How does string equality work in JavaScript?

Question

There are plenty of tutorials for == and === so please don't guide me to a basic tutorial, my question is a bit more specific:

For example http://www.w3schools.com/jsref/jsref_obj_string.asp states that:

Syntax:

var txt = new String("string");

// or more simply:
var txt = "string";

Good, but what about this?

alert(new String("a") == new String("a")); // false
alert("a" == "a"); // true

var a = new String("a");
var b = new String("a");
alert(a == b); // false

var c = "a";
var d = "a";
alert(c == d); // true

alert(c === d); // true
alert (a === c); // false

Of course nobody calls new String() anyway, but is it something about the equality failing because new String() is handled as an object not as a string?

And of course W3Schools is not the most trusted source but I would have expected all of the above alerts to say true.

Please explain.

Answer 1

The "surprising results" come from the way Javascript handles equality for Objects, plus the confusion that arises between string literals and String objects. From the Mozilla reference guide for the == operator:

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible. If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

You can experience the same behavior with numbers:

new Number(5) == new Number(5) // false

And clarify your mind by:

typeof "string" // string
typeof new String("string") // object