javascript gotcha: empty list to boolean conversion

Question

Why both

 [] == false

and

![] == false

are true?

Answer 1

The === operator is your friend. Never use the == operator; it will bite you, as you can see.

Answer 2

"For the ! operator, if expression is nonzero, result is zero. If expression is zero, result is 1." (link)

First, [] is inherently false.

Since the expression [] is nonzero, then the result of ![] is zero.

Therefore, ![] == false evaluates to 0 == false which evaluates to true.

Answer 3

An empty list, [] must be considered a "falsey", however in ![] the ! is then converting the operation to check for an object reference, where a non-null is not considered a "falsey".

Answer 4

Before [] is compared to false, it is coerced to a string which is coerced to a number, the empty string in the case of the empty array. The empty string coerces to 0 which is the same value that false coerces to numerically.

Use === instead of == to avoid this problem

These other arrays are also falsey:

  ['']
  [[[]]]
  (function () { var arr = []; arr[0] = arr; })()
  [0]
  ['-0.0']