Eth Token Tracker 
A JS module for tracking Ethereum tokens and their values over time.
Installation
yarn install '@metamask/eth-token-tracker'
Usage
const TokenTracker = require('@metamask/eth-token-tracker')
var tokenTracker = new TokenTracker({
userAddress: addresses[0], // whose balance to track
provider, // a web3-style provider
pollingInterval: 4000, // block polling interval (optional)
// Tell it about the tokens to track:
tokens: [
{
address: tokenAddress,
}
],
})
// You can use this method to check the state of the tokens
var balances = tokenTracker.serialize()
// You can also subscribe to updates
tokenTracker.on('update', function (balances) {
console.log(`Your balance of ${balances[0].symbol} is ${balances[0].string}`)
})
// You can add additional tokens after initialization:
tokenTracker.add({ address: otherTokenAddress })
// Make sure to clean up, or it will hold a reference:
tokenTracker.stop()Running tests
yarn test