JavaScript Object Iterations

Object Iteration Methods

for (let key in obj) {
  console.log(key, obj[key]);
}

Object.keys(obj).forEach(key => {
  console.log(key, obj[key]);
});

Object.values(obj).forEach(value => {
  console.log(value);
});

Object.entries(obj).forEach(([key, value]) => {
  console.log(key, value);
});

🔹 for...in Loop

The most basic way to iterate through object properties:

let student = {
    name: "Emma",
    age: 20,
    grade: "A",
    subject: "Computer Science"
};

console.log("Using for...in loop:");
for (let property in student) {
    console.log(property + ": " + student[property]);
}

// Only iterate through own properties (not inherited)
console.log("\nOwn properties only:");
for (let property in student) {
    if (student.hasOwnProperty(property)) {
        console.log(property + ": " + student[property]);
    }
}

🔹 Object.keys() Method

Get an array of all property names, then iterate:

let car = {
    brand: "Toyota",
    model: "Camry",
    year: 2022,
    color: "Blue"
};

// Get all keys
let keys = Object.keys(car);
console.log("Keys:", keys);

// Iterate using forEach
console.log("\nUsing Object.keys() with forEach:");
Object.keys(car).forEach(function(key) {
    console.log(key + " -> " + car[key]);
});

// Using for loop
console.log("\nUsing Object.keys() with for loop:");
for (let i = 0; i < keys.length; i++) {
    let key = keys[i];
    console.log(key + " = " + car[key]);
}

🔹 Object.values() Method

Get an array of all property values:

let book = {
    title: "JavaScript Guide",
    author: "John Doe",
    pages: 350,
    published: true
};

// Get all values
let values = Object.values(book);
console.log("Values:", values);

// Iterate through values
console.log("\nIterating through values:");
Object.values(book).forEach(function(value, index) {
    console.log("Value " + (index + 1) + ": " + value);
});

// Find specific values
console.log("\nChecking values:");
console.log("Has pages over 300?", Object.values(book).some(val => val > 300));
console.log("All values are truthy?", Object.values(book).every(val => val));

🔹 Object.entries() Method

Get an array of [key, value] pairs - the most powerful method:

let product = {
    name: "Laptop",
    price: 999,
    category: "Electronics",
    inStock: true
};

// Get all entries
let entries = Object.entries(product);
console.log("Entries:", entries);

// Iterate using destructuring
console.log("\nUsing destructuring:");
Object.entries(product).forEach(([key, value]) => {
    console.log(`${key}: ${value}`);
});

// Convert back to object
console.log("\nConverting back to object:");
let newObj = Object.fromEntries(entries);
console.log(newObj);

// Filter and transform
console.log("\nFiltered entries (only strings):");
Object.entries(product)
    .filter(([key, value]) => typeof value === 'string')
    .forEach(([key, value]) => {
        console.log(`${key} = "${value}"`);
    });

🔹 Practical Examples

Real-world scenarios for object iteration:

// Example 1: Count properties
let user = {name: "Bob", email: "[email protected]", age: 30};
let propertyCount = Object.keys(user).length;
console.log("User has " + propertyCount + " properties");

// Example 2: Find property by value
function findKeyByValue(obj, searchValue) {
    for (let [key, value] of Object.entries(obj)) {
        if (value === searchValue) {
            return key;
        }
    }
    return null;
}

let colors = {sky: "blue", grass: "green", sun: "yellow"};
console.log("Key for 'green':", findKeyByValue(colors, "green"));

// Example 3: Transform object
let prices = {apple: 1.2, banana: 0.8, orange: 1.5};
let discountedPrices = {};

Object.entries(prices).forEach(([item, price]) => {
    discountedPrices[item] = (price * 0.9).toFixed(2); // 10% discount
});

console.log("Original prices:", prices);
console.log("Discounted prices:", discountedPrices);