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Const variables in JavaScript have a behavior that I would not expect. Coming from a .NET/C# background const, behaves quite a bit differently than what I was used to.

Looking at the developer documentation for JavaScript, the declaration of a const “creates a constant whose scope can be either global or local to the block in which it is declared” (docs). That sounds all normal; well because it is so far.

As we read further, we get to this line: “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.” The VERY important piece here is that the VALUE IS NOT IMMUTABLE.

Immutable? What the heck does immutable mean? Well, if an object is immutable, for that instance the properties cannot be changed. But since const objects are NOT immutable; it means that a const object can have its properties changed within it.

That’s just how it is and we have to work with it but if you come from the C# world and you think of a const never changing, be prepared for a const to be non-immutable in JavaScript.

Valid Example

Looking at a code example, this is a valid usage of const where we change the non-immutable value of an underlying property.

Example of valid usage of const in JavaScript

Invalid Example

Now, it wouldn’t be called const if everything was not constant about our objects. In the docs, we see that “the variable identifier cannot be reassigned”. What this means is that we cannot completely reassign our const reference.

The following code is an example of what we CANNOT do with a const:

Example of a invalid reassign of const

const in JavaScript made me uneasy at first. The ability to change properties within a const was a new concept to me. However, I took the time to learn about it and understand it. I hope that after this const is a little less scary for you as well.

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