JavaScript: replace XML special characters

    XmlHelper = {};
  
    XmlHelper.getExp = function(str)
    {
        return new RegExp(str, 'g');
    }

    XmlHelper.encodeInvalidChars = function(str) {
        return str
                .replace(this.getExp('&'),'&')
                .replace(this.getExp('<'),'&lt;')
                .replace(this.getExp('>'),'&gt;')
                .replace(this.getExp('\''),'&apos;')
                .replace(this.getExp('"'),'&quot;')
    }
    XmlHelper.decodeInvalidChars = function(str) {
        return str
                .replace(this.getExp('&amp;'),'&')
                .replace(this.getExp('&lt;'),'<')
                .replace(this.getExp('&gt;'),'>')
                .replace(this.getExp('&apos;'),'\'')
                .replace(this.getExp('&quot;'),'"')
    }

C#: replace XML special characters

System.Security.SecurityElement.Escape(string str);

1. 
   & - &
   
2. 
   < - &lt;
   
3. 
   > - &gt;
   
4. 
   " - &quot;
   
5. 
   ' - &apos;