Below code snipped will show how to get the string using javascript and escape the special character using escape method.
Snippet:
var
mailtestTitle = 'Title of the email & special character';
var
testurl= "mailto:mailaddress@gyah.com;?" + "subject=" + escape(mailtestTitle)
+ "&Body=some text";
window.location.href = testurl;
This will open the mail box with specified title and body message.
This escape method will escape the special character and make it works
fine without breaking