Error when converting array to JSON in JQuery

Question

I have a number of strings:

var a = ['one','two'];
var b = ['three','four'];
var c = ['five'];
var d = ['six','seven','eight']; 
var e = ['nine','ten','eleven'];

a_str = JSON.stringify(a);
b_str = JSON.stringify(b);
c_str = JSON.stringify(c);
d_str = JSON.stringify(d);
e_str = JSON.stringify(e);

console.log(a_str);

From Inspect > Console, I get:

["one","two"]
> typeof(a)
<- "object"
>

How would I use AJAX to pass those variables to the backend (python/Flask)?