The JavaScript exception "TypeError: can't set prototype: it would cause a prototype chain cycle" occurs when an object's prototype is set to an object such that the prototype chain becomes circular (a and b both have each other in their prototype chains).

Message

TypeError: Cyclic __proto__ value (V8-based)
TypeError: can't set prototype: it would cause a prototype chain cycle (Firefox)
TypeError: cyclic __proto__ value (Safari)

Error type

What went wrong?

A loop, also called a cycle, was introduced in a prototype chain. That means that when walking this prototype chain, the same place would be accessed over and over again, instead of eventually reaching null.

This error is thrown at the time of setting the prototype. In an operation like Object.setPrototypeOf(a, b), if a already exists in the prototype chain of b, this error will be thrown.

Examples

const a = {};
Object.setPrototypeOf(a, a);
// TypeError: can't set prototype: it would cause a prototype chain cycle

const a = {};
const b = {};
const c = {};
Object.setPrototypeOf(a, b);
Object.setPrototypeOf(b, c);
Object.setPrototypeOf(c, a);
// TypeError: can't set prototype: it would cause a prototype chain cycle

See also

• Object Prototypes
• Inheritance and the prototype chain