This seems to be a very common misconception that just won’t die. I keep running into it in blog posts, Twitter discussions, and even books. Here’s my attempt at setting things straight.
ES6 const does not indicate that a value is ‘constant’ or immutable. A const value can definitely change. The following is perfectly valid ES6 code that does not throw an exception:
const foo = {}; foo.bar = 42; console.log(foo.bar); // → 42
The only thing that’s immutable here is the binding. const assigns a value ({}) to a variable name (foo), and guarantees that no rebinding will happen. Using an assignment operator or a unary or postfix -- or ++ operator on a const variable throws a TypeError exception:
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So, how to make a value immutable?
Primitive values, i.e. numbers, strings, booleans, symbols, null, or undefined, are always immutable.
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const vs. let
The only difference between const and let is that const makes the contract that no rebinding will happen.
Everything I wrote here so far are facts. What follows is entirely subjective, but bear with me.