JavaScript SyntaxError – Redeclaration of formal parameter “x”

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This JavaScript exception redeclaration of formal parameter occurs if a variable name is a function parameter and also declared again inside the function body using a let assignment.

Message:

SyntaxError: Let/Const redeclaration (Edge)
SyntaxError: redeclaration of formal parameter "x" (Firefox)
SyntaxError: Identifier "x" has already been declared (Chrome)

Error Type:

SyntaxError

Cause of Error: In a function declaration, a variable name in the function parameter and inside the function body, same variable name declared using a let assignment.

Example 1: In this example, the ‘let’ keyword is used to re-declare the parameter variable, So the error has occurred.

HTML

<!DOCTYPE html> 
<html> 
<head> 
    <title>Syntax Error</title> 
</head> 
<body> 
    <script> 
    function GFG(var_name) {  
      let var_name = 'This is GFG'; 
      return var_name; 
    } 
    document.write(GFG()); 
    </script> 
</body> 
</html>

Output(In console):

SyntaxError: Let/Const redeclaration

Example 2: In this example, the ‘const’ keyword is used to re-declare the parameter variable, So the error has occurred.

HTML

<!DOCTYPE html> 
<html> 
<head> 
    <title>Syntax Error</title> 
</head> 
<body> 
    <script> 
    function GFG(var_name) {  
      const var_name = 12345; 
      return var_name; 
    } 
    document.write(GFG()); 
    </script> 
</body> 
</html>

Output(In console):

SyntaxError: Let/Const redeclaration

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