Name: Connect MetaMask wallet #JavaScript
Rating: 1.3 (3065 reviews)
Author: inancakduvan

We will connect MetaMask wallet and get accounts with javascript basically.

Try demo here:
https://inancakduvan.github.io/connect-metamask-wallet/

Find source code on github:
https://github.com/inancakduvan/connect-metamask-wallet

1) Check if MetaMask plugin installed on browser
If not, redirect user to MetaMask download page.

const connectButton = document.getElementById("connectButton");

connectButton.addEventListener("click", () => {
   if (typeof window.ethereum !== "undefined") {
       console.log("MetaMask installed")
   } else {
       window.open("https://metamask.io/download/", "_blank");
   }
})

2) Get accounts if MetaMask installed
MetaMask plugin will automatically open when we send a request to get accounts.

const connectButton = document.getElementById("connectButton");
const walletID = document.getElementById("walletID");

if (typeof window.ethereum !== "undefined") {
    ethereum
      .request({ method: "eth_requestAccounts" })
      .then((accounts) => {
        const account = accounts[0]

        walletID.innerHTML = `Wallet connected: ${account}`;
      })
} else {
      window.open("https://metamask.io/download/", "_blank");
}

3) Handle errors
Handling errors is a good practise to guide the user correctly.
(For example: show a loader while user is connecting wallet and hide it if user cancel process)

const connectButton = document.getElementById("connectButton");
const walletID = document.getElementById("walletID");

connectButton.addEventListener("click", () => {
   if (typeof window.ethereum !== "undefined") {
        ethereum
         .request({ method: "eth_requestAccounts" })
         .then((accounts) => {
           const account = accounts[0]

           walletID.innerHTML = `Wallet connected: ${account}`;
        }).catch((error) => {
           // Handle error
           console.log(error, error.code);

           // 4001 - The request was rejected by the user
           // -32602 - The parameters were invalid
           // -32603- Internal error
       });
   } else {
       window.open("https://metamask.io/download/", "_blank");
   }
})

It is done here, we can say. But if you are interested in some extra suggestions, keep reading:

EXTRA SUGGESTIONS

4) Show a loader while user is connecting wallet via plugin

const connectButton = document.getElementById("connectButton");
const walletID = document.getElementById("walletID");

connectButton.addEventListener("click", () => {
   // Start loader while connecting
   connectButton.classList.add("loadingButton");

   if (typeof window.ethereum !== "undefined") {
      ethereum
        .request({ method: "eth_requestAccounts" })
        .then((accounts) => {
           const account = accounts[0]

           walletID.innerHTML = `Wallet connected: ${account}`;

           // Stop loader when connected
           connectButton.classList.remove("loadingButton");
      }).catch((error) => {
        // Handle error
        console.log(error, error.code);

        // Stop loader if error occured

        connectButton.classList.remove("loadingButton");

        // 4001 - The request was rejected by the user
        // -32602 - The parameters were invalid
        // -32603- Internal error
      });
   } else {
      window.open("https://metamask.io/download/", "_blank");
   }
})

5) Show a "Reload page" warning when user is redirected to download page
User needs to refresh page after plugin installed and so browser can re-fetch scripts. It would be a good practise to let user know about this.

const connectButton = document.getElementById("connectButton");
const walletID = document.getElementById("walletID");
const reloadButton = document.getElementById("reloadButton");
const installAlert = document.getElementById("installAlert");

connectButton.addEventListener("click", () => {
   // Start loader while connecting
   connectButton.classList.add("loadingButton");

   if (typeof window.ethereum !== "undefined") {
      ethereum
        .request({ method: "eth_requestAccounts" })
        .then((accounts) => {
           const account = accounts[0]

           walletID.innerHTML = `Wallet connected: ${account}`;

           // Stop loader when connected
           connectButton.classList.remove("loadingButton");
      }).catch((error) => {
        // Handle error
        console.log(error, error.code);

        // Stop loader if error occured
        // For example, when user cancelled request 
        // and closed plugin
        connectButton.classList.remove("loadingButton");

        // 4001 - The request was rejected by the user
        // -32602 - The parameters were invalid
        // -32603- Internal error
      });
   } else {
      window.open("https://metamask.io/download/", "_blank");

      // Show 'Reload page' warning to user
      installAlert.classList.add("show");
   }
})

