babel/babel · error · TypeError

"${name}" is read-only

Error message

"${name}" is read-only

What it means

This TypeError is thrown by _readOnlyError(name), emitted by Babel's block-scoping transform when a `const`-declared binding is assigned in source. Because the transform may downlevel `const` to `var` (losing native read-only enforcement), it inserts this helper at assignment sites to preserve the runtime TypeError that `const` would normally produce. The thrown message names the offending binding.

Source

Thrown at packages/babel-helpers/src/helpers/readOnlyError.ts:4

/* @minVersion 7.0.0-beta.0 */

export default function _readOnlyError(name: string) {
  throw new TypeError('"' + name + '" is read-only');
}

View on GitHub (pinned to 06b6eae39d)

Solutions

1. Change the declaration from `const` to `let` if reassignment is intended.
2. If the binding should be immutable, fix the logic so it never reassigns (mutate properties instead: `cfg.field = x`).
3. Enable a linter rule (prefer-const / no-const-assign) to catch this at edit time.

Example fix

// before
const total = 0;
for (const n of nums) total += n; // throws

// after
let total = 0;
for (const n of nums) total += n;

// Decide intent before declaring:
// - need reassignment? use `let`
// - immutable binding? use `const` and never reassign
let total = 0;
total += n;

When it happens

Trigger: Any assignment to a `const` variable: `const x = 1; x = 2;`, `const cfg = {}; cfg = newCfg;`, `++counter` where counter is const, or compound assignment `x += 1` on a const.

Common situations: Accidentally declaring with `const` instead of `let`; refactoring that changes a value's mutation pattern; copy-paste leaving `const` on a variable that needs reassignment; linter disabled so the mistake reaches the transpiler.

Related errors

• Invalid attempt to destructure non-iterable instance. In ord
• Invalid attempt to spread non-iterable instance. In order to
• Cannot set property of null or undefined.
• Cannot destructure ${obj}
• Derived constructors may only return object or undefined

AI-assisted analysis of babel/babel@06b6eae39d (2026-08-03). Data as JSON: /data/errors/390dd5cebf5dca1f.json. Report an issue: GitHub.