how to get csrf token in javascript

<input type="hidden" name="csrftoken" value="hgdillksdbgjksdbkvbskb">
csrftoken = document.mainform.csrftoken.value;
# Do something with the CSRF token, like add dynamic values, like sha256(csrftoken + "dynamicvalue");
document.mainform.csrftoken.value = csrftoken;