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

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subjective null

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

typeofnull// '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)// ReferenceError: abc is not defined

console.log(typeofabc)// '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> ]constarr=[1,,,2,,,3]console.log(arr.length)// 7arr.forEach((item,idx)=>console.log(item,idx))// 1 0// 2 3// 3 6console.log(JSON.stringify(arr))// '[1,null,null,2,null,null,3]'constarr2=[...arr]arr2.forEach((item,idx)=>console.log(item,idx))// 1 0// undefined 1// undefined 2// 2 3// undefined 4// undefined 5// 3 6arr[1000]=1console.log(arr.length)// 1001console.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)// 0parseInt(0.005)// 0parseInt(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

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object wrappers

letx='hi'x.lang// undefinedx.lang='eng'// 'eng'x.lang// undefined'lang'inx// TypeError: Cannot use 'in' operator to search for 'lang' in hi