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)

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.

In this case, the variable "arg" redeclares the argument.

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.

function f(arg) {
  arg = "foo";
}

function g(arg) {
  let bar = "foo";
}

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