedIncr, 90);
+    setTimeout(debouncedIncr, 120);
+    setTimeout(debouncedIncr, 150);
+    _.delay(function(){ equal(counter, 1, "incr was debounced"); start(); }, 220);
+  });
+
   test("functions: once", function() {
     var num = 0;
     var increment = _.once(function(){ num++; });

underscore.js

@@ -548,15 +548,17 @@
 
   // Returns a function, that, as long as it continues to be invoked, will not
   // be triggered. The function will be called after it stops being called for
-  // N milliseconds.
-  _.debounce = function(func, wait) {
+  // N milliseconds. If `immediate` is passed, trigger the function on the
+  // leading edge, instead of the trailing.
+  _.debounce = function(func, wait, immediate) {
     var timeout;
     return function() {
       var context = this, args = arguments;
       var later = function() {
         timeout = null;
-        func.apply(context, args);
+        if (!immediate) func.apply(context, args);
       };
+      if (immediate && !timeout) func.apply(context, args);
       clearTimeout(timeout);
       timeout = setTimeout(later, wait);
     };