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TypeError: Invalid operand to 'in' (Edge)
TypeError: right-hand side of 'in' should be an object, got 'x' (Firefox)
TypeError: cannot use 'in' operator to search for 'x' in 'y' (Firefox, Chrome)

TypeError

in

(Python ) in

"Hello" in "Hello World";
// TypeError: cannot use 'in' operator to search for 'Hello' in 'Hello World'

String.prototype.indexOf()

"Hello World".indexOf("Hello") !== -1;
// true

null undefined

null undefined

var foo = null;
"bar" in foo;
// TypeError: cannot use 'in' operator to search for 'bar' in 'foo' (Chrome)
// TypeError: right-hand side of 'in' should be an object, got null (Firefox)

in

var foo = { baz: "bar" };
"bar" in foo; // false

"PI" in Math; // true
"pi" in Math; // false

in Array in

var trees = ["redwood", "bay", "cedar", "oak", "maple"];
3 in trees; // true
"oak" in trees; // false

in

MDN

MDN 2024424

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*