TypeError: "x" is read-only

The JavaScript strict mode-only exception "is read-only" occurs when a global variable or object property that was assigned to is a read-only property.

Message#

TypeError: Cannot assign to read only property 'x' of #<Object> (V8-based)
TypeError: "x" is read-only (Firefox)
TypeError: Attempted to assign to readonly property. (Safari)

Error type#

{{jsxref("TypeError")}} in strict mode only.

What went wrong?#

The global variable or object property that was assigned to is a read-only property. (Technically, it is a non-writable data property.)

This error happens only in strict mode code. In non-strict code, the assignment is silently ignored.

Examples#

Invalid cases#

Read-only properties are not super common, but they can be created using {{jsxref("Object.defineProperty()")}} or {{jsxref("Object.freeze()")}}.

```js example-bad "use strict"; const obj = Object.freeze({ name: "Elsa", score: 157 }); obj.score = 0; // TypeError

("use strict"); Object.defineProperty(this, "LUNG_COUNT", { value: 2, writable: false }); LUNG_COUNT = 3; // TypeError

("use strict"); const frozenArray = Object.freeze([0, 1, 2]); frozenArray[0]++; // TypeError

There are also a few read-only properties built into JavaScript. Maybe you tried to
redefine a mathematical constant.

```js example-bad
"use strict";
Math.PI = 4; // TypeError

Sorry, you can't do that.

The global variable undefined is also read-only, so you can't silence the infamous "undefined is not a function" error by doing this:

```js example-bad "use strict"; undefined = function () {}; // TypeError: "undefined" is read-only

### Valid cases

```js example-good
"use strict";
let obj = Object.freeze({ name: "Score", points: 157 });
obj = { name: obj.name, points: 0 }; // replacing it with a new object works

See also#

• {{jsxref("Object.defineProperty()")}}
• {{jsxref("Object.freeze()")}}