How do i use the regular javascript script externalFile.js to set the background of my header(id=bgCanvas) using the useEffect hook to load the js: externalFile.js:   (function () {
      var requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame || window.msRequestAnimationFrame || function (callback) {
              window.setTimeout(callback, 1000 / 60);
          };
      window.requestAnimationFrame = requestAnimationFrame;
  })();

  // Terrain stuff.
  var background = document.getElementById("bgCanvas"),
      bgCtx = background.getContext("2d"),
      width = window.innerWidth,
      height = document.body.offsetHeight;

  (height < 400) ? height = 400 : height;

  background.width = width;
  background.height = height;

  function Terrain(options) {
      options = options || {};
      this.terrain = document.createElement("canvas");
      this.terCtx = this.terrain.getContext("2d");
      this.scrollDelay = options.scrollDelay || 90;
      this.lastScroll = new Date().getTime();

      this.terrain.width = width;
      this.terrain.height = height;
      this.fillStyle = options.fillStyle || "#191D4C";
      this.mHeight = options.mHeight || height;

      // generate
      this.points = [];

      var displacement = options.displacement || 140,
          power = Math.pow(2, Math.ceil(Math.log(width) / (Math.log(2))));

      // set the start height and end height for the terrain
      this.points[0] = this.mHeight;//(this.mHeight - (Math.random() * this.mHeight / 2)) - displacement;
      this.points[power] = this.points[0];

      // create the rest of the points
      for (var i = 1; i < power; i *= 2) {
          for (var j = (power / i) / 2; j < power; j += power / i) {
              this.points[j] = ((this.points[j - (power / i) / 2] + this.points[j + (power / i) / 2]) / 2) + Math.floor(Math.random() * -displacement + displacement);
          }
          displacement *= 0.6;
      }

      document.body.appendChild(this.terrain);
  }

  Terrain.prototype.update = function () {
      // draw the terrain
      this.terCtx.clearRect(0, 0, width, height);
      this.terCtx.fillStyle = this.fillStyle;
      
      if (new Date().getTime() > this.lastScroll + this.scrollDelay) {
          this.lastScroll = new Date().getTime();
          this.points.push(this.points.shift());
      }

      this.terCtx.beginPath();
      for (var i = 0; i <= width; i++) {
          if (i === 0) {
              this.terCtx.moveTo(0, this.points[0]);
          } else if (this.points[i] !== undefined) {
              this.terCtx.lineTo(i, this.points[i]);
          }
      }

      this.terCtx.lineTo(width, this.terrain.height);
      this.terCtx.lineTo(0, this.terrain.height);
      this.terCtx.lineTo(0, this.points[0]);
      this.terCtx.fill();
  }


  // Second canvas used for the stars
  bgCtx.fillStyle = '#05004c';
  bgCtx.fillRect(0, 0, width, height);

  // stars
  function Star(options) {
      this.size = Math.random() * 2;
      this.speed = Math.random() * .05;
      this.x = options.x;
      this.y = options.y;
  }

  Star.prototype.reset = function () {
      this.size = Math.random() * 2;
      this.speed = Math.random() * .05;
      this.x = width;
      this.y = Math.random() * height;
  }

  Star.prototype.update = function () {
      this.x -= this.speed;
      if (this.x < 0) {
          this.reset();
      } else {
          bgCtx.fillRect(this.x, this.y, this.size, this.size);
      }
  }

  function ShootingStar() {
      this.reset();
  }

  ShootingStar.prototype.reset = function () {
      this.x = Math.random() * width;
      this.y = 0;
      this.len = (Math.random() * 80) + 10;
      this.speed = (Math.random() * 10) + 6;
      this.size = (Math.random() * 1) + 0.1;
      // this is used so the shooting stars arent constant
      this.waitTime = new Date().getTime() + (Math.random() * 3000) + 500;
      this.active = false;
  }

  ShootingStar.prototype.update = function () {
      if (this.active) {
          this.x -= this.speed;
          this.y += this.speed;
          if (this.x < 0 || this.y >= height) {
              this.reset();
          } else {
              bgCtx.lineWidth = this.size;
              bgCtx.beginPath();
              bgCtx.moveTo(this.x, this.y);
              bgCtx.lineTo(this.x + this.len, this.y - this.len);
              bgCtx.stroke();
          }
      } else {
          if (this.waitTime < new Date().getTime()) {
              this.active = true;
          }
      }
  }

  var entities = [];

  // init the stars
  for (var i = 0; i < height; i++) {
      entities.push(new Star({
          x: Math.random() * width,
          y: Math.random() * height
      }));
  }

  // Add 2 shooting stars that just cycle.
  entities.push(new ShootingStar());
  entities.push(new ShootingStar());
entities.push(new Terrain({mHeight : (height/2)-120}));
entities.push(new Terrain({displacement : 120, scrollDelay : 50, fillStyle : "rgb(17,20,40)", mHeight : (height/2)-60}));
  entities.push(new Terrain({displacement : 100, scrollDelay : 20, fillStyle : "rgb(10,10,5)", mHeight : height/2}));

  //animate background
  function animate() {
      bgCtx.fillStyle = '#110E19';
      bgCtx.fillRect(0, 0, width, height);
      bgCtx.fillStyle = '#ffffff';
      bgCtx.strokeStyle = '#ffffff';

      var entLen = entities.length;

      while (entLen--) {
          entities[entLen].update();
      }
      requestAnimationFrame(animate);
  }
  animate();    Header.js: import React from 'react'
import { NavLink } from "react-router-dom";
import styled from "styled-components";
import Nav from "./Nav";


const Header = () => {
  return (
      <MainHeader id="bgCanvas">
        <NavLink to="/">
          <img src="../../Assets/shop_logo.png" width="128" alt="my logo img" className="logo"/>
        </NavLink>
        <Nav />
      </MainHeader>    
  );
};

const MainHeader = styled.header`
  padding: 0 4.8rem;
  height: 10rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
  position: relative;
  border-radius: 0 0 50px 50px;
  width: 100%;
  background: #333;
  overflow: hidden;
  background-size: cover;
  background-position: center center;
  z-index: 1;
}

  .logo {
    height: 5rem;
  }
`;
export default Header;      Example of useEffect hook from documentation: import React, { useEffect } from 'react';

function MyComponent() {
  useEffect(() => {
    const script = document.createElement('script');

    script.src = "https://externalwebsite.com/externalFile.js";
    script.async = true;

    document.body.appendChild(script);

    return () => {
      document.body.removeChild(script);
    }
  }, []);

  return (
    // JSX goes here
  );
}