Equality in Javascript

Relevant Links

• Flanagan’s book, “Relational Expressions” section of chapter 4
• MDN’s description of equality rules. In particular check the table for loose equality cases.
• The official description of strict equality, from the standard

Equality

• Javascript has two different “equality” operations, == (loose equality) and === (strict equality).
• You should almost never use ==.
• Stick to ===, which is a more strict equality test.

• Essentially == does a “type conversion” before comparing. This leads to some weird behavior (for instance it is no longer transitive). Some examples:

  0 == "0"          // true
  0 == ""           // true
  "" == "0"         // false
  0 === "0"         // false
  false == "false"  // false
  0 == false        // true

• Two objects are only equal if they are literally the same object:

  var o = { foo: 2 }; var o2 = { foo: 2 };
  o == o2;          // false
  o == o;           // true

Exception: There is one case where using loose equality works well: If you want to capture both “undefined” and “null”. So o == null is going be true both when o is undefined and when it is null.