TypeError: can't define property "x": "obj" is not extensible

Message

TypeError: Cannot add property x, object is not extensible (V8-based)
TypeError: Cannot define property x, object is not extensible (V8-based)
TypeError: can't define property "x": Object is not extensible (Firefox)
TypeError: Attempting to define property on object that is not extensible. (Safari)

Error type

What went wrong?

Usually, an object is extensible and new properties can be added to it. However, in this case the object isn't extensible, so that it will never have properties beyond the ones it had at the time it was marked as non-extensible. You could have marked the object as non-extensible by calling Object.preventExtensions(), Object.seal(), or Object.freeze(), or a library you are using could have done that for you.

Examples

Adding new properties to a non-extensible objects

In strict mode, attempting to add new properties to a non-extensible object via assignment throws a TypeError. In sloppy mode, the addition of the "x" property is silently ignored.

"use strict";

const obj = {};
Object.preventExtensions(obj);

obj.x = "foo";
// TypeError: can't define property "x": Object is not extensible

In both strict mode and sloppy mode, a call to Object.defineProperty() throws when adding a new property to a non-extensible object.

const obj = {};
Object.preventExtensions(obj);

Object.defineProperty(obj, "x", { value: "foo" });
// TypeError: can't define property "x": Object is not extensible

Non-extensible objects created by other means

The Object.seal() and Object.freeze() methods also create non-extensible objects—they just have additional restrictions on modifying existing properties as well.

"use strict";

const obj = { y: "bar" };
Object.seal(obj);
obj.x = "foo";
// TypeError: can't define property "x": Object is not extensible

Fixing the error

There are three ways to fix this error: you can remove the property addition entirely if you don't need it, you can copy the existing properties to a new extensible object, or you can add the property before making the object non-extensible.

"use strict";

const obj = {};
obj.x = "foo"; // add property first and only then prevent extensions

Object.preventExtensions(obj);

See also

• Object.preventExtensions()
• Object.seal()
• Object.freeze()