jsSHA default export and flow

hide.js

/* @flow */
import jsSHA from 'jssha';

export function mkHideLines(getSecret: () => string) : (pt: string) => string {
    return plainText => {
        var secret = getSecret();
        return plainText
            .split("\n")
            .map(line => hmac(line, secret))
            .join("\n");
    };
}

function hmac(plainText, secret) {
    var sha = new jsSHA(plainText, "TEXT");
    return sha.getHMAC(secret, "TEXT", "SHA-256", "HEX");
}

interfaces_jssha.js

declare module jssha {
    declare class jsSHA {
        getHMAC: (secret: string, ty: string,
                  alg: string, out: string) => string;
    }

    declare var exports = typeof jsSHA;
}