Backport #3397

src/Extension/CoreExtension.php

@@ -1233,15 +1233,18 @@ function _twig_escape_js_callback($matches)
         return $shortMap[$char];
     }
 
-    // \uHHHH
-    $char = twig_convert_encoding($char, 'UTF-16BE', 'UTF-8');
-    $char = strtoupper(bin2hex($char));
-
-    if (4 >= \strlen($char)) {
-        return sprintf('\u%04s', $char);
+    $codepoint = mb_ord($char);
+    if (0x10000 > $codepoint) {
+        return sprintf('\u%04X', $codepoint);
     }
 
-    return sprintf('\u%04s\u%04s', substr($char, 0, -4), substr($char, -4));
+    // Split characters outside the BMP into surrogate pairs
+    // https://tools.ietf.org/html/rfc2781.html#section-2.1
+    $u    = $codepoint - 0x10000;
+    $high = 0xD800 | ($u >> 10);
+    $low  = 0xDC00 | ($u & 0x3FF);
+
+    return sprintf('\u%04X\u%04X', $high, $low);
 }
 
 function _twig_escape_css_callback($matches)