How to Iterate Over Object Properties in JavaScript

To iterate over object properties in JavaScript, use for...in to loop through all enumerable keys, or use Object.keys() or Object.entries() with forEach or for...of for more control. These methods let you access keys and values easily.

Syntax

Here are common ways to loop over object properties:

• for...in: loops over all enumerable keys.
• Object.keys(obj).forEach(): loops over keys array.
• Object.entries(obj).forEach(): loops over [key, value] pairs.

javascript

for (const key in obj) {
  // use obj[key]
}

Object.keys(obj).forEach(key => {
  // use obj[key]
});

for (const [key, value] of Object.entries(obj)) {
  // use key and value
}

Example

This example shows how to print each property and its value using different methods.

javascript

const person = { name: 'Alice', age: 30, city: 'Paris' };

console.log('Using for...in:');
for (const key in person) {
  console.log(key + ': ' + person[key]);
}

console.log('\nUsing Object.keys().forEach():');
Object.keys(person).forEach(key => {
  console.log(key + ': ' + person[key]);
});

console.log('\nUsing Object.entries() with for...of:');
for (const [key, value] of Object.entries(person)) {
  console.log(key + ': ' + value);
}

Output

Using for...in:
name: Alice
age: 30
city: Paris

Using Object.keys().forEach():
name: Alice
age: 30
city: Paris

Using Object.entries() with for...of:
name: Alice
age: 30
city: Paris

Common Pitfalls

Common mistakes include:

• Using for...in without hasOwnProperty, which can include inherited properties.
• Assuming property order is guaranteed (it is mostly insertion order but not always).
• Trying to use forEach directly on objects (it works only on arrays).

javascript

const obj = Object.create({ inherited: 'yes' });
obj.own = 'no';

console.log('Wrong way (includes inherited):');
for (const key in obj) {
  console.log(key);
}

console.log('\nRight way (only own properties):');
for (const key in obj) {
  if (Object.prototype.hasOwnProperty.call(obj, key)) {
    console.log(key);
  }
}

Output

Wrong way (includes inherited):
inherited
own

Right way (only own properties):
own

Quick Reference

Summary of methods to iterate object properties:

Method	Description	Use Case
for...in	Loops over all enumerable keys including inherited	Use with hasOwnProperty to filter own keys
Object.keys()	Returns array of own enumerable keys	Use with forEach or for...of to loop keys
Object.entries()	Returns array of [key, value] pairs	Use with for...of to access keys and values directly

Key Takeaways

Use for...in with hasOwnProperty to avoid inherited properties.

Object.keys() and Object.entries() provide arrays for easy looping.

forEach works on arrays, not directly on objects.

Property order is mostly insertion order but not guaranteed.

Choose the method based on whether you need keys only or keys with values.