The Coder Bros — Master Modern Development

Code Cheatsheets

Search and browse syntax references for all supported languages. Copy code snippets with one click.

Variables

PythonBasics

Python variables are dynamically typed

name = "Alice"
age = 25
height = 1.75
is_student = True

Data Types

PythonBasics

Common Python data types

type(42)       # <class 'int'>
type(3.14)     # <class 'float'>
type("hello")  # <class 'str'>
type(True)     # <class 'bool'>
type([1,2,3])  # <class 'list'>

String Methods

PythonStrings

Common string operations

"hello".upper()        # HELLO
"HELLO".lower()        # hello
"  hi  ".strip()       # hi
"a,b,c".split(",")    # ['a','b','c']
"-".join(["a","b"])   # a-b

f-strings

PythonStrings

String interpolation

name = "Alice"
age = 25
print(f"{name} is {age} years old")
print(f"Pi = {3.14159:.2f}")

List Operations

PythonCollections

Common list methods

nums = [1, 2, 3]
nums.append(4)
nums.insert(0, 0)
nums.pop()
nums.sort()
[x*2 for x in nums]

Dict Operations

PythonCollections

Common dictionary methods

d = {"a": 1, "b": 2}
d["c"] = 3
d.get("x", 0)
d.keys()
d.values()
d.items()
{k: v*2 for k,v in d.items()}

Conditionals

PythonControl Flow

If-elif-else and ternary

if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

# Ternary
res = "even" if x % 2 == 0 else "odd"

Loops

PythonControl Flow

Loop patterns

for i in range(5):
for i, x in enumerate(lst):
for k, v in d.items():
while x > 0:

[x**2 for x in range(10) if x % 2 == 0]

Function Patterns

PythonFunctions

Function definition patterns

def add(a, b=0, *args, **kwargs):
    return a + b

# Lambda
square = lambda x: x**2

# Decorator
def timer(fn):
    def wrapper(*a, **kw):
        import time
        start = time.time()
        result = fn(*a, **kw)
        print(f"took {time.time()-start}s")
        return result
    return wrapper

Class Basics

PythonOOP

Class definition structure

class Dog:
    species = "Canine"
    def __init__(self, name):
        self.name = name
    def bark(self):
        return f"{self.name} says Woof!"

File Operations

PythonFile I/O

Reading and writing files

with open("file.txt", "r") as f:
    content = f.read()
    lines = f.readlines()
    for line in f:
        print(line.strip())

with open("file.txt", "w") as f:
    f.write("Hello\n")

Variables & Types

JavaScriptBasics

Variable declarations and type checking

let name = "Alice"
const age = 25
var old = "avoid"

typeof "hello"  // "string"
typeof 42       // "number"
typeof true     // "boolean"
typeof {}       // "object"
typeof []       // "object"

String Methods

JavaScriptStrings

String manipulation

"hello".toUpperCase()
"HELLO".toLowerCase()
"  hi  ".trim()
"a,b,c".split(",")
["a","b"].join("-")
`Hello, ${name}!`

Array Methods

JavaScriptArrays

Essential array operations

nums.push(4)
nums.pop()
nums.unshift(0)
nums.shift()
nums.slice(1,3)
nums.splice(1,1)
nums.map(x => x*2)
nums.filter(x => x > 2)
nums.reduce((a,b) => a+b, 0)

Arrow Functions

JavaScriptFunctions

Arrow function syntax

const add = (a, b) => a + b
const square = x => x * x
const greet = name => `Hello ${name}`

// Higher-order
const twice = fn => x => fn(fn(x))

Object Operations

JavaScriptObjects

Object manipulation

const user = { name: "Alice", age: 25 }
user.email = "a@b.com"
const { name, age } = user
const clone = { ...user }
Object.keys(user)
Object.values(user)
Object.entries(user)

Promises & Async/Await

JavaScriptAsync

Async patterns

fetch(url)
  .then(res => res.json())
  .catch(err => console.error(err))

async function getData() {
    try {
        const res = await fetch(url)
        return await res.json()
    } catch (err) {
        console.error(err)
    }
}

const [a, b] = await Promise.all([p1, p2])

DOM Manipulation

JavaScriptDOM

DOM selection and manipulation

document.querySelector('.class')
document.querySelectorAll('div')
elem.textContent = 'text'
elem.classList.add('active')
elem.style.color = '#D946EF'
elem.addEventListener('click', e => {
    console.log(e.target)
})

Document Structure

HTML/CSSHTML

Standard HTML5 template

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Page</title>
</head>
<body>
    <header>Nav</header>
    <main>Content</main>
    <footer>Footer</footer>
</body>
</html>

Flexbox

HTML/CSSCSS

Flexbox layout reference

.container {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-wrap: wrap;
    gap: 1rem;
}

