JSON.parse(...).logout is not a function in keycloak

Question

When I going to use keycloak.logout(), it shows a error JSON.parse(...).logout is not a function I added and got keycloak object using sessionStorage.but when I logged it in the console , the logout function was not in the keyloack object.So that the the above error occured.

This is my code to set the keycloak object.

const keycloak = Keycloak('/keycloak.json');
      keycloak.init({onLoad: 'login-required'}).then(authenticated => {
        this.setState({ keycloak: keycloak, authenticated: authenticated })

        sessionStorage.setItem('keycloak', JSON.stringify(keycloak))

This is my code to get the keycloak object. and call logout()

JSON.parse(sessionStorage.getItem('keycloak')).logout()