To detect if a user has already installed Phantom, a web application should check for the existence of a phantom object. Phantom’s browser extension and mobile in-app browser will both inject a phantom object into the window of any web application the user visits, provided that site is using https://, on localhost, or is 127.0.0.1. Phantom will not inject the provider into iframes or sites using http://. If a phantom object exists, EVM dApps can interact with Phantom via the API found at window.phantom.ethereum. This ethereum provider is also made available at window.ethereum but is prone to namespace collisions from other injected wallets. To detect if Phantom is installed, an application should check for an additional isPhantom flag. 
const isPhantomInstalled = window?.phantom?.ethereum?.isPhantom
If Phantom is not installed, we recommend you redirect your users to our website https://phantom.com/. Altogether, this may look like the following:

​
window.phantom

const getProvider = () => {
  if ('phantom' in window) {
    const anyWindow: any = window;
    const provider = anyWindow.phantom?.ethereum;
   
    if (provider) {
      return provider;
    }
  }

  window.open('https://phantom.com/', '_blank');
};

​
window.ethereum

const getProvider = () => {
  if ('ethereum' in window) {
    const anyWindow: any = window;
    const provider = anyWindow.ethereum;
   
    if (provider?.isPhantom) {
      return provider;
    }
  }

  window.open('https://phantom.com/', '_blank');
};
For an example of how a React application can detect Phantom, refer to getProvider in our sandbox.