How do I use a for-in loop with objects in JavaScript?

How do I use a for-in loop with objects in JavaScript?

You can use a for-in loop to iterate over the properties of an object. Here's a simple example:

const person = { name: 'Alice', age: 25, city: 'New York' }; 
for (let key in person) { 
    console.log(key + ': ' + person[key]); 
}

This will log each property and its value.

What if the object has nested objects?

If the object has nested objects, you can use a for-in loop recursively. Here's an example:

const person = { 
    name: 'Alice', 
    age: 25, 
    address: { city: 'New York', zip: '10001' } 
}; 

function printProperties(obj) { 
    for (let key in obj) { 
        if (typeof obj[key] === 'object') { 
            printProperties(obj[key]); 
        } else { 
            console.log(key + ': ' + obj[key]); 
        } 
    } 
}
printProperties(person);

Are there any performance concerns with using for-in loops?

Yes, for-in loops can iterate over inherited properties as well. It's a good practice to use hasOwnProperty to filter out those properties. Here's how:

for (let key in person) { 
    if (person.hasOwnProperty(key)) { 
        console.log(key + ': ' + person[key]); 
    } 
}