When you make something const in JavaScript, you can't reassign the variable itself to reference something else. However, the variable can still reference a mutable object.

const x = {a: 123};

// This is not allowed.  This would reassign `x` itself to refer to a
// different object.
x = {b: 456};

// This, however, is allowed.  This would mutate the object `x` refers to,
// but `x` itself hasn't been reassigned to refer to something else.
x.a = 456;

In the case of primitives such as strings and numbers, const is simpler to understand, since you don't mutate the values but instead assign a new value to the variable.

MDN sums it up nicely:

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered.

More succinctly: const creates an immutable binding.

In other words: const, like var, gives you a mutable chunk of memory in which you're storing something. However, const dictates that you must keep referring to that same chunk of memory – you can't reassign the variable to a different chunk of memory, because the variable reference is constant.

To really make something constant and unchanging after you've declared it, you need to use something like Object.freeze(). However, that's shallow and only works on key/value pairs. Freezing an entire object takes a bit more effort. To do so repeatedly in a performant way is yet more challenging. If you really have a need for that, I'd recommend checking out something like Immutable.js

Rebinding

const and let declarations control whether rebindings (aka reassignments) between identifiers and values are allowed:

const x = "initial value";
let y = "initial value";

// rebinding/reassignment

try { x = "reassignment" } catch(e) { console.log(x) } // fails

y = "reassignment"; // succeeds
console.log(y);

Immutability

Immutability is controlled at the type level. Object is a mutable type, whereas String is an immutable type:

const o = {mutable: true};
const x = "immutable";

// mutations

o.foo = true; // succeeds
x[0] = "I"; // fails

console.log(o); // {mutable: true, foo: true}
console.log(x); // immutable