How do I loop through or enumerate a JavaScript object?

To loop through or enumerate the properties of a JavaScript object, you can use several methods depending on your needs. Here are the most common approaches:

1. for...in Loop

Iterates over enumerable properties (including inherited ones, unless filtered).

const obj = { a: 1, b: 2, c: 3 };

for (const key in obj) {
  if (obj.hasOwnProperty(key)) { // Check to exclude inherited properties
    console.log(`Key: ${key}, Value: ${obj[key]}`);
  }
}

Output:

Key: a, Value: 1
Key: b, Value: 2
Key: c, Value: 3

2. Object.keys()

Returns an array of the object’s own enumerable property names.

const obj = { a: 1, b: 2, c: 3 };

Object.keys(obj).forEach(key => {
  console.log(`Key: ${key}, Value: ${obj[key]}`);
});

3. Object.values()

Returns an array of the object’s own enumerable property values (ES2017+).

const obj = { a: 1, b: 2, c: 3 };

Object.values(obj).forEach(value => {
  console.log(`Value: ${value}`);
});

4. Object.entries()

Returns an array of [key, value] pairs (ES2017+).

const obj = { a: 1, b: 2, c: 3 };

// Using for...of with destructuring:
for (const [key, value] of Object.entries(obj)) {
  console.log(`Key: ${key}, Value: ${value}`);
}

// Or with forEach:
Object.entries(obj).forEach(([key, value]) => {
  console.log(`Key: ${key}, Value: ${value}`);
});

Key Differences

Method	Returns	Includes Inherited Properties?	ES Version
for...in	Keys of enumerable properties	Yes (unless filtered)	ES1
Object.keys()	Array of keys	No (only own properties)	ES5
Object.values()	Array of values	No	ES2017
Object.entries()	Array of [key, value] pairs	No	ES2017

When to Use Which?

• for...in: Rarely needed today. Use only if you need to include inherited properties (and filter with hasOwnProperty).
• Object.keys/values/entries: Preferred for modern code. Safer and more declarative.
• Object.entries: Best for iterating over both keys and values.

Example: Convert Object to Map

const obj = { a: 1, b: 2 };
const map = new Map(Object.entries(obj));
console.log(map.get('a')); // 1

Notes

• Non-enumerable properties (e.g., methods from Object.defineProperty) won’t appear in these loops. Use Object.getOwnPropertyNames() to include them.
• Symbols: Use Object.getOwnPropertySymbols() to iterate over symbol-keyed properties.