[Ethereum] How to convert bignumber to normal number using ethers.js

If your token have 18 decimals

like an ether you can use

then(function(balance) {
    console.log(web3.fromWei(balance.toNumber(), "ether" ) );
})

Use fromWei to convert your bigNumber to ether and then convert it to Number. Be aware that unnecessary 0 after the floating point will not be displayed.

In web3 ^1.0 it may change to web3.utils.fromWei instead of web3.fromWei doc. Note that truffle ^5.0 uses web3 ^1.0.

If you have a different token decimal

Let's say 6. You'll have to do the conversion manually with something like:

var tokenBalance = balance.ToNumber() / Math.pow(10, tokenDecimals)

Contract function calls in Truffle return promises so you have to add a callback to get the response. If getAge() is declared as either constant, pure or view in the Solidity contract you can do instance.getAge().then(age => age.toNumber()) if it doesn't have any of those modifiers then you need to do instance.getAge.call().then(age => age.toNumber())