.item {
    flex: 1 1 200px;
}

Grid

HTML/CSSCSS

CSS Grid layout patterns

.grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 1rem;
}

.featured {
    grid-column: span 2;
}

.responsive {
    grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
}

Animations

HTML/CSSCSS

Transitions and keyframe animations

.element {
    transition: all 0.3s ease;
}

@keyframes slideIn {
    from { opacity: 0; transform: translateX(-20px); }
    to { opacity: 1; transform: translateX(0); }
}

Hello World

JavaBasics

Basic Java program structure

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Collections & Streams

JavaCollections

Java Collections Framework

List<String> list = new ArrayList<>();
list.add("a");

Map<String, Integer> map = new HashMap<>();
map.put("a", 1);

list.stream()
    .filter(s -> s.startsWith("a"))
    .map(String::toUpperCase)
    .collect(Collectors.toList());

Hello World

C#Basics

Basic C# program

using System;

class Program {
    static void Main(string[] args) {
        Console.WriteLine("Hello, World!");
    }
}

LINQ Queries

C#LINQ

LINQ query and method syntax

var numbers = new[] { 1, 2, 3, 4, 5 };

var evens = from n in numbers
            where n % 2 == 0
            select n;

var squares = numbers
    .Where(n => n % 2 == 0)
    .Select(n => n * n);

Hello World

C++Basics

Basic C++ program

#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

Pointers & References

C++Memory

Pointer and reference syntax

int value = 42;
int* ptr = &value;
int& ref = value;

*ptr = 100;

auto unique = std::make_unique<int>(42);
auto shared = std::make_shared<int>(100);

STL Containers

C++STL

STL algorithms and containers

#include <vector>
#include <algorithm>

std::vector<int> nums = {3, 1, 4, 1, 5};
std::sort(nums.begin(), nums.end());

auto it = std::find(nums.begin(), nums.end(), 4);

std::count_if(nums.begin(), nums.end(),
    [](int n) { return n % 2 == 0; });

Error Handling

PythonErrors

Try/except/else/finally flow

try:
    result = 10 / int(x)
except ZeroDivisionError:
    print("cannot divide by zero")
except ValueError as e:
    print(f"bad input: {e}")
except Exception:
    print("unexpected")
else:
    print("no error")
finally:
    print("always runs")

Sets & Tuples

PythonCollections

Set operations and tuple unpacking

s = {1, 2, 3}
s.add(4)
s.union({5, 6})
s.intersection({2, 3})
s.difference({1})

t = (1, 2, 3)
a, b, c = t

unique = list(set([1, 1, 2, 3, 3]))  # [1, 2, 3]

Modules & Imports

PythonModules

Import forms and entry-point pattern

import math
from datetime import datetime
import numpy as np

math.sqrt(16)              # 4.0
datetime.now()             # current time
np.array([1, 2, 3]).sum()  # 6

if __name__ == "__main__":
    main()

Date & Time

PythonBasics

datetime formatting and arithmetic

from datetime import datetime, timedelta

now = datetime.now()
now.strftime("%Y-%m-%d %H:%M")   # format
parsed = datetime.fromisoformat("2026-01-15")
parsed + timedelta(days=7)        # add days
tz = now.astimezone()

ts = now.timestamp()              # unix seconds

Regex

PythonStrings

Common regular expression operations

import re

re.search(r"\d+", "abc 123").group()   # "123"
re.findall(r"\b\w{3}\b", "one two three")
re.sub(r"\s+", "-", "a  b")            # "a-b"
re.match(r"^\d", "1abc")               # match at start

email = r"[\w.+-]+@[\w-]+\.[\w.]+"

Destructuring & Spread

JavaScriptObjects

Array and object destructuring

const [a, b, ...rest] = [1, 2, 3, 4]
const { name, age = 18, email: mail } = user

const clone = { ...user, age: 26 }
const merged = [...nums1, ...nums2]

// Function params
function draw({ x = 0, y = 0, color = "red" }) {}

Closures & Scope

JavaScriptFunctions

Closures, IIFE, and encapsulation

function counter() {
    let count = 0
    return () => ++count
}
const next = counter()
next()  // 1

// Module pattern
const store = (() => {
    let data = []
    return {
        add: (x) => data.push(x),
        all: () => data,
    }
})()

Set & Map

JavaScriptCollections

Unique values and key-value stores

const set = new Set([1, 2, 2, 3])
set.has(2)          // true

const map = new Map()
map.set("a", 1)
map.get("a")        // 1
map.has("b")        // false

const counts = new Map()
for (const w of words)
    counts.set(w, (counts.get(w) || 0) + 1)

Error Handling

JavaScriptErrors

try/catch/finally and custom errors

try {
    const res = await fetch(url)
    if (!res.ok) throw new Error(`HTTP ${res.status}`)
    const data = await res.json()
} catch (err) {
    console.error("fetch failed:", err)
} finally {
    cleanup()
}

