Question

In JavaScript, I noticed that []==![] result is true. demo

I don’t understand this result. Can you explain why?

Answer 1

[] is an array, but ![] is a boolean value. Whenever you try to compare two objects with different types with == both objects should be transformed into comparable objects (using ToNumber, see step 7. in 11.9.3). This is why [] == ![] yields true, the first empty array gets evaluated to false.

11.9.3 The Abstract Equality Comparison Algorithm

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as
follows:

1. […]
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.
4. If Type(x) is Number and Type(y) is String,
   return the result of the comparison x == ToNumber(y).
5. If Type(x) is String and Type(y) is Number,
   return the result of the comparison ToNumber(x) == y.
6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
8. If Type(x) is either String or Number and Type(y) is Object,
   return the result of the comparison x == ToPrimitive(y).
9. If Type(x) is Object and Type(y) is either String or Number,
   return the result of the comparison ToPrimitive(x) == y.
10. Return false.

However, if you use the strict type comparison operator === the result is false, since both types differ:

11.9.6 The Strict Equality Comparison Algorithm

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed
as follows:

1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is Number, then
   1. If x is NaN, return false.
   2. If y is NaN, return false.
   3. If x is the same Number value as y, return true.
   4. If x is +0 and y is -0, return true.
   5. If x is -0 and y is +0, return true.
   6. Return false.
5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and
   same characters in corresponding positions); otherwise, return false.
6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
7. Return true if x and y refer to the same object. Otherwise, return false.