"Imperfection is beauty, madness is genius and it's better to be absolutely ridiculous than absolutely boring."

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subjective null

{} instanceof null
// Uncaught TypeError: Right-hand side of 'instanceof' is not an object

typeof null
// 'object'

Explanation

null is not an object, but a primitive value. typeof null returning 'object' is a bug, which is not fixed due to backward-compatability.

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'2' + '2' - '2' = 20

'2' + '2' - '2'
// 20

Explanation

• + is both concatenation and addition ('2' + '2' is '22');
• - is only subtraction and arguments are coerced to numbers ('22' - 2 is 20).

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typeof undeclared

console.log(abc)
// Uncaught ReferenceError: abc is not defined

console.log(typeof abc)
// 'undefined'

Explanation

typeof operator can check whether the variable has been declared.

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sparse arrays

console.log(Array(5))
// [ <5 empty items> ]

const arr = [1,,,2,,,3]

console.log(arr.length)
// 7

arr.forEach((item, idx) => console.log(item, idx))
// 1 0
// 2 3
// 3 6

console.log(JSON.stringify(arr))
// '[1,null,null,2,null,null,3]'

const arr2 = [...arr]
arr2.forEach((item, idx) => console.log(item, idx))
// 1 0
// undefined 1
// undefined 2
// 2 3
// undefined 4
// undefined 5
// 3 6

arr[1000] = 1

console.log(arr.length)
// 1001

console.log(arr)
// [ 1, <2 empty items>, 2, <2 empty items>, 3, <993 empty items>, 1 ]

Story

Someday I was writing a Telegram bot and spent several hours debugging one issue.

Bot had logging functionality and logged something JSON.stringified, and after some changes I got an error:

FATAL ERROR: JS Allocation failed - process out of memory

In short, the problem was that I was setting some array's item by Telegram user's ID (which was big like 987654321). So JS tried to stringify an array with nine hundred eighty-seven million six hundred fifty-four thousand three hundred twenty-one null and ran out of memory:

arr[user.id] = 'hi'
JSON.stringify(arr)
// FATAL ERROR: JS Allocation failed - process out of memory

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parseInt

parseInt(0.5)
// 0

parseInt(0.005)
// 0

parseInt(0.0000005)
// 5

Explanation

• The parseInt function converts its first argument to a string, parses that string, then returns an integer or NaN.
• If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point.
• (0.5).toString() -> '0.5'
• (0.0000005).toString() -> '5e-7'

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018 === 022

018 === 022
// true

Explanation

• A leading 0 on an integer literal, or a leading 0o (or 0O) indicates it is in octal. Octal integer literals can include only the digits 0 – 7. (MDN Web Docs)
• 022 starts with 0 => it's in octal
• 018 contains 8 => it's not in octal, but in decimal
• 22 in octal = 18 in decimal