cryptozombies-lesson-code

Here I organize different concepts, terminologies I learnt during my study of solidity.

Terms

tokens

a smart contract that follows some common rules. all ERC20 tokens share the same set of functions with the same names

ERC-20

fungible tokens (tradable, divisible)

ERC-721

non-fungible tokens (NFT). not interchangeable since each one is assumed to be unique, and are not divisible. You can only trade them in whole units, and each one has a unique ID. eg art piece, game character,

msg.sender

it's a global variable available to all functions - refers to the address of the sender

Wei

the smallest sub-unit of Ether — there are 10^18 wei in one ether

Web3.js

a JavaScript library from the Ethereum Foundation. Ethereum nodes only speak a language called JSON-RPC, which isn't very human-readable. A query to tell the node you want to call a function on a contract looks something like this: {"jsonrpc":"2.0","method":"eth_sendTransaction","params":[{"from":"0xb60e8dd61c5d32be8058bb8eb970870f07233155","to":"0xd46e8dd67c5d32be8058bb8eb970870f07244567","gas":"0x76c0","gasPrice":"0x9184e72a000","value":"0x9184e72a","data":"0xd46e8dd67c5d32be8d46e8dd67c5d32be8058bb8eb970870f072445675058bb8eb970870f072445675"}],"id":1}

call

used for view and pure functions. It only runs on the local node, and won't create a transaction on the blockchain. view and pure functions are read-only and don't change state on the blockchain. They also don't cost any gas, and the user won't be prompted to sign a transaction with MetaMask.

send

creates a transaction and change data on the blockchain. You'll need to use send for any functions that aren't view or pure.

assert vs require

assert is similar to require, where it will throw an error if false. require will refund the user the rest of their gas when a function fails, whereas assert will not. assert is typically used when something has gone horribly wrong with the code (like a uint overflow).

Web3 Provider

Setting a Web3 Provider in Web3.js tells our code which node we should be talking to handle our reads and writes. It's kind of like setting the URL of the remote web server for your API calls in a traditional web app.

Infura

Infura is a service that maintains a set of Ethereum nodes with a caching layer for fast reads, which you can access for free through their API. Using Infura as a provider, you can reliably send and receive messages to/from the Ethereum blockchain without needing to set up and maintain your own node.

Metamask

browser extension for Chrome and Firefox that lets users securely manage their Ethereum accounts and private keys, and use these accounts to interact with websites that are using Web3.js. Metamask uses Infura's servers under the hood as a web3 provider, just like we did above — but it also gives the user the option to choose their own web3 provider. So by using Metamask's web3 provider, you're giving the user a choice, and it's one less thing you have to worry about in your app. Metamask uses Infura's servers under the hood as a web3 provider, just like we did above — but it also gives the user the option to choose their own web3 provider. So by using Metamask's web3 provider, you're giving the user a choice, and it's one less thing you have to worry about in your app.

ABI

ABI stands for Application Binary Interface. Basically it's a representation of your contracts' methods in JSON format that tells Web3.js how to format function calls in a way your contract will understand.

When you compile your contract to deploy to Ethereum (which we'll cover in Lesson 7), the Solidity compiler will give you the ABI,

Ropsten(https://ropsten.etherscan.io/)

A proof-of-work blockchain that most closely resembles Ethereum; you can easily mine faux-Ether.

Kovan

A proof-of-authority blockchain, started by the Parity team. Ether can’t be mined; it has to be requested.

Rinkeby

A proof-of-authority blockchain, started by the Geth team. Ether can’t be mined; it has to be requested.

Types

uint

equivalent to uint256

string

struct

Visibility modifier

private

it's only callable from other functions inside the contract

internal

it's like private, but can also be called by contracts that inherit from this one

external

can only be called outside the contract

public

can be called anywhere, both internally and externally. In Solidity, when you declare a variable public, it automatically creates a public "getter" function with the same name.

indexed

in order to filter events and only listen for changes related to the current user, the Solidity contract would have to use the indexed keyword

State modifier

view

no data is saved/changed. no gas cost if called externally from outside the contract (but do coast gas if called internally by another function)

pure

doesn't read any data from the blockchain. no gas cost if called externally from outside the contract(but do coast gas if called internally by another function)

Custom modifier

define custom logic to determine how they affect a function eg validations in onlyOwner, aboveLevel.

Payable modifier

a special type of function that can receive Ether.