JavaScript cheat sheet and syntax reference

JavaScript quick reference by topic

1. Variables and constants

   Use let for values that change and const for bindings that should not be reassigned.

   const name = "Ada";
   let count = 0;
   count += 1;

   • JavaScript variables
   • let const
   • declare variable
   Study Variables

2. Data types

   JavaScript values include strings, numbers, booleans, null, undefined, bigint, symbols, and objects.

   typeof "hello";
   typeof 42;
   value === null;

   • JavaScript data types
   • typeof
   • primitive values
   Study Data types

3. Operators and comparisons

   Combine arithmetic, assignment, logical operators, and strict equality to write predictable expressions.

   total += price;
   value === expected;
   isReady && run();

   • JavaScript operators
   • strict equality
   • logical operators
   Study Basic operators, maths

4. Conditions and loops

   Control program flow with if, switch, while, for, break, and continue.

   if (score >= 90) { grade = "A"; }
   for (const item of items) { render(item); }

   • JavaScript if else
   • JavaScript loops
   • switch statement
   Study Loops: while and for

5. Functions

   Package reusable behavior with declarations, expressions, parameters, return values, and arrow functions.

   function add(a, b) { return a + b; }
   const double = (n) => n * 2;

   • JavaScript functions
   • arrow function
   • return value
   Study Functions

6. Arrays

   Store ordered values and transform lists with map, filter, reduce, find, some, and every.

   items.map((item) => item.name);
   items.filter(Boolean);
   items.reduce((sum, n) => sum + n, 0);

   • JavaScript arrays
   • array methods
   • map filter reduce
   Study Array methods

7. Objects

   Group related data with object literals, properties, methods, destructuring, spread, and optional chaining.

   const user = { name: "Ada" };
   const { name } = user;
   user.profile?.email;

   • JavaScript objects
   • object properties
   • destructuring
   Study Objects

8. Classes

   Create reusable object models with class syntax, constructors, methods, inheritance, static fields, and private fields.

   class User { constructor(name) { this.name = name; } }
   class Admin extends User {}

   • JavaScript classes
   • class syntax
   • inheritance
   Study Class basic syntax

9. Modules

   Split JavaScript into files with named exports, default exports, and imports.

   export function format(value) { return String(value); }
   import { format } from "./format.js";

   • JavaScript modules
   • import export
   • ES modules
   Study Modules, introduction

10. Promises and async/await

    Represent asynchronous results with promises and write promise flow clearly with async functions and await.

    const result = await fetchData();
    promise.then(handleResult).catch(handleError);

    • JavaScript promises
    • async await
    • asynchronous JavaScript
    Study Async/await

11. DOM and events

    Select elements, update the page, and respond to user actions with browser event listeners.

    const button = document.querySelector("button");
    button.addEventListener("click", onClick);

    • JavaScript DOM
    • querySelector
    • event listener
    Study Event loop: microtasks and macrotasks

12. Fetch and JSON

    Call APIs with fetch, parse JSON responses, stringify data, and handle network errors.

    const url = "/api/items";
    const data = await response.json();

    • JavaScript fetch
    • JSON parse
    • API request
    Study Fetch

13. Regular expressions

    Search, validate, extract, and replace text with RegExp patterns and string methods.

    /\d+/.test(value);
    text.replace(/\s+/g, " ");

    • JavaScript regex
    • regular expression
    • RegExp
    Study Patterns and flags