JavaScript SyntaxError – Malformed formal parameter

This JavaScript exception malformed formal parameter occurs if the argument list of a Function() constructor call is not valid.

Message:

SyntaxError: Expected {x} (Edge)
SyntaxError: malformed formal parameter (Firefox)

Error Type:

SyntaxError

Cause of Error: The argument list passed to the function is not valid. The if or var can not be picked as an argument name, or there’s some stray punctuation in the argument list. The invalid value passed can cause a problem, like a number or an object.

Example 1: In this example, the number is used as an argument name. So the error has occurred.

Javascript

// error here
let f = Function(45, "alert('This is alert')");

Output(in console):

SyntaxError: Expected identifier

Example 2: In this example, there is a missing comma, So the error occurred.

Javascript

let f = Function('x y', 'return x + y;');

Output(in console):

SyntaxError: Expected ')'