How to Use Object.keys in JavaScript

Consider an object:

var user = {
  name: "Jagathish",
  age: 20
}      

In the user object, the name and age are the keys of the object. Keys are also referred to as object “properties”. We can access the property value by obj.propertyName or obj[propertyName].

The Object.keys() method returns an array of strings of a given object’s own property/key names. The following is what we get for our user object:

Object.keys(user); // ["name", "user"]

Let’s look at another example:

var user = {
name : "Jagathish",
  age  : 20,
  getAge() {
    return this.age;
  }
}

Object.keys(user); //  ["name", "age", "getAge"]

The key names are returned for all properties, whether it’s a function or primitive variable type. The order of key names in the array will be the same as they were in the object.

Syntax

Object.keys(obj)

Parameter: obj

The only parameter the Object.keys() function takes is an object itself.

• The object of which the enumerable’s own properties are to be returned.
• If we pass an empty object, then it returns an empty array.
• If we don’t pass any argument (which is equivalent to passing undefined) or if we pass null, then it throws an error.

Return value: Array of strings

An array of strings that represent all the enumerable properties of the given object.

var array = ['a', 'b', 'c'];

console.log(Object.keys(array)); // ['0', '1', '2']

var funObj = {
      fun : function () {
                ...
      }
}
console.log(Object.keys(funObj)) // ["fun"]

When we pass a non-object except undefined, it will be coerced to an object.

Object.keys(123) // []

Object.keys(123.34) // []

Object.keys("hi") // ["0" , "1"]

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