Geo.js

const key = "b8a59b301233a16e12819fc3b17ebc57";

const showPosition = position => {
  updateByGeo(position.coords.latitude, position.coords.longitude);
};

export const updateByGeo = (lat, lon) => {
  let url =
    "https://api.openweathermap.org/data/2.5/forecast?" +
    "lat=" +
    lat +
    "&lon=" +
    lon +
    "&APPID=" +
    key;
  return url;
};

window.addEventListener("load", () => {
  if (navigator.geolocation) {
    navigator.geolocation.getCurrentPosition(showPosition);
  } else {
    alert("No internet connection");
  }
});

Weather

import React, { Component } from "react";
import { updateByGeo } from "./Geo";

class Weather extends Component {
  constructor(props) {
    super(props);
    this.state = {
      city: "London",
      url:
        "https://api.openweathermap.org/data/2.5/forecast?lat=56&lon=14&APPID=b8a59b301233a16e12819fc3b17ebc57"
    };
  }

  sendRequest = async url => {
    const link = await fetch(url);
    const data = await link.json();
    let city = data.city.name;
    this.setState({ city });
  };

  async componentDidMount() {
    this.sendRequest(updateByGeo);
  }
  render() {
    return (
      <div>
        /* content */
      </div>
    );
  }
}

export default Weather;