Source:

False Values in JS

• "" (empty string)
• 0, -0, NaN (invalid number)
• null, undefined

Simple Rules Using ==, ===

• If either value (aka side) in a comparison could be the true or false value, avoid == and use ===.
• If either value in a comparison could be of these specific values (0, "", or [] -- empty array), avoid == and use ===.
• In all other cases, you're safe to use ==. Not only is it safe, but in many cases it simplifies your code in a way that improves readability.

Object(including function, array) Comparsion

Because those values are actually held by reference, both == and ===comparisons will simply check whether the references match, not anything about the underlying values.

arrays are by default coerced to strings by simply joining all the values with commas (,) in between.

var a = [1,2,3];var b = [1,2,3];var c = "1,2,3";a == c;     // trueb == c;     // truea == b;     // false

Inequality

• string values can also be compared for inequality, using typical alphabetic rules ("bar" < "foo")

var a = 41;var b = "42";var c = "43";a < b;      // trueb < c;      // true

•  If both values in the < comparison are strings, as it is with b < c, the comparison is made lexicographically.

• But if one or both is not a string, as it is with a < b, then both values are coerced to be numbers, and a typical numeric comparison occurs.

var a = 42;var b = "foo";a < b;      // falsea > b;      // falsea == b;     // false

• b value is being coerced to the "invalid number value" NaN in the < and > comparisons.
• NaN is neither greater-than nor less-than, equal-to any other value. Hence, it returns false.

Source:

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

1. 1. If (x) is the same as (y), then
      1. If (x) is Undefined, return true.
      2. If (x) is Null, return true.
      3. If (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 (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 (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 (x) is Number and (y) is String,
      return the result of the comparison x == (y).
   5. If (x) is String and (y) is Number,
      return the result of the comparison (x) == y.
   6. If (x) is Boolean, return the result of the comparison (x) == y.
   7. If (y) is Boolean, return the result of the comparison x == (y).
   8. If (x) is either String or Number and (y) is Object,
      return the result of the comparison x == (y).
   9. If (x) is Object and (y) is either String or Number,
      return the result of the comparison (x) == y.
   10. Return false.

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

1. If (x) is different from (y), return false.
2. If (x) is Undefined, return true.
3. If (x) is Null, return true.
4. If (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 (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 (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.