To develop for MetaMask, install MetaMask in the browser of your choice on your development machine. Download here (opens new window).
A quick note...
This guide assumes intermediate knowledge of HTML, CSS, and JavaScript.
Once MetaMask is installed and running (make sure you back up your Secret Recovery Phrase), you should find that new browser tabs have a window.ethereum object available in the developer console.This is how your website will interact with MetaMask.
You can review the full API for that object here.Note that over the course of 2020, we introduced significant changes to this API, and we recommend that you refer to its documentation.
# Basic Considerations
# Web3 Browser Detection
To verify if the browser is running MetaMask, copy and paste the code snippet below in the developer console of your web browser:
if (typeof window.ethereum !== 'undefined') { console.log('MetaMask is installed!');}You can review the full API for the window.ethereum object here.
# Running a Test Network
In the top right menu of MetaMask, select the network that you are currently connected to. Among several popular defaults, you'll find Custom RPC and Localhost 8545. These are both useful for connecting to a test blockchain, like ganache (opens new window). You can quickly install and start Ganache if you have npm installed with npm i -g ganache-cli && ganache-cli.
Ganache has some great features for starting your application with different states. If your application starts with the -m flag, you can feed it the same seed phrase you have in your MetaMask, and the test network will give each of your first 10 accounts 100 test ether, which makes it easier to start work.
Since your seed phrase has the power to control all your accounts, it is probably worth keeping at least one seed phrase for development, separate from any that you use for storing real value. One easy way to manage multiple seed phrases with MetaMask is with multiple browser profiles, each of which can have its own clean extension installations.
# Resetting Your Local Nonce Calculation
If you're running a test blockchain and restart it, you can accidentally confuse MetaMask because it calculates the next noncebased on both the network state and the known sent transactions.
To clear MetaMask's transaction queue, and effectively reset its nonce calculation, you can use the Reset Account button in Settings (available in the top-right identicon menu).
# Detecting MetaMask
If you want to differentiate MetaMask from other Ethereum-compatible browsers, you can detect MetaMask using ethereum.isMetaMask.
# User State
Currently there are a few stateful things to consider when interacting with this API:
- What is the current network?
- What is the current account?
Both of these are available synchronously as ethereum.networkVersion and ethereum.selectedAddress.You can listen for changes using events too, see (the API reference).
# Connecting to MetaMask
"Connecting" or "logging in" to MetaMask effectively means "to access the user's Ethereum account(s)".
You should only initiate a connection request in response to direct user action, such as clicking a button.You should always disable the "connect" button while the connection request is pending.You should never initiate a connection request on page load.
We recommend that you provide a button to allow the user to connect MetaMask to your dapp.Clicking this button should call the following method:
ethereum.request({ method: 'eth_requestAccounts' });Example:
This promise-returning function resolves with an array of hex-prefixed Ethereum addresses, which can be used as general account references when sending transactions.
Over time, this method is intended to grow to include various additional parameters to help your site request everything it needs from the user during setup.
Since it returns a promise, if you're in an async function, you may log in like this:
const accounts = await ethereum.request({ method: 'eth_requestAccounts' });const account = accounts[0];// We currently only ever provide a single account,// but the array gives us some room to grow.Example:
# Choosing a Convenience Library
Convenience libraries exist for a variety of reasons.
Some of them simplify the creation of specific user interface elements, some entirely manage the user account onboarding, and others give you a variety of methods of interacting with smart contracts, for a variety of API preferences, from promises, to callbacks, to strong types, and so on.
The provider API itself is very simple, and wrapsEthereum JSON-RPC (opens new window)formatted messages, which is why developers usually use a convenience library for interactingwith the provider, like ethers (opens new window), web3.js (opens new window),truffle (opens new window), Embark (opens new window), or others. From those tools,you can generally find sufficient documentation to interact with the provider, without reading this lower-level API.
