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MetaMask is a crypto wallet that can be used in a web browser and on mobile devices to interact with the Ethereum blockchain. It allows you to run Ethereum Dapps (Decentralized Apps) right in your browser without running a full Ethereum node.

Type: Non-custodial/HD
Private Key Storage: User’s local browser storage
Communication to Ethereum Ledger: Infura
Private key encoding: Mnemonic


Please Backup your Secret Recovery Phrase. If your device breaks, is lost, stolen, or has data corruption, there is no other way to recover it. The Secret Recovery Phrase is the only way to recover your MetaMask accounts. Check more Basic Safety and Security Tips for MetaMask.

Guide to set up MetaMask for Polygon

1. Set up Web3

Step 1.1

Install the following in your DApp:

npm install --save web3

Create a new file, name it web3.js and insert the following code in it:

import Web3 from 'web3';

const getWeb3 = () => new Promise((resolve) => {
  window.addEventListener('load', () => {
    let currentWeb3;

    if (window.ethereum) {
      currentWeb3 = new Web3(window.ethereum);
      try {
        // Request account access if needed
        // Acccounts now exposed
      } catch (error) {
        // User denied account access...
        alert('Please allow access for the app to work');
    } else if (window.web3) {
      window.web3 = new Web3(web3.currentProvider);
      // Acccounts always exposed
    } else {
      console.log('Non-Ethereum browser detected. You should consider trying MetaMask!');

export default getWeb3;

The above file exports a function called getWeb3() - the purpose of which is to request MetaMask account’s access via detecting a global object (ethereum or web3) injected by Metamask.

According to Metamask’s API documentation:

MetaMask injects a global API into websites visited by its users at window.ethereum. This API allows websites to request users’ Ethereum accounts, read data from blockchains the user is connected to, and suggest that the user sign messages and transactions. The presence of the provider object indicates an Ethereum user.

In simpler terms, it basically means that having Metamask’s extension/add-on installed in your browser, you’d have a global variable defined, called ethereum (web3 for older versions), and using this variable we instantiate our web3 object.

Step 1.2

Now, in your client code, import the above file:

  import getWeb3 from '/path/to/web3';

and call the function:

    .then((result) => {
      this.web3 = result;// we instantiate our contract next

2. Set up account

Now to send transactions (specifically those that alter the state of the blockchain) we’ll need an account to sign those transactions. We instantiate our contract instance from the web3 object we created above:

  .then((accounts) => {
    this.account = accounts[0];

The getAccounts() function returns an array of all the accounts on user’s MetaMask, and accounts[0] is the one currently selected by the user.

3. Instantiate your contracts

Once we have our web3 object in place, we’ll next instantiate our contracts, assuming you have your contract ABI and address already in place:

  const myContractInstance = new this.web3.eth.Contract(myContractAbi, myContractAddress)

4. Call functions

Now for any function you’d want to call from your contract, we directly interact with our instantiated contract object (which is myContractInstance declared in Step 2).


  • Functions that alter the state of the contract are called send() functions.
  • Functions that do not alter the state of the contract are called call() functions.

Calling call() Functions

  .then (
    // do stuff with returned values

Calling send() Functions

    from: this.account,gasPrice: 0
  .then (
    (receipt) => {
      // returns a transaction receipt}

Last update: December 22, 2023
Authors: kmurphypolygon (99.28%), Nadim Kobeissi (0.72%)