How to Use for in Loop in JavaScript: Syntax and Examples

Use the for in loop in JavaScript to iterate over all enumerable properties of an object. The syntax is for (key in object) { /* code */ }, where key is each property name. This loop is useful to access keys in objects but not recommended for arrays.

Syntax

The for in loop syntax is:

• key: a variable that holds the current property name (key) during each loop cycle.
• object: the object whose properties you want to loop through.
• The loop runs once for each enumerable property in the object.

javascript

for (const key in object) {
  // code to run for each key
}

Example

This example shows how to use for in to print all keys and values of an object.

javascript

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

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

Output

name: Alice
age: 30
city: New York

Common Pitfalls

Common mistakes when using for in include:

• Using it to loop over arrays, which can lead to unexpected order and inherited properties.
• Not checking if the property belongs directly to the object (not inherited) using hasOwnProperty.

Here is a wrong and right way example:

javascript

// Wrong: looping over array with for in
const arr = ['a', 'b', 'c'];
for (const index in arr) {
  console.log(arr[index]); // works but order and extra properties may cause issues
}

// Right: use for of for arrays
for (const value of arr) {
  console.log(value);
}

// Checking own properties
const obj = Object.create({ inherited: 'no' });
obj.own = 'yes';
for (const key in obj) {
  if (Object.prototype.hasOwnProperty.call(obj, key)) {
    console.log(key + ': ' + obj[key]);
  }
}

Output

a
b
c
own: yes

Quick Reference

• Use for in to loop over object keys.
• Use hasOwnProperty to avoid inherited keys.
• Do not use for in for arrays; use for of instead.

Key Takeaways

Use for in to loop over all enumerable keys of an object.

Always check hasOwnProperty to avoid inherited properties.

Avoid using for in to loop over arrays; use for of instead.

The loop variable holds the property name as a string.

for in loops over keys, not values directly.