in Javascript

Different ways of setting a value to a Javascript object

Different ways of setting a value to a Javascript object

//create an object
var myVar = {};

//1. set value using dot
myVar.key = "myvalue";

//2. set value using square brackets
myVar["key2"] = "myValue2"

//3. set value using defineProperty
Object.defineProperty(myVar,"key3",{ 
             value : "myValue3"  
 });

Setting values using the dot syntax or square brackets is pretty easy to understand but using defineProperty seems unusual. As you can see in the code, the defineProperty function of Object takes three parameters.

Object.defineProperty(obj, prop, descriptor)

obj - The object variable
prop - the name of the property that needs to be updated in our variable
descriptor - this parameter helps us in making our object immutable or mutable.

The descriptor is an object that can take four parameters

Object.defineProperty(obj, 'key', {
 enumerable: false, //default is false
 configurable: false, //default is false
 writable: false, //default is false
 value: 'static'
});

About writable

When you set a value to a property using defineProperty, you are making it an immutable property, i.e you cannot change its value. Take a look at the snap below.

javascript immutable object key

Javascript object when immutable

As you can see, the value of key never changes even when I try to set it explicitly.

Javascript object when mutable

Javascript object when mutable

When i make writable to true, the object key behaves like a normal key which can be changed.

About enumerable

Enumerable simply means “can be looped”. This property is used to restrict the set property to show up when using the for…in loop.

Enumerable javascript object property

Enumerable javascript object property

As you can see in the code above, our property “key” is enumerable.

About Configurable 

Configurable restricts any change to the property, like deleting or changing its behaviour except writable.

Failed to delete the key

Failed to delete the key

When configurable is false, I cannot delete the property from my object.

Cannot change the enumerable property

Cannot change the enumerable property

The code throws exception if I try to change the property’s enumerable behavior.

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