How to Fix SyntaxError: arguments is not valid in Fields in JavaScript?

A Syntax Error argument is not valid in fields occurs in JavaScript when the arguments object is used improperly within the context of a class field or similar construct where it's not allowed, the arguments object is an array-like object accessible within the scope of a function that contains the values of the arguments passed to that function, in order to provide clarity on the subject matter, we shall discuss about this mistake with regards to its causes and how it can be corrected by using illustrations.

Understanding the Error

The arguments object is an array-like object accessible only within the scope of a function, it contains the values of the arguments passed to that function, using arguments outside of a function scope, such as in-class field definitions or object initializers, is not allowed in JavaScript and it will result in a Syntax Error arguments is not valid in fields.

Case 1: Error Cause: Using arguments in a Class Field Initializer

When you are using arguments directly in a class field definition will result in a Syntax Error.

Example:

function makeOne() {
  class C {
    args = { ...arguments }; // SyntaxError: arguments is not valid in fields
  }
  return new C();
}

Output:

SyntaxError: arguments is not valid in fields

Resolution of error:

If you want to fix this then you have to save parameters in a variable and use that variable to initialize class field.

function makeOne() {
  const _arguments = arguments;
  class C {
    args = { ..._arguments }; // Only the identifier is forbidden
  }
  return new C();
}

const instance = makeOne(1, 2, 3);
console.log(instance.args); 

{ '0': 1, '1': 2, '2': 3 }

Case 2: Error Cause: Using arguments in a Static Initialization Block

When you are using arguments in a static initializer will also lead to a Syntax Error.

Example:

let CArgs;
class C {
  static {
    CArgs = arguments; // SyntaxError: arguments is not valid in fields
  }
}

Output:

SyntaxError: arguments is not valid in fields

Resolution of error:

You cannot access arguments within a static initialization block rather move your code into either a method or constructor where you can get at these arguments.

let CArgs;
class C {
    static initialize(...args) {
        CArgs = args;
    }
}

C.initialize(1, 2, 3);
console.log(CArgs); // [1, 2, 3]

[ 1, 2, 3 ]

Conclusion

In conclusion to keep away from the "arguments is not valid in fields " mistake in JavaScript, one must ensure that arguments object is always used within any given function’s context since it cannot be directly accessed from an object literal or class field initializer, this enables maintaining correctness in writing clean codes at all times by enabling one to understand where to apply this method accordingly.