How would one store crypto data on client side persistently

Question

I am using libsignal for some personal project. The library says i need to store the generated identity key pair for each client on the client side. But considering development in a web browser, the only place this could happen is localstorage.

But considering that the Identity key pair generated is in the form of

// keyPair -> { pubKey: ArrayBuffer, privKey: ArrayBuffer }
// Store identityKeyPair somewhere durable and safe.

Localstorage only stores strings, and objects need to be 'stringify'ed.

Could anyone give insight as to how to would be done or how clients like whatsapp do it.