javascript - Keyword 'const' does not make the value immutable. What does it mean?

Question

There's the const definition in Exploring ES6 by Dr. Axel Rauschmayer:

const works like let, but the variable you declare must be immediately initialized, with a value that can’t be changed afterwards. […]

const bar = 123;
bar = 456;  // TypeError: `bar` is read-only

and then he writes

Pitfall: const does not make the value immutable

const only means that a variable always has the same value, but it does not mean that the value itself is or becomes immutable.

I am little confused with this pitfall. Can any one clearly define the const with this pitfall?

Answer 1

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.