Combine surrogate pairs into one escape sequence when encoding. (#369)

lib/js-yaml/dumper.js

@@ -461,11 +461,21 @@ function foldLine(line, width) {
 // Escapes a double-quoted string.
 function escapeString(string) {
   var result = '';
-  var char;
+  var char, nextChar;
   var escapeSeq;
 
   for (var i = 0; i < string.length; i++) {
     char = string.charCodeAt(i);
+    // Check for surrogate pairs (reference Unicode 3.0 section "3.7 Surrogates").
+    if (char >= 0xD800 && char <= 0xDBFF/* high surrogate */) {
+      nextChar = string.charCodeAt(i + 1);
+      if (nextChar >= 0xDC00 && nextChar <= 0xDFFF/* low surrogate */) {
+        // Combine the surrogate pair and store it escaped.
+        result += encodeHex((char - 0xD800) * 0x400 + nextChar - 0xDC00 + 0x10000);
+        // Advance index one extra since we already used that char here.
+        i++; continue;
+      }
+    }
     escapeSeq = ESCAPE_SEQUENCES[char];
     result += !escapeSeq && isPrintable(char)
       ? string[i]