Solidity Fast Track: Learn Solidity Fast

'Learn X in Y minutes' this time with X = Solidity 0.7 and Y = 20

You might be familiar with the Learn X in Y minutes. For example you could learn JavaScript in 20 minutes at Unfortunately there is no equivalent for Solidity, but this is about to change.

Do you have 20 minutes to learn all of the basics? We even include the required blockchain basics.

Fast Meme

So we have no time to loose. Let's start.

What is Bitcoin? (Blockchain 101)

Bitcoin fundamentally is a digital currency, but there is one major issue with any digital currency. It can be copied an infinite amount of times by anyone. This is what Bitcoin solves.

Technically a 'Bitcoin' doesn't actually exist. It's not an actual coin someone owns on their hard drive. Rather it's a long list of transactions. The first transaction for a created Bitcoin comes from mining, the details here are not important now, just know that a miner is rewarded for doing hard work.

So to obtain Bitcoin, someone has to send it to me. This someone creates a transaction saying 'Send 1 BTC to Markus'. He publishes this transaction. Nodes in the Bitcoin network will eventually see it. They collect transactions and store them together in a block. Those blocks will be put together sequentially into a chain, also known as 'blockchain'. To prevent spamming the network, any transaction must also pay a fee.

In other words, a blockchain is just a list of strictly ordered transactions. To obtain the balance of an address, all we need to do is go through this long list until the end and keep track of all the incoming Bitcoins and all the outgoing Bitcoins. The current balance of an address equals the sum of all incoming minus the sum of all outgoing Bitcoins. Pretty simple right?

Upgrade Button

What's different in Ethereum? (Blockchain 102)

Now that we know how Bitcoin works, let's explore what's new in Ethereum. And that's the capability to create smart contracts. I like to think of smart contracts as essentially automated, verifiable addresses. In Bitcoin, every address is just a human sending Bitcoin from A to B, or a software acting on behalf of a human. Let's compare the difference between those two automations:

  1.  Let's imagine I run a computer software that automatically trades Bitcoin. You could think of it as an automated account. It listens to external output (who is sending me how much money, what is the current Bitcoin price etc.) and reacts by sending Bitcoins. Who knows and can change the behavior of this software? Only me, it's running only on my personal computer.
  2. Now compare that to an Ethereum smart contract. Just like before it listens to external output and reacts by sending Ether (or other things). Who knows what the software is doing? Everyone. Who can change the behavior? Nobody (unless its designed to be changed). The software is running on every Ethereum node, and in many cases, for all eternity.

So that's a smart contract in a nutshell. An automated account that is immutable and verifiable, because it runs on every machine of the network.

Archer Solidity

Let's get to Solidity!

Now it's time to talk about Solidity. Those automated accounts (as mentioned about, let's just call them smart contracts from now on) can be programmed. If you're a super nerd, you can just write direct machine code operations for it, also known as OPCODES. But if you are a normal person, you will want to use a higher level abstraction.

Solidity is here to help you. Just like many other languages, given the Solidity source code a compiler is generating the machine code for the Ethereum Virtual Machine (EVM).

Smart Contracts 101

What exactly defines a smart contract?

You can send a transaction to this address, just like you can to a normal address. One difference here is that you can send data along in your transaction which will define
  1. what function code on the contract to execute
  2. what are the input parameters to the function

Another difference is the concept of gas costs. They are essentially just the transaction fees, but in a normal transaction that just sends money from A to B, we know exactly how many resources are required by Ethereum nodes to process and verify the transaction (not a lot). But in a smart contract, you could do very heavy computations and store large amount of data. Every Ethereum node would have to compute and store this data. So to prevent spamming the network, we need a way to determine the resource impact of a transaction to a contract. Every OPCODE has a pre-defined gas cost.

In general you can know that there are four tiers of gas costs:

  1. Writing to Storage: Very expensive.
  2. Reading from Storage: Average.
  3. In-memory data and computations: Cheap.
  4. Deleting (freeing up) storage data: You get money back.

One very important thing for you to know is that there is a maximum amount of gas you can use in a single transaction. You can see here that it's changing over time and tends to go slowly up. But always know that there is a max. So you should never loop over arbitrary large arrays. This is bad design for contracts. Instead find a way to split the computation into smaller steps.

For example instead of sending money to all defined addresses, have a single function that sends the money to only who just called the contract. Then everyone who wants their money, needs to call the function. This is called the withdrawal pattern. (If the last part was a bit over your head still, don't worry. Read the rest and come back here afterwards.)

1. Let's talk about Functions

Let's look at our first example. This would be a pretty stupid smart contract to actually have, but it makes a great example. Anyone could send Ether to this contract and it would automatically send only half the Ether back. A money eating machine. Don't actually use this.

