diff --git a/src/core/core.js b/src/core/core.js
index 689078433..13f64454c 100644
--- a/src/core/core.js
+++ b/src/core/core.js
@@ -291,11 +291,13 @@ var PDFDocument = (function PDFDocumentClosure() {
   function find(stream, needle, limit, backwards) {
     var pos = stream.pos;
     var end = stream.end;
-    var str = '';
+    var strBuf = [];
     if (pos + limit > end)
       limit = end - pos;
-    for (var n = 0; n < limit; ++n)
-      str += String.fromCharCode(stream.getByte());
+    for (var n = 0; n < limit; ++n) {
+      strBuf.push(String.fromCharCode(stream.getByte()));
+    }
+    var str = strBuf.join('');
     stream.pos = pos;
     var index = backwards ? str.lastIndexOf(needle) : str.indexOf(needle);
     if (index == -1)
diff --git a/src/core/fonts.js b/src/core/fonts.js
index 1d92856fa..d27cd7b57 100644
--- a/src/core/fonts.js
+++ b/src/core/fonts.js
@@ -2382,11 +2382,11 @@ var Font = (function FontClosure() {
   }
 
   function arrayToString(arr) {
-    var str = '';
-    for (var i = 0, ii = arr.length; i < ii; ++i)
-      str += String.fromCharCode(arr[i]);
-
-    return str;
+    var strBuf = [];
+    for (var i = 0, ii = arr.length; i < ii; ++i) {
+      strBuf.push(String.fromCharCode(arr[i]));
+    }
+    return strBuf.join('');
   }
 
   function int16(bytes) {
@@ -2801,10 +2801,11 @@ var Font = (function FontClosure() {
     for (var i = 0, ii = strings.length; i < ii; i++) {
       var str = proto[1][i] || strings[i];
 
-      var strUnicode = '';
-      for (var j = 0, jj = str.length; j < jj; j++)
-        strUnicode += string16(str.charCodeAt(j));
-      stringsUnicode.push(strUnicode);
+      var strBufUnicode = [];
+      for (var j = 0, jj = str.length; j < jj; j++) {
+        strBufUnicode.push(string16(str.charCodeAt(j)));
+      }
+      stringsUnicode.push(strBufUnicode.join(''));
     }
 
     var names = [strings, stringsUnicode];
diff --git a/src/shared/util.js b/src/shared/util.js
index 343ac14e3..6ffd41985 100644
--- a/src/shared/util.js
+++ b/src/shared/util.js
@@ -386,11 +386,12 @@ var XRefParseException = (function XRefParseExceptionClosure() {
 
 
 function bytesToString(bytes) {
-  var str = '';
+  var strBuf = [];
   var length = bytes.length;
-  for (var n = 0; n < length; ++n)
-    str += String.fromCharCode(bytes[n]);
-  return str;
+  for (var n = 0; n < length; ++n) {
+    strBuf.push(String.fromCharCode(bytes[n]));
+  }
+  return strBuf.join('');
 }
 
 function stringToBytes(str) {
@@ -715,19 +716,20 @@ var PDFStringTranslateTable = [
 ];
 
 function stringToPDFString(str) {
-  var i, n = str.length, str2 = '';
+  var i, n = str.length, strBuf = [];
   if (str[0] === '\xFE' && str[1] === '\xFF') {
     // UTF16BE BOM
-    for (i = 2; i < n; i += 2)
-      str2 += String.fromCharCode(
-        (str.charCodeAt(i) << 8) | str.charCodeAt(i + 1));
+    for (i = 2; i < n; i += 2) {
+      strBuf.push(String.fromCharCode(
+        (str.charCodeAt(i) << 8) | str.charCodeAt(i + 1)));
+    }
   } else {
     for (i = 0; i < n; ++i) {
       var code = PDFStringTranslateTable[str.charCodeAt(i)];
-      str2 += code ? String.fromCharCode(code) : str.charAt(i);
+      strBuf.push(code ? String.fromCharCode(code) : str.charAt(i));
     }
   }
-  return str2;
+  return strBuf.join('');
 }
 
 function stringToUTF8String(str) {