validateInput = () => {
        var emailError = '';
        var passwordError = '';

        // password check
        if (this.state.password === '') {
            passwordError = `Password shouldn't be empty`;
        } else if (this.state.password.length < 6) {
            passwordError = `Password should containt at least 6 characters`;
        }

        // email check
        if (this.state.email === '') {
            emailError = `Email shouldn't be empty`;
        } else if(!validateEmail(this.state.email)) {
            emailError = `Wrong format of email address`;
        }

        if(emailError !== '' || passwordError !== '') {
            this.setState({
                emailError,
                passwordError
            }, () => {
                return false;
            });
        } else {
            return true;
        }

    }