javaScript == vs ===

To quote Douglas Crockford’s excellent JavaScript: The Good Parts :

JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:

” == ‘0’           // false

0 == ”             // true

0 == ‘0’            // true

 

false == ‘false’    // false

false == ‘0’        // true

 

false == undefined  // false

false == null       // false

null == undefined   // true

 

‘ \t\r\n ‘ == 0     // true

The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use ===and !==. All of the comparisons just shown produce false with the === operator.

If we read ECMA-262, we will shortly discover very interesting 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. If Type(x) is different from Type(y), go to step 14.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is not Number, go to step 11.
  5. If x is NaN, return false.
  6. If y is NaN, return false.
  7. If x is the same number value as y, return true.
  8. If x is +0 and y is -0, return true.
  9. If x is -0 and y is +0, return true.
  10. Return false.
  11. 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.
  12. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
  13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
  14. If x is null and y is undefined, return true.
  15. If x is undefined and y is null, return true.
  16. If Type(x) is Number and Type(y) is String, return the result of the comparison x== ToNumber(y).
  17. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x)== y.
  18. If Type(x) is Boolean, return the result of the comparison ToNumber(x)== y.
  19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  20. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparisn x == ToPrimitive(y).
  21. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x)== y.
  22. Return false.
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