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Discussion on: Share a code snippet in any programming language and describe what's going on

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crowdozer profile image
crowdozer • 
// javascript 

{
  const x = 1
  {
    const x = 2 // no error
    {
      console.log(x) // 2
      const x = 3 
      console.log(x) // 3
    }
  }
}

console.log(x) // undefined
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In javascript, const denotes a read-only variable. If you try to reassign it, you'll get a "SyntaxError: Identifier has already been declared"

However, each braced segment is treated as a new scope. This is most obvious with functions and control structures like if (true) { ... }, but it's not immediately obvious to people that you can arbitrarily create new scopes anywhere. Switch statements are a good place to use them.

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pothieug profile image
Gilles Pothieu •

Wow, thanks for the hint !

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moopet profile image
Ben Sinclair •

Const doesn't denote read-only, it only means you can't reassign it in the same scope. You can mutate it all you want (provided it's a type that's mutable in the first place)

const x = [1, 2, 3];

x.push(4);
delete x[0];

// [ <empty slot>, 2, 3, 4 ]
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crowdozer profile image
crowdozer • • Edited

I worded it a bit slopy, a "read-only reference to a value" according to MDN, but it's all semantics as long as we understand how it works =]