JavaScript

Assignment to Constant Variable: What It Means and How to Fix It

Error reference

Assignment to Constant Variable

Language

JavaScript

Severity

Medium

When it happens

This TypeError is thrown at runtime when you attempt to reassign a variable declared with const.

Potential fixes

Use let for variables whose values will change. Reserve const for values that are truly fixed. Remember that const objects can still have their properties mutated; use Object.freeze() if you need true immutability.

Related errors

TypeError TS2540: Readonly Violation

Deep dive

This TypeError is thrown at runtime when you attempt to reassign a variable declared with const. const creates a binding that cannot be reassigned after initialization. Note that for objects and arrays, the binding is constant but the contents can still be mutated.

Why it happens

•Directly reassigning a const variable with = after its initial declaration.
•Using += or ++ on a const variable.
•Accidentally declaring a loop counter with const instead of let.
•Mistakenly thinking const makes a full deep-freeze of objects.

Potential fixes

Use let for variables whose values will change. Reserve const for values that are truly fixed. Remember that const objects can still have their properties mutated; use Object.freeze() if you need true immutability.

Examples

JavaScript

Code that triggers the error

const MAX_RETRIES = 3;
MAX_RETRIES = 5;

Error output

TypeError: Assignment to constant variable.

Fixed code

// Use let if the value needs to change
let maxRetries = 3;
maxRetries = 5;

// Or keep const if the value is truly fixed
const MAX_RETRIES = 3;

Practice in English

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