Making Decisions: Conditionals and Loops

Making Decisions: Conditionals and Loops

So far, your code has run top to bottom. Real programs need to make decisions and repeat work. A conditional is a statement that runs code only when a condition is true. A loop is a statement that repeats code.

const age = 20;

if (age >= 18) {
  console.log('Adult');
} else {
  console.log('Minor');
}

for (let i = 1; i <= 3; i++) {
  console.log('Lap ' + i);
}

The if statement checks a condition. If it is true, the first block runs; otherwise, the else block runs. The for loop repeats three times: it starts i at 1, keeps going while i <= 3, and adds 1 each pass.

Functions: Reusing Logic

A function is a named block of code you can run whenever you need it. Functions let you write logic once and reuse it.

function double(n) {
  return n * 2;
}

console.log(double(4));  // 8
console.log(double(21)); // 42

The word return sends a value back to whoever called the function.

Working with Collections of Data: Arrays

An array is an ordered list of values. You often want to do the same thing to every item — that is where loops and array methods shine.

const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(n => n * 2);
const evens = numbers.filter(n => n % 2 === 0);

console.log(doubled); // [2, 4, 6, 8, 10]
console.log(evens);   // [2, 4]

map builds a new array by transforming each item. filter builds a new array with only the items that pass a test. An arrow function like n => n * 2 is a short way to write a small function.

Modeling Data with Objects

An object is a collection of named values, called properties. Objects are how you model one thing, like a book.

const book = {
  title: 'Exploring JS',
  edition: 2,
  describe() {
    return `${this.title} (edition ${this.edition})`;
  }
};

console.log(book.title);        // Exploring JS
console.log(book['edition']);   // 2
console.log(book.describe());   // Exploring JS (edition 2)

Use a dot (book.title) or brackets (book['edition']) to read a property. this inside a method refers to the object itself.

Scope and Closures

Scope is where a variable can be seen in your code. Variables declared with let and const inside a block or function exist only there.

function counter() {
  let count = 0;
  return function () {
    count++;
    return count;
  };
}

const next = counter();
console.log(next()); // 1
console.log(next()); // 2

A closure is a function that remembers the variables from where it was created. next remembers count, so each call keeps counting.