How to convert a regular checkout id to Storefront CheckoutID?

[Atiqur_Rahman]

So, I was trying to addLineItem to checkout using storefront api on a regular checkout. What I tried is - 

var ck_id = "{{ checkout.id }}";
ck_id = btoa('gid://shopify/Checkout/'+ck_id);

But in response it says, "Checkout doesn't exist" which means my checkout id conversion isn't really working. Any thoughts?