Function returning undefined when using Github API

I am using Github API to return the sha of a branch by branch name. Why is my function returning undefined?

const branches = (b) => {
        let theName;
        octokit.request(`GET /repos/owner/repo/branches`, {
            owner: "owner",
            repo: "repo"
        }).then(response => {
            response.data.forEach(d => {
                if (d.name === b)  theName = d.commit.sha;
            });
        });
        return theName;
    }
    console.log(branch('main'));

What am I missing here? How do I get my function to return the value stored in theName.