Use toInteger in pad methods and make nullish chars checks in pad and trim methods undefined checks.

lodash.js

@@ -3341,13 +3341,13 @@
      */
     function createPadding(string, length, chars) {
       var strLength = string.length;
-      length = +length;
+      length = toInteger(length);
 
-      if (strLength >= length || !nativeIsFinite(length)) {
+      if (!length || strLength >= length) {
         return '';
       }
       var padLength = length - strLength;
-      chars = chars == null ? ' ' : (chars + '');
+      chars = chars === undefined ? ' ' : (chars + '');
       return repeat(chars, nativeCeil(padLength / chars.length)).slice(0, padLength);
     }
 
@@ -10018,10 +10018,10 @@
      */
     function pad(string, length, chars) {
       string = baseToString(string);
-      length = +length;
+      length = toInteger(length);
 
       var strLength = string.length;
-      if (strLength >= length || !nativeIsFinite(length)) {
+      if (!length || strLength >= length) {
         return string;
       }
       var mid = (length - strLength) / 2,
@@ -10466,7 +10466,7 @@
       if (!string) {
         return string;
       }
-      if (guard || chars == null) {
+      if (guard || chars === undefined) {
         return string.slice(trimmedLeftIndex(string), trimmedRightIndex(string) + 1);
       }
       chars = (chars + '');
@@ -10497,7 +10497,7 @@
       if (!string) {
         return string;
       }
-      if (guard || chars == null) {
+      if (guard || chars === undefined) {
         return string.slice(trimmedLeftIndex(string));
       }
       return string.slice(charsLeftIndex(string, (chars + '')));
@@ -10527,7 +10527,7 @@
       if (!string) {
         return string;
       }
-      if (guard || chars == null) {
+      if (guard || chars === undefined) {
         return string.slice(0, trimmedRightIndex(string) + 1);
       }
       return string.slice(0, charsRightIndex(string, (chars + '')) + 1);
@@ -10587,7 +10587,7 @@
         return omission;
       }
       var result = string.slice(0, end);
-      if (separator == null) {
+      if (separator === undefined) {
         return result + omission;
       }
       if (isRegExp(separator)) {

test/test.js

@@ -11381,7 +11381,7 @@
     });
 
     test('`_.' + methodName + '` should treat nullish values as empty strings', 6, function() {
-      _.each([null, '_-'], function(chars) {
+      _.each([undefined, '_-'], function(chars) {
         var expected = chars ? (isPad ? '__' : chars) : '  ';
         strictEqual(func(null, 2, chars), expected);
         strictEqual(func(undefined, 2, chars), expected);
@@ -15839,11 +15839,10 @@
       });
     });
 
-    test('`_.' + methodName + '` should work with nullish or empty string values for `chars`', 3, function() {
+    test('`_.' + methodName + '` should work with `undefined` or empty string values for `chars`', 2, function() {
       var string = whitespace + 'a b c' + whitespace,
           expected = (index == 2 ? whitespace : '') + 'a b c' + (index == 1 ? whitespace : '');
 
-      strictEqual(func(string, null), expected);
       strictEqual(func(string, undefined), expected);
       strictEqual(func(string, ''), string);
     });