// Reload the page on reload button click
reloadButton.addEventListener("click", () => {
  window.location.reload();
});

6) Check if user is on mobile device
If user is on mobile device, MetaMask application's browser should be used to be connected.

const connectButton = document.getElementById("connectButton");
const walletID = document.getElementById("walletID");
const reloadButton = document.getElementById("reloadButton");
const installAlert = document.getElementById("installAlert");
const mobileDeviceWarning = document.getElementById("mobileDeviceWarning");

connectButton.addEventListener("click", () => {
      // Start loader while connecting
      connectButton.classList.add("loadingButton");

      if (typeof window.ethereum !== "undefined") {
        ethereum
          .request({ method: "eth_requestAccounts" })
          .then((accounts) => {
             const account = accounts[0]

             walletID.innerHTML = `Wallet connected: ${account}`;

             // Stop loader when connected
             connectButton.classList.remove("loadingButton");
         }).catch((error) => {
           // Handle error
           console.log(error, error.code);

           // Stop loader if error occured
           // For example, when user cancelled request 
           // and closed plugin
           connectButton.classList.remove("loadingButton");

           // 4001 - The request was rejected by the user
           // -32602 - The parameters were invalid
           // -32603- Internal error
         });
      } else { 
          // Find the function which checks if device is mobile
          // at my github repo or anywhere
         if (isMobile()) {
           mobileDeviceWarning.classList.add("show");
         } else {
           window.open("https://metamask.io/download/", "_blank");

           // Show 'Reload page' warning to user
           installAlert.classList.add("show");
         }
      }
})

// Reload the page on reload button click
reloadButton.addEventListener("click", () => {
  window.location.reload();
});

Here is html and css:

 <div class="container">
      <button class="button" id="connectButton">
        Connect wallet

        <span id="loading">
         <span>&bull;</span><span>&bull;</span><span>&bull;</span> 
        </span>
      </button>

      <div id="walletID">Your wallet is not connected yet.</div>

     <div id="mobileDeviceWarning">
        If you are on a mobile phone, please use MetaMask 
        application's browser to connect.
     </div>
 </div>

<div class="alert" id="installAlert">
  Please refresh your browser after installing the Metamask plugin

  <button class="button" id="reloadButton">Reload page</button>
</div>

.container {
    position: relative;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    width: 100%;
    height: 100vh;
}

.button {
    position: relative;
    display: flex;
    align-items: center;
    justify-content: center;
    border: none;
    outline: none;
    box-shadow: 0 1px 5px 0 #e6e9ef;
    background: #ffffff;
    color: #000;
    border-radius: 99px;
    padding: 16px 32px;
    font-family: 'Quicksand', sans-serif;
    font-size: 24px;
    font-weight: 500;
    cursor: pointer;
}

#connectButton {
    margin-bottom: 16px;
    color: #eb874b;
}

#walletID span{
    max-width: 90%;
    margin: 0 auto;
    font-size: 12px;
    font-weight: 600;
}

#mobileDeviceWarning {
    display: none;
    margin-top: 24px;
    color: rgb(247, 47, 47);
}

#mobileDeviceWarning.show {
    display: block;
}

.alert {
    z-index: 10;
    position: fixed;
    left: 0;
    top: 0;
    display: none;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    width: 100%;
    height: 100vh;
    background: #ffffff;
}

.alert.show {
    display: flex;
}

.alert .button {
    margin-top: 24px;
}

/* Loading dots styles */
@keyframes opacity {
    0% { opacity: 1; }
    100% { opacity: 0; }
}

.loadingButton {
    pointer-events: none;
}

.loadingButton #loading {
    display: flex;
}

#loading {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: none;
    align-items: center;
    justify-content: center;
    background: #fff;
    border-radius: 99px;
}

#loading span:not(:last-child) {
    margin-right: 5px;
}

#loading span {
    animation-name: opacity;
    animation-duration: 1s;
    animation-iteration-count: infinite;
}

#loading span:nth-child(2) {
    animation-delay: 100ms;
    animation-delay: 100ms;
}

#loading span:nth-child(3) {
    animation-delay: 300ms;
    animation-delay: 300ms;
}

Find source code on github:
https://github.com/inancakduvan/connect-metamask-wallet

Follow me on:

Github: https://github.com/inancakduvan/
Twitter: https://twitter.com/InancAkduvan

Thank you for reading 🙂