Constants in Javascript - ECMAScript 6

Const

In ECMAScript 6 we have const variable arrived. Does it work the same as C++ const?
Not exactly. I would even say, “not at all”.
When you declare something with const in ECMAScript 6, it means that the identifier can’t be reassigned.

What does it mean?

• It means that you cannot declare that variable more than once. Because its binding is immutable.
  
• You have guaranty that no rebinding will happen to it.
  
• You will not be able to change its type.
  
• If your const variable is a simple value, like string, number, boolean etc. you also wouldn’t be able to change its value.
  

So let’s look how does it work:

Declaration

Very simple. Just use const instead of var :

    const brandColor = 'turquoise';  

The value of a constant doesn’t have to to be known at compile time, but you must assign it a value exactly once.

Const and scopes

Redeclaring:

Constants can’t be redeclared.

    const num = 1;
    //1
    const num = 2;
    //Uncaught TypeError: Identifier 'num' has already been declared(…)

Function scope:

You can declare const inside the function scope with the same as outside it, but you actually have local scoped const.

    const num = 1;
    
    function setNum(){
        const num = 2;
        console.log(num);
    }
    
    setNum();
    //2;
    
    console.log(num);
    //1;

You cannot reach const outside scope:

    if(!num){
        const num = 2;
    }
    console.log(num);
    //Uncaught ReferenceError: num is not defined(…)

Ok that’s easy. But also you cannot reach const by this, because it is block-scoped:

    const date = new Date();
    
    console.log(this.date);
    //undefined

Deleting and changing value of const

You cannot change value or delete a simple constant variable.

    const a = 1;
    
    a++;
    //Uncaught TypeError: Assignment to constant variable.(…)
    
    delete a;
    //true
    
    console.log(a);
    //1

Const Objects

If you declare const Object, you will not be able to delete it, or make it array or string, but you will be able to declare, change and even delete values of your const Object.

    const params = {
        size: 2,
        color: 'gray'
    }
    
    params = {};
    //Uncaught TypeError: Assignment to constant variable.(…)
    
    params.size++;
    delete params.color;
    
    console.log(params)
    //Object {size: 3}

But there is the way to freeze the Object with its values:

    const obj = Object.freeze({
        'num': 3
    });
    obj.num = 4; //throws error

So where to use const ?

Use const when you don’t need reassign the identifier. In practice it is becoming the brand new var for objects and arrays.
Because if you need to change type of any of them, it seems you wrote bad, not semantic code.

And what about let ?

Read more in the LET in ECMAScript 6

ES 2015 provided const and let to let us write var-free javascript. So are you going to use var again? :)