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JavaScript SyntaxError – Malformed formal parameter

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  • Last Updated : 31 Jul, 2020

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


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

Error Type:


Cause of Error: The argument list passed to 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 the problem, like a number or object.

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


    // error here
    var 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.


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

Output(in console):

SyntaxError: Expected ')'
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