SyntaxError: redeclaration of formal parameter "x"
The JavaScript exception "redeclaration of formal parameter" occurs when the same
variable name occurs as a function parameter and is then redeclared using a
let
assignment in a function body again.
Message
SyntaxError: Identifier "x" has already been declared (V8-based) SyntaxError: redeclaration of formal parameter "x" (Firefox) SyntaxError: Cannot declare a let variable twice: 'x'. (Safari)
Error type
What went wrong?
The same variable name occurs as a function parameter and is then redeclared using a
let
assignment in a function body again. Redeclaring the
same variable within the same function or block scope using let
is not
allowed in JavaScript.
Examples
Redeclared argument
In this case, the variable "arg" redeclares the argument.
js
function f(arg) {
let arg = "foo";
}
// SyntaxError: redeclaration of formal parameter "arg"
If you want to change the value of "arg" in the function body, you can do so, but you
do not need to declare the same variable again. In other words: you can omit the
let
keyword. If you want to create a new variable, you need to rename it as
conflicts with the function parameter already.
js
function f(arg) {
arg = "foo";
}
function g(arg) {
let bar = "foo";
}