SyntaxError: property name __proto__ appears more than once in object literal

The JavaScript exception "property name __proto__ appears more than once in object literal" occurs when an object literal contains multiple occurrences of the __proto__ field, which is used to set the prototype of this new object.

Message

SyntaxError: Duplicate __proto__ fields are not allowed in object literals (V8-based)
SyntaxError: property name __proto__ appears more than once in object literal (Firefox)
SyntaxError: Attempted to redefine __proto__ property. (Safari)

Error type

SyntaxError

What went wrong?

The __proto__ key, unlike other property keys, is a special syntax in an object literal. It is used to set the prototype of the object being created and is not allowed to appear more than once in an object literal. Note that this restriction only applies to the __proto__ prototype setter syntax: if it actually has the effect of creating a property called __proto__, then it can appear multiple times. See prototype setter for the exact syntax restrictions.

Worth noting that the __proto__ key in object literals is a special syntax and is not deprecated, unlike the Object.prototype.__proto__ accessor property.

Examples

Invalid cases

js
const obj = { __proto__: {}, __proto__: { a: 1 } };

Valid cases

js
// Only setting the prototype once
const obj = { __proto__: { a: 1 } };

// These syntaxes all create a property called "__proto__" and can coexist
// They would overwrite each other and the last one is actually used
const __proto__ = null;
const obj2 = {
  ["__proto__"]: {},
  __proto__,
  __proto__() {},
  get __proto__() {},
};

See also