escape.js

// 0.

'<script><\/script>' // deprecated
'<' + 'script>' + '<' + '/script>' // trash

// 1.

var script = document.createElement('script');
script.textContent = 'var foo = 1';

var serializer = new XMLSerializer();
var string = serializer.serializeToString(script);

alert(string); // <script>var foo = 1</script>

// 2.
var escape_slash = function(value) {
   return value.replace(/<|>/g, function(chars, value) {
      return chars[value];
   }
   .bind(null, {
        '<' : '\x3C',
        '>' : '\x3E'
   }));
};

alert(escape_slash('<script>var foo = 1</script>')); // <script>var foo = 1</script>