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Discussion on: Declaring Variables in JavaScript

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Daniel Brady Author • Edited on

Someone asked me a really good question about const offline, so I recapped my answer in the comments:

Regarding this statement I made:

Sometimes, I want to give a name that never changes meaning, to a value that never changes. No single construct in JavaScript can enforce this.

Someone offline (to DEV, anyway) gave me a really good "But what about this..." example, so I wanted to share it and elaborate a bit on why what my statement still holds.

Basically, they wanted to know why this simple declaration doesn't provide complete immutability:

const x = 42;
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My answer is that, at least according to my own understanding of JavaScript, it does. However, it is not because of const alone. This declaration creates a box with contents that can be neither replaced nor modified because of two things:

  1. const prevents re-assignments
  2. numbers are immutable value types

If you replaced the initial value 42 with an object (arrays are a kind of JS object), then all of a sudden you've got a mutable value in an immutable variable, and the contents of your const box can be modified because the value itself can be modified:

/* Totally valid but very confusing so I don't like using `const` for this 🙂*/
const config = { some_setting: 42 }; // immutable variable
config.some_setting = "cat"; // mutable value
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This is what I meant when I said that no single construct in JavaScript can enforce complete immutability. Hope this helps!