Object.key() in JavaScript

In JavaScript, Object.keys() is a built-in method that returns an array of a given object's own enumerable property names, in the same order as a for...in loop would iterate them.

Example:-

Here's an example:

Output

[ 'name', 'age', 'job' ]

In this example, the Object.keys() method is used to return an array of the myObject object's property names. The resulting array contains the strings 'name', 'age', and 'job'.

Note that the method only returns the object's own enumerable properties, which means that the properties that are directly defined on the object are not properties inherited from its prototype chain. To include all properties, enumerable and non-enumerable, you can use Object.getOwnPropertyNames() method instead.

Example:-

Here's another example of using Object.keys() to iterate over an object's properties:

Output:

The output of this code would be:

name: John
age: 30
job: Developer

In this example, we first get an array of the myObject object's property names using Object.keys(). After that, we use for loop to iterate over the keys array and log each property name and its corresponding value to the console.

Note that the order of the properties in the output may not necessarily match the order in which they were defined in the object literal.

Example:-

This is another example that demonstrates how to use Object.keys() to manipulate an object's properties:

Output

{ name: 'John', age: 31, location: 'USA' }
name: John
age: 31
job: undefined

In this example, we start by getting an array of the myObject object's property names using Object.keys(). After that, we add a new property (location) to the object, delete an existing property (job), and update an existing property (age). We then log the updated object to the console.

Example:-

This example shows how to use Object.keys() to create a new object that contains only certain properties of an existing object:

Output

{ name: 'John', age: 30 }

In this example, we start by defining an array selectedKeys that contains the names of the properties we want to include in the new object. After that, we use Object.keys() to get an array of all the property names in myObject.

We use the Array.filter() method to remove any property names that are not included in selectedKeys. After that, we use the Array.reduce() method to create a new object containing only the selected properties. The reduce() method takes an initial value of an empty object ({}) and a callback function that adds each selected property to the object.

Here are some more important points to note about Object.keys():

• keys() is a static method, which means it's called directly on the Object constructor function, not on individual objects.
• The order of the keys returned by keys() is determined by the engine implementation and may not be consistent across different engines or versions.
• If you pass an argument that is not an object (e.g. null, undefined, a primitive value), keys() will throw a TypeError.
• If you pass an object with no own enumerable properties (i.e. an object with no properties defined directly on it), keys() will return an empty array.
• keys() only returns enumerable property names. To include non-enumerable properties, use Object.getOwnPropertyNames().
• If you want to iterate over an object's properties and you don't care about the property names, you can use Object.values() to get an array of the property values, or Object.entries() to get an array of [key, value] pairs.