Javascript logical "!==" operator?

You can find === and !== operators in several other dynamically-typed languages as well. It always means that the two values are not only compared by their "implied" value (i.e. either or both values might get converted to make them comparable), but also by their original type.

That basically means that if 0 == "0" returns true, 0 === "0" will return false because you are comparing a number and a string. Similarly, while 0 != "0" returns false, 0 !== "0" returns true.

It's != without type coercion. See the MDN documentation for comparison operators.

Also see this StackOverflow answer, which includes a quote from "JavaScript: The Good Parts" about the problems with == and !=. (null == undefined, false == "0", etc.)

Short answer: always use === and !== unless you have a compelling reason to do otherwise. (Tools like JSLint, JSHint, ESLint, etc. will give you this same advice.)

Copied from the formal specification: ECMAScript 5.1 section 11.9.5

11.9.4 The Strict Equals Operator ( === )

The production EqualityExpression : EqualityExpression === RelationalExpression is evaluated as follows:

1. Let lref be the result of evaluating EqualityExpression.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating RelationalExpression.
4. Let rval be GetValue(rref).
5. Return the result of performing the strict equality comparison rval === lval. (See 11.9.6)

11.9.5 The Strict Does-not-equal Operator ( !== )

The production EqualityExpression : EqualityExpression !== RelationalExpression is evaluated as follows:

1. Let lref be the result of evaluating EqualityExpression.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating RelationalExpression.
4. Let rval be GetValue(rref). Let r be the result of performing strict equality comparison rval === lval. (See 11.9.6)
5. If r is true, return false. Otherwise, return true.

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. Type(x) is Undefined, return true.
3. Type(x) is Null, return true.
4. 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.