Empty array is false, but in IF statement it returns true [duplicate]

Answer 1

this is because == in JS forces conversion and if one type can be converted to another the return value is true and here because [] cant be changed or compared with bool it is false

where as if([]) checks for null and undefined values and because [] is neither null or undefined it is returning true

check this Which equals operator (== vs ===) should be used in JavaScript comparisons?

Answer 2

According to section 11.9.3 of the ECMAScript® Language Specification, any == comparison is done as follows:

1. If Type(x) is the same as Type(y), then
   1. If Type(x) is Undefined, return true.
   2. If Type(x) is Null, return true.
   3. 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.
   4. 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.
   5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
   6. Return true if x and y refer to the same object. Otherwise, return false.
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.

In the first step, ToNumber() is applied to false and yields Number(0). In the second step, rule #9 applies ToPrimitive() to the empty array and yields "" which, cast to a numeric value, becomes Number(0) as well.

Additionally section 9.2 says this about using an object in an expression:

The abstract operation ToBoolean converts its argument to a value of type Boolean according to this table:

• Undefined -> false
• Null -> false
• Boolean -> The result equals the input argument (no conversion).
• Number -> The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
• String -> The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
• Object -> true