Things you may not know about Javascript Objects

Object Properties:

An Object is a non primitive data with collection of properties(key, value). Each property(key) of an Object has following configurable properties,

1. value - value of the property. Default: undefined
2. enumerable - if true, the property can be accessed using a for..in loop and Object.keys. Default: false
3. writable - if true, the property value can be modified. Default: false
4. get - defines a getter function. Default: undefined
5. set - defines a setter function. Default: undefined
6. configurable - if true, all the properties can be modified and deleted. Default: false

With Object.defineProperty and Object.defineProperties we can set the above configurable properties for an Object property(key).

With Object.getOwnPropertyDescriptor and Object.getOwnPropertyDescriptors we can get the above configurable properties associated with an Object property(key).

example:

var x = { a:1, b:2 };

Object.getOwnPropertyDescriptor(x, 'a'); 
// {value: 1, writable: true, enumerable: true, configurable: true} 

Object.freeze()

This method prevents an Object from,

1. New Properties being added
2. Modify existing properties
3. Delete existing properties
4. Configuring the object properties

But still the inner object properties can be modified. To deep freeze the whole object, we have to recursively freeze the non primitive values (Object).

So the property description for each property in an object will be,

writable: false
enumerable: true
configurable: false

example:

var x = {
a:1, 
b:2, 
c:{
d:3
}
};

var y = Object.freeze(x);
    
    Object.getOwnPropertyDescriptor(x, 'a'); //{value: 1, writable: false, enumerable: true, configurable: false}

x===y; //true

x.a; //1
x.a = 5; //silently fails. In strict mode - throws a TypeError
x.a; //1
x.m = 5; //silently fails. In strict mode - throws a TypeError
x.m; //undefined

x.c.d; //3
x.c.d = 5; //since it is not a primitive value, still it allows to modify internal properties of 'c'
x.c.d; //5

Object.defineProperty(x, 'n', { value: 10 }); //throws a TypeError
Object.defineProperty(x, 'a', { value: 20 }); //throws a TypeError

Object.isFrozen(x); //true
Object.isExtensible(x); //false

Object.seal()

This method prevents an Object from,

1. New Properties being added
2. Delete existing properties
3. Configuring the object properties

It allows the value of an existing properties to be modified.

So the property description for each property in an object will be,

writable: true
enumerable: true
configurable: false

example:

var x = {
a:1, 
b:2, 
c:{
d:3
}
};

var y = Object.seal(x);

Object.getOwnPropertyDescriptor(x, 'a'); //{value: 1, writable: true, enumerable: true, configurable: false}

x===y; //true

x.a; //1
x.a = 5; //since it is writable. the value can be changed
x.a; //5

x.m = 5; //silently fails. In strict mode - throws a TypeError
x.m; //undefined

Object.defineProperty(x, 'n', { value: 10 }); //throws a TypeError
Object.defineProperty(x, 'a', { value: 20 }); //allows to modify value

x.a; //20

Object.isSealed(x); //true
Object.isExtensible(x); //false