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SyntaxError: Unexpected '#' used outside of class body

The JavaScript exception "Unexpected '#' used outside of class body" occurs when a hash ("#") is encountered in an unexpected context, most notably outside of a class declaration. Hashes are valid at the beginning of a file as a hashbang comment, or inside of a class as part of a private field. You may encounter this error if you forget the quotation marks when trying to access a DOM identifier as well.

Message

SyntaxError: Unexpected '#' used outside of class body.

Error type

SyntaxError

What went wrong?

We encountered a # somewhere unexpected. This may be due to code moving around and no longer being part of a class, a hashbang comment found on a line other than the first line of a file, or accidentally forgetting the quotation marks around a DOM identifier.

Examples

Missing quotation marks

For each case, there might be something slightly wrong. For example

js
document.querySelector(#some-element)

This can be fixed via

js
document.querySelector("#some-element");

Outside of a class

js
class ClassWithPrivateField {
  #privateField;

  constructor() {}
}

this.#privateField = 42;

This can be fixed by moving the private field back into the class

js
class ClassWithPrivateField {
  #privateField;

  constructor() {
    this.#privateField = 42;
  }
}

See also

© 2005–2023 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Hash_outside_class