highly efficient solution

index.js

@@ -1,17 +1,23 @@
 module.exports = leftpad;
 
 function leftpad (str, len, ch) {
-  str = String(str);
+  //convert the `str` to String
+  str = str +''; 
 
-  var i = -1;
+  //needn't to pad
+  len = len - str.length;
+  if (len <= 0) return str;
 
+  //convert the `ch` to String
   if (!ch && ch !== 0) ch = ' ';
+  ch = ch + ''; 
 
-  len = len - str.length;
-
-  while (++i < len) {
-    str = ch + str;
+  var pad = '';
+  while (true) {
+    if (len & 1) pad += ch;
+    len >>= 1;
+    if (len) ch += ch;
+    else break;
   }
-
-  return str;
+  return pad + str;
 }

package.json

@@ -4,9 +4,11 @@
   "description": "String left pad",
   "main": "index.js",
   "scripts": {
-    "test": "node test"
+    "test": "node test",
+    "bench": "node perf_test.js"
   },
   "devDependencies": {
+    "benchmark": "^2.1.0",
     "tape": "*"
   },
   "keywords": [

perf_test.js

@@ -0,0 +1,69 @@
+function leftpad_orginal (str, len, ch) {
+  str = String(str);
+
+  var i = -1;
+
+  if (!ch && ch !== 0) ch = ' ';
+
+  len = len - str.length;
+
+  while (++i < len) {
+    str = ch + str;
+  }
+
+  return str;
+}
+
+
+function leftpad_es6_repeat(str, len, ch) {
+  str = String(str);
+  if (!ch && ch !== 0) ch = ' ';
+  var l = len - str.length; 
+  if (l > 0) return ch.repeat(l)+str;
+  return str;
+}
+
+
+
+function leftpad_bit_ops(str, len, ch) {
+  str = String(str);
+
+  if (!ch && ch !== 0) ch = ' ';
+
+  len = len - str.length;
+  if (len <= 0) return str;
+
+  ch = ch + '';
+  var pad = '';
+  while (true) {
+    if (len & 1) pad += ch;
+    len >>= 1;
+    if (len) ch += ch;
+    else break;
+  }
+  return pad + str;
+}
+
+
+var Benchmark = require('benchmark');
+
+var t_str = "abcd"
+var t_len = 100;
+
+var suite01 = new Benchmark.Suite;
+suite01.add('Long - Original',        function() {    leftpad_orginal(t_str, t_len, ' ');})
+       .add('Long - ES6 Repeat',      function() { leftpad_es6_repeat(t_str, t_len, ' ');})
+       .add('Long - Bit Operation',   function() {    leftpad_bit_ops(t_str, t_len, ' ');})
+       .on('cycle', function(event) { console.log(String(event.target)); })
+       .on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); })
+       .run();
+
+t_len=10;
+var suite02 = new Benchmark.Suite;
+suite02.add('Normal - Original',      function() {    leftpad_orginal(t_str, t_len, ' ');})
+       .add('Normal - ES6 Repeat',    function() { leftpad_es6_repeat(t_str, t_len, ' ');})
+       .add('Normal - Bit Operation', function() {    leftpad_bit_ops(t_str, t_len, ' ');})
+       .on('cycle', function(event) { console.log(String(event.target)); })
+       .on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); })
+       .run();
+

test.js

@@ -2,9 +2,12 @@ var leftpad = require("./");
 var test = require("tape");
 
 test('left pad', function (assert) {
-  assert.plan(4);
+  assert.plan(7);
   assert.strictEqual(leftpad('foo', 5), '  foo');
   assert.strictEqual(leftpad('foobar', 6), 'foobar');
   assert.strictEqual(leftpad(1, 2, 0), '01');
   assert.strictEqual(leftpad(1, 2, '-'), '-1');
+  assert.strictEqual(leftpad('foo', 2, ' '), 'foo');
+  assert.strictEqual(leftpad('foo', -1, ' '), 'foo');
+  assert.strictEqual(leftpad('foo', 7, 1), '1111foo');
 });