Fix JavaScript TypeError: Assignment to constant variable (2025 Guide)

Fix TypeError: Assignment to constant variable in JavaScript - 2025 Guide

Posted on: March 14, 2025

If you’ve hit a "TypeError: Assignment to constant variable" in JavaScript, it’s because you tried to reassign a const variable. Let’s fix it fast in this 2025 guide!

What Causes "TypeError: Assignment to constant variable"?

This error happens when you try to change the value of a variable declared with const. Common causes include:

• Reassignment: Directly assigning a new value to a const variable.
• Misunderstanding Scope: Accidentally reassigning in a loop or function.
• Object Mutation Confusion: Thinking const prevents all changes (it doesn’t).

Check this demo (open console with F12):

Here, myVar is const, so reassigning it triggers the error.

How to Fix It: 3 Solutions

Let’s solve this with quick fixes:

(Diagram: Developer runs code, gets error, checks const, fixes with let or mutation.)

Solution 1: Use let for Reassignment

Switch to let if you need to reassign:

// Wrong
const myVar = 10;
myVar = 20; // TypeError

// Fixed
let myVar2 = 10;
myVar2 = 20; // Works fine
console.log(myVar2);

Use let for variables that need reassignment.

Solution 2: Mutate Objects/Arrays

const allows object/array mutation, not reassignment:

// Wrong
const myObj = { value: 10 };
myObj = { value: 20 }; // TypeError

// Fixed
const myObj2 = { value: 10 };
myObj2.value = 20; // Works fine
console.log(myObj2.value);

Modify properties instead of reassigning the object.

Solution 3: Check Scope Issues

Avoid accidental reassignment in loops or functions:

// Wrong
for (const i = 0; i < 5; i++) { // TypeError in some contexts
  console.log(i);
}

// Fixed
for (let i = 0; i < 5; i++) {
  console.log(i);
}

Use let in loops where reassignment is needed.

Quick Checklist

• Is the variable meant to be reassigned? (Use let)
• Can you mutate instead of reassign? (Check objects/arrays)
• Is it a scope issue? (Fix loop or function logic)

Conclusion

The "TypeError: Assignment to constant variable" is easy to fix with the right approach. These 2025 solutions will get you sorted fast. Got another JavaScript error to solve? Let us know in the comments!