问使用fetch api发送get请求失败

EN

componentDidMount() {
    // fetch all categories from the server
    fetch('http:localhost:5555/categories')
        .then(res => res.json())
        .then(categories => {
            this.setState({ loading: false });
            console.log(categories);
            this.setState({ categories });
        })
        .catch(err => {
            this.setState({ loading: false });
            console.log(err);
        });
}