We could deploy this contract, meaning we put it on the blockchain and it will get an address for everyone to send transactions to. Once it's deployed, nobody can stop it, it will run forever, always with the same pre-defined behavior.

Let's break down each line.

// SPDX-License-Identifier: MIT
pragma solidity 0.7.5;

contract FastTrack {
  function payMeBackHalf() external payable {
    uint256 halfAmount = msg.value / 2;
    (bool success, ) ={value: halfAmount}("");

    require(success, "return transaction failed");
  • // SPDX-License-Identifier: MIT: Just a software license identifier that's required since 0.6.8. Check out available identifiers here.
  • pragma solidity 0.7.5: Specify which version of the Solidity compiler the contract is written for.
  • contract FastTrack: Specify the name of your smart contract.
  • function payMeBackHalf() external payable: A function we define that other people can use when interacting with out contract. Here we specify the name, visibility and declare it as payable. The visibility is external, meaning the function can only be called from outside of this contract. The opposite would be private, meaning the function is just a helper function we use in our contract to make it more readable. Declaring the function as payable means someone calling this function is allowed to send Ether along with the call. If it's not declared as payable, people couldn't send any Ether at all when calling this particular function.
  • uint256 halfAmount = msg.value / 2: This line is simple. The msg.value is a pre-defined variable and just holds the amount of Ether that is being sent to this contract. Now we set an unsigned integer with 256 bits to half of this value. Why unsigned? Because the value must be positive. Why 256 bits? The EVM operates on 32 bytes == 256 bits registers. You could use smaller types (like uint32), but this makes only sense in very specific situations. Don't worry about it now and just use 32 bytes types.
  • (bool success, ) ={value: halfAmount}(""): Okay this one might look a little bit complicated, but it's not. We also could have used the previous way to write it which was msg.transfer(halfAmount). Same effect, but its usage is discouraged and the call way is also great for explaining more since the .transfer just does a lot under the hood. The is creating another transaction. Remember a contract is just an automated address, so of course it can create transactions itself. In this case we send it to msg.sender, another pre-defined variable which is the current person (or contract) sending the transaction to this contract. The .call returns two values bool success and bytes result. Bool is just a value that is either true or false and bytes is just any undefined data (0's and 1's). However we do not care about the result data and just ignore it. We also send only an empty string via .call(""), meaning we don't send any data with our call, but we set value to halfAmount.. This is effectively just a transfer of Ether transaction.
  • require(success, "return transaction failed"):  Now this require you will see a lot of times in contracts. It's a short form for if (!success) revert("return transaction failed"). And it reverts a transaction with the given error message. If a transaction is reverted, it's considered invalid. So anytime this would happen in your smart contract, the transaction is just ignored by Ethereum nodes and it will never be integrated into the blockchain.
Exploding Head

2. Let's talk about Storage

In the above example we have only used data that was sent to in the transaction itself, but that alone would be pretty useless. We want to be able to store and read data inside the contract to enable more advanced use cases.

Let's build an escrow contract that enables a buyer and a seller that don't trust each other to do a trade online. They choose a trusted third-party arbitrator in case of a conflict. The contract will introduce the concept of

  1. Storage
  2. Modifiers
  3. Two more data types: address and mappings
  4. Constructors

Storage variables are defined at the top of the contract. You define the data type, the visibility and the name.

  • Type: address
  • Visibility: public
  • Name: seller

Types: We've already introduced the most important types, namely bool, integers and bytes. Each of those can be either a single value or a list of values (arrays). The two new types we added are:

  • address: This type represents an Ethereum address. This could be a human-controlled account address, also known as externally-owned account (EOA), or a smart contract address. There are a few other types, but don't worry about those for now.
  • mapping: This is just a key-value storage. You define the types for the key and the value, in our case address and bool. Now we can read and write to this variable:
    • read:   myMapping[myAddressKey]
    • write:  myMapping[myAddressKey] = myBool

Visibility: In storage you can choose between private, internal or public. Internal has to do with inheritance which is out of the scope of this introduction, so just think about private vs. public. Most of the times you probably want a public variable. This is just a convenience for people interacting with the contract to easily read the value. A private value does not mean the data is secret. A clever person could still find out the value.

Constructor: In our first contract we just deployed it without any additional data. But sometimes you might want to define some parameters when creating the contract. In our escrow we want to set the addresses for our three entities, seller, buyer and arbitrator.

Modifiers: We introduce the concept of modifiers here. You don't have to use them, but they can make the code more readable. You can define a bunch of requirements for a function call in them via the require statement. The '_;' indicates that here the function code will be executed here.

How does our Escrow Contract work?

Look at the example on the right. You should have all the knowledge to understand the code now. But take your time. Can you see what is happening?

