Constants in Javascript - ECMAScript 6


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?

So let’s look how does it work:


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


Constants can’t be redeclared.

    const num = 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;

You cannot reach const outside scope:

        const num = 2;
    //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();

Deleting and changing value of const

You cannot change value or delete a simple constant variable.

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

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.(…)
    delete params.color;
    //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? :)