My Favorite JavaScript Oddities

3 min readOct 30, 2019

There are so many oddities in an interpreted language like JavaScript. I’m sharing a list of my favorites. To make this article more accessible, I skipped the more complex quirks, the ones requiring a deeper understanding of JavaScript and its syntax. I’m also skipping the complex behavior of the “==” operator, a source of so many jokes in the JS community.

If you have a JS oddity you would like to add to this list, please let me know.

Curiosity #1

> ('b' + 'a' + + 'a' + 'a').toLowerCase()  "banana"

The middle + “a” (since it’s preceded by another +) is a unary coercion of “a” into a number, which fails and results in NaN, which is then concatenated in and lower-cased. If we remove the .lowercase() the error becomes evident because we get baNaNa.

Curiosity #2

> [] + []  ""

Adding two empty arrays returns an empty string (Gary Bernhardt). I don’t have any explanation for this.

Curiosity #3

> [] + {}
  [object Object] > {} + []
  0

Adding an empty array to an empty object returns an object, while the reverse, adding an empty object to an empty array returns zero (Gary Bernhardt). I don’t have any explanation for this.

Curiosity #4

> 1 - []
  1> 1 - {}
  NaN> 1 + []
  "1"

Subtracting an empty array from a number returns the number, while subtracting an empty object from a number returns NaN. Interestingly, adding an empty array to a number returns the string version of the number.

Curiosity #5

> [] == [] 
  false> [] != []
  true> ![]
  false

Apparently, an empty array is not the same as an empty array, but not an empty array is false (Brian Leroux).

Curiosity #6

> [3, 2, 1].sort()
  [1, 2, 3]> [3, 2, 10].sort()
  [10, 2, 3]

By default, the sorting of an array uses the lexical comparator converting every item to a string.

Curiosity #7

> 0.1 + 0.1
  0.2> 0.1 + 0.2
  0.30000000000000004

This is an error common to several languages and is related to the way JavaScript represents floating points.

Curiosity #8

> (3).toString()
  "3"> 3.toString()
  Uncaught SyntaxError: Invalid or unexpected token> 3..toString()
  "3"> 3.0.toString()
  "3"> Number(3).toString()
  "3"

This time the trick is to convince JS to convert the number 3 to an object before converting it to a string. The behavior remains anti-intuitive at best.

Curiosity #9

> typeof 3
  "number"> typeof NaN
  "number"

Apparently, NaN (Not a Number) is a number, somehow.

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