// Custom errors
class ValidationError extends Error {}
throw new ValidationError("bad input")

Selectors

HTML/CSSCSS

Selector reference with combinators

/* Basic */
.class, .other { }
#id { }
[data-type="a"] { }

/* Combinators */
.parent > .child      /* direct child */
.a + .b               /* adjacent sibling */
.a ~ .b               /* general sibling */

/* Pseudo */
li:nth-child(odd) { }
button:hover { }
input:focus { }
.link::after { }

Custom Properties

HTML/CSSCSS

CSS variables with fallbacks

:root {
    --accent: #6366F1;
    --radius: 12px;
    --space: 1rem;
}

.card {
    border: 1px solid color-mix(in srgb, var(--accent) 40%, transparent);
    border-radius: var(--radius);
    padding: var(--space);
    background: var(--card-bg, #1a1a1a);  /* fallback */
}

Responsive Patterns

HTML/CSSCSS

Fluid grids and breakpoints

.grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
    gap: 1rem;
}

@media (max-width: 768px) {
    .nav { flex-direction: column; }
    .hero { font-size: 1.5rem; }
}

@media (prefers-reduced-motion: reduce) {
    * { animation: none !important; transition: none !important; }
}

Forms

HTML/CSSHTML

Accessible form markup reference

<form novalidate>
    <label for="email">Email</label>
    <input id="email" name="email" type="email" required
           pattern="[^@]+@[^@]+\.[^@]+">

    <select name="plan">
        <option value="free">Free</option>
        <option value="pro" selected>Pro</option>
    </select>

    <textarea name="msg" rows="4" maxlength="500"></textarea>

    <fieldset>
        <legend>Level</legend>
        <input type="radio" name="level" value="beginner" checked>
        <input type="radio" name="level" value="advanced">
    </fieldset>

    <button type="submit">Send</button>
</form>

Classes & Objects

JavaOOP

Class structure and instantiation

public class Car {
    private String model;

    public Car(String model) {
        this.model = model;
    }

    public String getModel() {
        return model;
    }

    public static void main(String[] args) {
        Car tesla = new Car("Model 3");
        System.out.println(tesla.getModel());
    }
}

Exceptions

JavaErrors

Exception handling and try-with-resources

try {
    int result = 10 / Integer.parseInt(input);
} catch (ArithmeticException e) {
    System.err.println("division by zero");
} catch (NumberFormatException e) {
    System.err.println("bad number: " + e.getMessage());
} finally {
    closeResources();
}

// Try-with-resources

try (BufferedReader reader = new BufferedReader(new FileReader("f.txt"))) {
    System.out.println(reader.readLine());
}

Arrays & Loops

JavaLoops

Loop styles and control flow

int[] nums = {1, 2, 3};

for (int i = 0; i < nums.length; i++) { }
for (int n : nums) { }                 // enhanced for

List<String> names = new ArrayList<>(List.of("a", "b"));
names.forEach(System.out::println);

for (int i = 0; i < 10; i++) {
    if (i == 3) continue;
    if (i == 8) break;
}

Classes & Properties

C#OOP

Properties, auto-props, and expression bodies

public class Product {
    public string Name { get; set; }
    public decimal Price { get; private set; } = 0m;

    public Product(string name, decimal price) {
        Name = name;
        Price = price;
    }

    public decimal Total(int qty) => Price * qty;
}

var p = new Product("Keyboard", 49.99m);
Console.WriteLine(p.Total(2));  // 99.98

Async & Await

C#Async

Async/await patterns and Task composition

public async Task<string> FetchAsync(HttpClient client, string url)
{
    var response = await client.GetAsync(url);
    response.EnsureSuccessStatusCode();
    return await response.Content.ReadAsStringAsync();
}

// Fire-and-forget
_ = SomeLongTaskAsync();

// Parallel
var tasks = urls.Select(u => FetchAsync(client, u));
var results = await Task.WhenAll(tasks);

Classes

C++OOP

Encapsulation with member functions

class BankAccount {
private:
    double balance = 0.0;

public:
    BankAccount(double initial) : balance(initial) {}

    void deposit(double amount) { balance += amount; }
    double getBalance() const { return balance; }
};

int main() {
    BankAccount account(100.0);
    account.deposit(50.0);
    std::cout << account.getBalance() << std::endl;
}

Lambdas & Range-for

C++Modern

Modern C++ iteration and algorithms

#include <algorithm>
#include <numeric>

std::vector<int> v = {3, 1, 4, 1, 5};

for (const auto& x : v)          // range-based
    std::cout << x << " ";

std::sort(v.begin(), v.end());

int total = std::accumulate(v.begin(), v.end(), 0);

auto is_even = [](int n) { return n % 2 == 0; };
int evens = std::count_if(v.begin(), v.end(), is_even);