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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)

SyntaxError

let JavaScript let

"arg"

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

// SyntaxError: redeclaration of formal parameter "arg"

"arg" let

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

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

MDN

MDN 2026514

GitHub

JavaScript
JavaScript
1. (+)
2. (+=)
3. (=)
4. async function
5. async function*
6. await
7. (&)
8. (&=)
9. (~)
10. (|)
11. (|=)
12. (^)
13. (^=)
14. class
15. (,)
17. (--)
18. delete
20. (/)
21. (/=)
22. (==)
23. (**)
24. (**=)
25. function
26. function*
27. (>)
28. (>=)
29. ( )
30. import.meta
  1. import.meta.resolve()
31. import()
32. in
33. (++)
34. (!=)
35. instanceof
36. (<<)
37. (<<=)
38. (<)
39. (<=)
40. (&&)
41. (&&=)
42. (!)
43. (||)
44. (||=)
45. (*)
46. (*=)
47. new
48. new.target
49. null
50. (??=)
51. (??)
54. (?.)
56. (%)
57. (%=)
58. (>>)
59. (>>=)
60. (...)
61. (===)
62. (!==)
63. (-)
64. (-=)
65. super
66. this
67. typeof
68. (-)
69. (+)
70. (>>>)
71. (>>>=)
72. void
73. yield
74. yield*