Arrays

What is an array?

An array is an ordered list that can hold multiple values.
Each element is identified by an index (starting at 0).

Example: ["apple", "banana", "pear"]

• fruits[0] → “apple”
• fruits[1] → “banana”

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Creating an array

You can create an array using square brackets [].

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Main methods

Method	Example	Result
push()	fruits.push("kiwi")	Adds to the end
pop()	fruits.pop()	Removes the last item
shift()	fruits.shift()	Removes the first item
unshift()	fruits.unshift("strawberry")	Adds to the beginning
map()	fruits.map(f => f.toUpperCase())	Transforms the elements
filter()	fruits.filter(f => f.startsWith("p"))	Filters elements
reduce()	[1,2,3].reduce((a,b)=>a+b)	Reduces to one value

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Iterating over an array

With a classic for

Loop through using indexes:

for (let i = 0; i < fruits.length; i++) {
  console.log(fruits[i]);
}

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With for...of

Loop directly through values (more readable):

for (let fruit of fruits) {
  console.log(fruit);
}

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With forEach

Calls a function on each element:

fruits.forEach(fruit => console.log(fruit));

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Focus: the map() method

The map() method creates a new array by applying a function to each element of the original array.

Important: map() does not modify the original array.

Example: transform all items to uppercase.

let fruits = ["apple", "banana", "pear"];
let upper = fruits.map(fruit => fruit.toUpperCase());

console.log(upper);  // ["APPLE", "BANANA", "PEAR"]
console.log(fruits); // ["apple", "banana", "pear"] (unchanged)

Another example: extract a property from an array of objects.

let users = [
  { name: "Alice", age: 25 },
  { name: "Bob", age: 30 },
  { name: "Charlie", age: 35 }
];

let names = users.map(u => u.name);
console.log(names); // ["Alice", "Bob", "Charlie"]

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Practical example

Filter even numbers from an array:

let numbers = [1, 2, 3, 4, 5];
let evens = numbers.filter(n => n % 2 === 0);
console.log(evens); // [2, 4]

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Best practices

• Use for...of or forEach for readability.
• Prefer map, filter, reduce to write more functional and concise code.
• Avoid mixing different types in the same array (for clarity).