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Documentation Index

Fetch the complete documentation index at: https://js.maxbraglia.com/llms.txt

Use this file to discover all available pages before exploring further.

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The methods you’ll use every day

JavaScript arrays have dozens of methods, but you’ll use about six of them 90% of the time. These are the ones that matter for web development and React. Every method here takes a callback function — an arrow function that runs once for each item in the array.

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.map() — transform each item

.map() creates a new array by transforming every item. Use it when you need to convert data from one shape to another. 
const prices = [10, 20, 30, 40];
const withTax = prices.map(price => price * 1.08);
console.log(withTax); // [10.8, 21.6, 32.4, 43.2]

const names = ["sarah", "john", "alice"];
const capitalized = names.map(name => name.charAt(0).toUpperCase() + name.slice(1));
console.log(capitalized); // ["Sarah", "John", "Alice"]

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Real-world: formatting API data

const apiUsers = [
  { id: 1, first_name: "Sarah", last_name: "Chen" },
  { id: 2, first_name: "John", last_name: "Smith" },
];

const displayNames = apiUsers.map(user => ({
  id: user.id,
  fullName: `${user.first_name} ${user.last_name}`,
}));

console.log(displayNames);
// [{ id: 1, fullName: "Sarah Chen" }, { id: 2, fullName: "John Smith" }]
In React, you’ll use .map() to render lists: 
{users.map(user => (
  <li key={user.id}>{user.fullName}</li>
))}
.map() always returns a new array with the same number of items. It never modifies the original. This is exactly what React needs for rendering lists.

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.filter() — keep matching items

.filter() creates a new array with only the items that pass a test. The callback must return true or false. 
const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

const evens = numbers.filter(n => n % 2 === 0);
console.log(evens); // [2, 4, 6, 8, 10]

const bigNumbers = numbers.filter(n => n > 5);
console.log(bigNumbers); // [6, 7, 8, 9, 10]

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Real-world: filtering a user list

const users = [
  { name: "Sarah", role: "admin", active: true },
  { name: "John", role: "editor", active: false },
  { name: "Alice", role: "admin", active: true },
  { name: "Bob", role: "viewer", active: true },
];

const activeAdmins = users.filter(user => user.role === "admin" && user.active);
console.log(activeAdmins);
// [{ name: "Sarah", role: "admin", active: true }, { name: "Alice", role: "admin", active: true }]
.filter() can return fewer items, the same number, or even an empty array. It never modifies the original.

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.find() — get the first match

.find() returns the first item that matches. Unlike .filter(), it returns a single item (or undefined if nothing matches). 
const users = [
  { id: 1, name: "Sarah", role: "admin" },
  { id: 2, name: "John", role: "editor" },
  { id: 3, name: "Alice", role: "viewer" },
];

const admin = users.find(user => user.role === "admin");
console.log(admin); // { id: 1, name: "Sarah", role: "admin" }

const manager = users.find(user => user.role === "manager");
console.log(manager); // undefined
Use .find() when you need one specific item — looking up a user by ID, finding a product by slug, getting the current route. 
// Common pattern: find by ID
const userId = 2;
const user = users.find(u => u.id === userId);
.find() returns undefined if no item matches — not null, not an error. Always handle the “not found” case.

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.forEach() — do something with each item

.forEach() runs a function for each item but doesn’t return anything. Use it for side effects like logging or DOM manipulation. 
const tasks = ["Buy groceries", "Write code", "Walk the dog"];

tasks.forEach(task => {
  console.log(`- ${task}`);
});
// - Buy groceries
// - Write code
// - Walk the dog

// With index
const scores = [85, 92, 78, 95];
scores.forEach((score, index) => {
  console.log(`Student ${index + 1}: ${score}`);
});
If you need a new array, use .map(). If you need to do something with each item (log, send, update DOM), use .forEach(). The rule: .map() transforms, .forEach() performs actions.

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.includes() — check if item exists

const permissions = ["read", "write", "delete"];

console.log(permissions.includes("write"));  // true
console.log(permissions.includes("admin"));  // false

// Use in conditionals
if (permissions.includes("delete")) {
  console.log("User can delete items");
}
Simple and readable. Works great for checking roles, features, or any “is this in the list?” question.

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.reduce() — combine into a single value

.reduce() takes an array and reduces it down to a single value — a total, an object, a string, whatever you need. 
const prices = [29.99, 49.99, 9.99, 79.99];

const total = prices.reduce((sum, price) => sum + price, 0);
console.log(total); // 169.96
The callback takes two arguments:
  1. Accumulator (sum) — the running result
  2. Current item (price) — the current array element
The second argument to .reduce() (0) is the initial value of the accumulator.

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More reduce examples

// Count occurrences
const votes = ["yes", "no", "yes", "yes", "no", "yes"];
const tally = votes.reduce((counts, vote) => {
  counts[vote] = (counts[vote] || 0) + 1;
  return counts;
}, {});
console.log(tally); // { yes: 4, no: 2 }

// Find the maximum
const scores = [72, 95, 88, 61, 93];
const highest = scores.reduce((max, score) => score > max ? score : max, 0);
console.log(highest); // 95
.reduce() is powerful but can be hard to read. If a .map() + .filter() combination does the job, prefer that over .reduce(). Readability matters more than cleverness.

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Chaining methods

Array methods return arrays, so you can chain them: 
const products = [
  { name: "Laptop", price: 999, inStock: true },
  { name: "Phone", price: 699, inStock: false },
  { name: "Tablet", price: 499, inStock: true },
  { name: "Watch", price: 299, inStock: true },
];

// Get names of in-stock products under $500
const affordable = products
  .filter(p => p.inStock)
  .filter(p => p.price < 500)
  .map(p => p.name);

console.log(affordable); // ["Tablet", "Watch"]
Read chains top to bottom: start with all products → keep only in-stock → keep only under $500 → extract names.

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Common mistakes

// ❌ Wrong: Curly braces but no return
const doubled = [1, 2, 3].map(n => {
  n * 2; // Computes but doesn't return
});
console.log(doubled); // [undefined, undefined, undefined]

// ✅ Correct: Add return with curly braces
const doubled = [1, 2, 3].map(n => {
  return n * 2;
});

// ✅ Or use implicit return (no curly braces)
const doubled = [1, 2, 3].map(n => n * 2);
This is the same arrow function gotcha from earlier. With curly braces, you need return. Without curly braces, the return is implicit. This mistake causes arrays of undefined.
// ❌ Wrong: Using map for side effects (the returned array is wasted)
users.map(user => {
  console.log(user.name);
});

// ✅ Correct: Use forEach for side effects
users.forEach(user => {
  console.log(user.name);
});
.map() creates and returns a new array. If you don’t need that array, use .forEach() instead. Using .map() for side effects is wasteful and signals to other developers that you intended to transform data.
// ❌ Wrong: .sort() mutates the original array
const scores = [85, 92, 78, 95];
const sorted = scores.sort((a, b) => a - b);
console.log(scores); // [78, 85, 92, 95] — original is modified!

// ✅ Correct: Copy first, then sort
const sorted = [...scores].sort((a, b) => a - b);
console.log(scores); // [85, 92, 78, 95] — original unchanged
.sort(), .reverse(), .splice(), .push(), and .pop() all mutate the original array. If you need the original intact, spread it into a new array first: [...arr].sort().

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What’s next?

Arrays handle lists. Now let’s learn about objects — JavaScript’s key-value data structure for representing entities like users, products, and API responses.

Objects

Store data as key-value pairs