When doing an online trade, the seller can discuss with the buyer which arbitrator to choose. Once decided, someone can deploy this contract. Now the buyer can call depositIntoEscrow and transfer the purchase price amount into the contract. Then the money is locked in our contract.

In the normal case the seller would now ship the product, the buyer receives it and calls confirmPurchase. Then the seller can also call confirmPurchase and the money will be sent to him.

In the case of a refund, the buyer would send the product back to the seller. Now the seller would call confirmRefund. Next the buyer can also call confirmRefund and the money will be sent back to him.

In the case of a dispute, a buyer might claim to never have received a product. So the buyer calls confirmRefund, while the seller would call confirmPurchase. Now nobody gets any money, but the arbitrator can be called. The arbitrator can investigate the case and then decide if to call confirmPurchase or confirmRefund.

Job for you: Try to improve on this for the dispute case by allowing the arbitrator to split the remaining funds in a ratio that he defines. So for example the arbitrator could decide, send 30% to the buyer and 70% to the seller.

contract Escrow {
  address public seller;
  address public buyer;
  address public arbitrator;

  uint256 public purchasePrice;
  uint256 public purchaseConfirmationCount;
  uint256 public refundConfirmationCount;

  mapping(address => bool) public hasConfirmedPurchase;
  mapping(address => bool) public hasConfirmedRefund;

    address seller_,
    address buyer_,
    address arbitrator_
  ) {
    seller = seller_;
    buyer = buyer_;
    arbitrator = arbitrator_;

  modifier isRegisteredParticipant {
      msg.sender == seller ||
        msg.sender == buyer ||
        msg.sender == arbitrator,
      "Only registered participants can call this"


  function depositIntoEscrow() external payable {
    require(purchasePrice == 0, "Already deposited");
      msg.sender == buyer,
      "Only buyer should deposit into escrow"
    purchasePrice = msg.value;

  function confirmPurchase() external isRegisteredParticipant {
      "Already confirmed purchase"

    hasConfirmedPurchase[msg.sender] = true;
    purchaseConfirmationCount = purchaseConfirmationCount + 1;

    if (purchaseConfirmationCount >= 2) {

  function confirmRefund() external isRegisteredParticipant {
      "Already confirmed refund"

    hasConfirmedRefund[msg.sender] = true;
    refundConfirmationCount = refundConfirmationCount + 1;

    if (refundConfirmationCount >= 2) {

  function _sendFundsTo(address recipient) private {
    (bool success, ) ={value: purchasePrice}("");
    require(success, "Sending funds transaction failed");
The Expert Meme

Congratulations. That's it.

I know you're already an expert, but even experts can still learn more right? A few ideas on what to look at next would be:

  • Events: A special type of storage that cannot be accessed by contracts themselves.
  • Structs: Custom types you can define that are a collection of sub-types.
  • ERC-20: A very popular contract standard. See here for how you can implement one easily.
  • Libraries: Special contracts that have no storage, but can be used only for implementation code.
  • Inheritance: Solidity supports inheritance including polymorphism. A great way to structure your contracts.

And last but not least, check out my overview about Solidity development and all the relevant tools and infrastructure here.

Markus Waas

Solidity Developer

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    It's April 2019 in Sydney. Here I am looking for the Edcon Hackathon inside the massive Sydney university complex. It feels like a little city within a city. Of course, I am at the wrong end of the complex and I realize to get to the venue hosting the Hackathon I need to walk 30 minutes to the...

  • Waffles

    Smart Contract Testing with Waffle 3

    What are the features of Waffle and how to use them.

    Waffle has been a relatively recent new testing framework, but has gained a lot of popularity thanks to its simplicity and speed. Is it worth a try? Absolutely. I wouldn't run and immediately convert every project to it, but you might want to consider it for new ones. It's also actively being...

  • xDai

    How to use xDai in your Dapp

    Deploying and onboarding users to xDai to avoid the high gas costs

    Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: Today we will take a closer look at xDai as a solution for your Dapp. What are...

  • 15 Stacks

    Stack Too Deep

    Three words of horror

    You just have to add one tiny change in your contracts. You think this will take you only a few seconds. And you are right, adding the code took you less than a minute. All happy about your coding speed you enter the compile command. With such a small change, you are confident your code is...

  • Chainlink Thumbnail

    Integrating the new Chainlink contracts

    How to use the new price feeder oracles

    By now you've probably heard of Chainlink. Maybe you are even participating the current hackathon? In any case adding their new contracts to retrieve price feed data is surprisingly simple. But how does it work? Oracles and decentralization If you're confused about oracles, you're not alone. The...

  • TheGraph

    TheGraph: Fixing the Web3 data querying

    Why we need TheGraph and how to use it

    Previously we looked at the big picture of Solidity and the create-eth-app which already mentioned TheGraph before. This time we will take a closer look at TheGraph which essentially became part of the standard stack for developing Dapps in the last year. But let's first see how we would do...

