JavaScript Cheat Sheet — FreeCodeCamp

Basics

• Use let to declare variables you may reassign.

• Use const for variables that shouldn’t change.

• Variable names: letters, numbers, _, $ — can’t start with a number.

• Initialization = assigning value at declaration.

• Uninitialized variables default to undefined.

let name = "John"; // initialized
let age;           // uninitialized → undefined

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Data Types

• Primitive Types (7): string, number, boolean, null, undefined, symbol, bigint

• Strings are immutable (can't change characters, only reassign the whole value).

• Non-Primitive Data Types

  • Objects → { name: "John" }

  • Arrays → [1, 2, 3]

  • Functions → function greet() {}

  • Others → Date, RegExp, Map, Set, etc.

These are reference types, not copied by value.

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Console & Comments

• console.log() — display output.

• Single-line: // comment

• Multi-line: /* comment */

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Naming Convention

• Use camelCase for multi-word variables: userName, totalCount

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Math & Operators

• Arithmetic: +, -, *, /, %

• Increment: x++, Decrement: x--

• PEMDAS order of operations applies.

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Arrays

• Arrays: let arr = [1, 2, 3]

• Access: arr[0] (first element)

• Last item: arr[arr.length - 1]

• Mutate: arr[0] = 99

• .push() — add to end

• .pop() — remove from end and return it

• shift() — removes first item from array

• unshift() — adds item at the beginning of array

• repeat() — repeats a string N times

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Loops

// For loop
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// For...of loop
for (const val of arr) {
  console.log(val);
}

• Off-by-one error: Watch index starts at 0.

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while Loop

jsCopyEditlet i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

• Runs as long as the condition is true.

• Don't forget to update the variable (i++), or it may become an infinite loop

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String Methods

"abc".repeat(3); // "abcabcabc"

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Functions

function greet(name) {
  return "Hello, " + name;
}
greet("Alex"); // Hello, Alex

• function defines it.

• Call it using greet().

• Use parameters to make it reusable.

• Use return to pass back a value and stop function execution.

• Arguments are values passed in during the function call.

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Scope

• Global scope: declared outside any block/function.

• Local (block) scope: declared inside function/loop — not accessible outside.

const globalVar = "hi";

function sayHi() {
  const localVar = "bye";
  return globalVar; // can access globalVar
}
console.log(sayHi());
console.log(localVar); // ❌ Error

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Logic & Conditions

if (condition) {
  // run if truthy
}

• Truthy: most values (e.g., "text", 1, [])

• Falsy: false, 0, "", null, undefined, NaN

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Reusability Example

function getName() {
  const name = "Camper";
  return name;
}

const value = getName(); // use returned value

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If anything is missing, please let me know in the comments — I’ll definitely add it to the cheat sheet.