encoding struct in polkadot.js api

Question

I have a question regarding the frontend polkadot.js api. Suppose I have a struct

pub struct B {
    b: i32,
}
pub struct A {
    a: Vec<u8>,
    b: B,
}

I want to encode struct A into byte array and then pass it in substrate dispatch call, then I will decode it from byte array. What is the api to achieve this?

In short:

 frontend in polkadot.js api: encode structure to Vec<u8>
 backend in substrate module dispatch call: decode Vec<u8> back to struct.

I didn't find the corresponding APIs and examples on polkadot.js document. Thanks!