One of the cornerstones of functional programming is that so called variables can be assigned only once. This is known as immutability. ECMAScript 2015 supports a new keyword, const. The const keyword can be used in the same way as var except that variables assigned with const will be immutable. For instance, the following code shows a variable and a constant that are both manipulated in the same way:

let numberOfQueens = 1;
const numberOfKings = 1;
numberOfQueens++;
numberOfKings++;
console.log(numberOfQueens);
console.log(numberOfKings);

The output of running this is the following:

2
1

As you can see, the results for the constant and variable are different.

If you're using an older browser without support, then const won't be available to you. A possible workaround is to make use of the Object.freeze functionality which is more widely adopted:

let consts = Object.freeze({ pi : 3.141});
consts.pi = 7;
console.log(consts.pi);//outputs 3.141

As you can see, the syntax here is not very user-friendly. Also an issue is that attempting to assign to an already assigned const simply fails silently instead of throwing an error. Failing silently in this fashion is not at all a desirable behavior; a full exception should be thrown. If you enable strict mode, a more rigorous parsing mode is added in ECMAScript 5, and an exception is actually thrown:

"use strict";
var consts = Object.freeze({ pi : 3.141});
consts.pi = 7;

The preceding code will throw the following error:

consts.pi = 7;
          ^
TypeError: Cannot assign to read only property 'pi' of #<Object>

An alternative is the object.Create syntax we spoke about earlier. When creating properties on the object, one can specify writable: false to make the property immutable:

var t = Object.create(Object.prototype,
{ value: { writable: false,
  value: 10}
});
t.value = 7;
console.log(t.value);//prints 10

However, even in strict mode no exception is thrown when attempting to write to a non-writable property. Thus I would claim that the const keyword is not perfect for implementing immutable objects. You're better off using freeze.