The for…in and Object.keys() in JavaScript are used to iterate over the properties of an object. While they might seem similar at first glance they have distinct usage, behavior, and characteristics. This article will explore these differences in detail.

These are the following topics that we are going to discuss:

What is for…in?

The for…in statement is a looping construct that iterates over the enumerable properties of an object. It goes through each property name (key) in the object and allows to perform the actions with these properties.

Characteristics:

• Iterates over all enumerable properties including the inherited properties.
• Can iterate over properties added to the object via the prototype chain.
• Useful for general property enumeration when dealing with both own and inherited properties.

Applications:

• Iterating over all properties of an object.
• Useful when we want to include the inherited properties in the iteration.

Example: In this example, the for…in loop iterates over the properties name and age of the person object as well as the gender property inherited from the Object.prototype.

let person = {
    name: 'kumar',
    age: 25
};
Object.prototype.gender = 'Male';
for (let key in person) {
    console.log(key + ': ' + person[key]);
}

name: kumar
age: 25
gender: Male

What is Object.keys()?

The Object.keys() method returns an array of the given object’s own enumerable property names in the same order as provided by the for…in loop but without including the properties from the prototype chain.

Characteristics:

• Returns an array of the object’s own enumerable properties.
• Does not include the inherited properties.
• Useful for obtaining a list of the property names for an object’s own properties.

Applications:

• Extracting a list of the object’s own property names.
• Avoiding the enumeration of the properties from the prototype chain.

Example: In this example Object.keys(person) returns an array containing only the name and age properties excluding the inherited gender property.

let person = {
    name: 'kumar',
    age: 30
};
Object.prototype.gender = 'Male';
let keys = Object.keys(person);
console.log(keys);
keys.forEach(key => {
    console.log(key + ': ' + person[key]);
});

[ 'name', 'age' ]
name: kumar
age: 30

Difference Between for…in and Object.keys()