  • truffle buidler typescript

    Adding Typescript to Truffle and Buidler

    How to use TypeChain to utilize the powers of Typescript in your project

    Unlike compiled languages, you pretty much have no safeguards when running JavaScript code. You'll only notice errors during runtime and you won't get autocompletion during coding. With Typescript you can get proper typechecking as long as the used library exports its types. Most Ethereum...

  • Balance Rope

    Integrating Balancer in your contracts

    What is Balancer and how to use it

    What is Balancer? Balancer is very similar to Uniswap. If you're not familiar with Uniswap or Balancer yet, they are fully decentralized protocols for automated liquidity provision on Ethereum. An easier-to-understand description would be that they are decentralized exchanges (DEX) relying on...

  • mousetrap

    Navigating the pitfalls of securely interacting with ERC20 tokens

    Figuring out how to securely interact might be harder than you think

    You would think calling a few functions on an ERC-20 token is the simplest thing to do, right? Unfortunately I have some bad news, it's not. There are several things to consider and some errors are still pretty common. Let's start with the easy ones. Let's take a very common token: ... Now to...

  • Aave

    Why you should automatically generate interests from user funds

    How to integrate Aave and similar systems in your contracts

    If you're writing contracts that use, hold or manage user funds, you might want to consider using those funds for generating free extra income. What's the catch? That's right, it's basically free money and leaving funds unused in a contract is wasting a lot of potential. The way these...

  • Matic Logo

    How to use Polygon (Matic) in your Dapp

    Deploying and onboarding users to  Polygon  to avoid the high gas costs

    Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: Today we will take a closer look at Polygon (previously known as Matic) as a...

  • Migrating from Truffle to Buidler

    And why you should probably keep both.

    Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...

  • Factory

    Contract factories and clones

    How to deploy contracts within contracts as easily and gas-efficient as possible

    The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...

  • IPFS logo

    How to use IPFS in your Dapp?

    Using the interplanetary file system in your frontend and contracts

    You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...

  • tiny-kitten

    Downsizing contracts to fight the contract size limit

    What can you do to prevent your contracts from getting too large?

    Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...


    Using EXTCODEHASH to secure your systems

    How to safely integrate anyone's smart contract

    What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...

  • Uniswap

    Using the new Uniswap v2 in your contracts

    What's new in Uniswap v2 and how to integrate Uniswap v2

    Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...

  • Continuous Integration

    Solidity and Truffle Continuous Integration Setup

    How to setup Travis or Circle CI for Truffle testing along with useful plugins.

    Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...

  • Devcon 6

    Upcoming Devcon 2021 and other events

    The Ethereum Foundation just announced the next Devcon in 2021 in Colombia

    Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...

  • ERC-2020

    The Year of the 20: Creating an ERC20 in 2020

    How to use the latest and best tools to create an ERC-20 token contract

    You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...

  • hiring

    How to get a Solidity developer job?

    There are many ways to get a Solidity job and it might be easier than you think!

    You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...

  • People making fun

    Design Pattern Solidity: Mock contracts for testing

    Why you should make fun of your contracts

    Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...

  • React and Ethereum

    Kickstart your Dapp frontend development with create-eth-app

    An overview on how to use the app and its features

    Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...

  • Solidity Overview

    The big picture of Solidity and Blockchain development in 2020

    Overview of the most important technologies, services and tools that you need to know

    Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...

  • Design Pattern Solidity: Free up unused storage

    Why you should clean up after yourself

    You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...

  • How to setup Solidity Developer Environment on Windows

    What you need to know about developing on Windows

    Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...

  • Avoiding out of gas for Truffle tests

    How you do not have to worry about gas in tests anymore

    You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...

  • Design Pattern Solidity: Stages

    How you can design stages in your contract

    Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...

  • Web3 1.2.5: Revert reason strings

    How to use the new feature

    A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...

  • Gaining back control of the internet

    How Ocelot is decentralizing cloud computing

    I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...

  • Devcon 5 - Review

    Impressions from the conference

    I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...

  • Devcon 5 - Information, Events, Links, Telegram

    What you need to know

    Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...

  • Design Pattern Solidity: Off-chain beats on-chain

    Why you should do as much as possible off-chain

    As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...

  • Design Pattern Solidity: Initialize Contract after Deployment

    How to use the Initializable pattern

    There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...

  • Consensys Blockchain Jobs Report

    What the current blockchain job market looks like

    Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...

  • Provable — Randomness Oracle

    How the Oraclize random number generator works

    One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...

  • Solidity Design Patterns: Multiply before Dividing

    Why the correct order matters!

    There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...

  • Devcon 5 Applications closing in one week

    Devcon 5 Applications closing

    Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...

  • Randomness and the Blockchain

    How to achieve secure randomness for Solidity smart contracts?

    When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...