diff --git a/underscore.js b/underscore.js
index 31b07a4de..8d9748936 100644
--- a/underscore.js
+++ b/underscore.js
@@ -483,7 +483,7 @@
 
   // Is a given value an array?
   _.isArray = function(obj) {
-    return obj && obj.concat && obj.unshift;
+    return !!(obj && obj.concat && obj.unshift);
   };
 
   // Is a given variable an arguments object?
@@ -493,12 +493,12 @@
 
   // Is a given value a function?
   _.isFunction = function(obj) {
-    return obj && obj.constructor && obj.call && obj.apply;
+    return !!(obj && obj.constructor && obj.call && obj.apply);
   };
 
   // Is a given value a string?
   _.isString = function(obj) {
-    return obj === '' || (obj && obj.charCodeAt && obj.substr);
+    return !!(obj === '' || (obj && obj.charCodeAt && obj.substr));
   };
 
   // Is a given value a number?
@@ -508,12 +508,12 @@
 
   // Is a given value a date?
   _.isDate = function(obj) {
-    return obj && obj.getTimezoneOffset && obj.setUTCFullYear;
+    return !!(obj && obj.getTimezoneOffset && obj.setUTCFullYear);
   };
 
   // Is the given value a regular expression?
   _.isRegExp = function(obj) {
-    return obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false);
+    return !!(obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false));
   };
 
   // Is the given value NaN -- this one is interesting. NaN != NaN, and