Logical operator && and two strings in javascript

in the expression

"Cat" && "Dog"
// => "Dog"

Because you're using &&, JavaScript is promising you that it will verify that both sides of the expression are true. In this case, "Dog" is the just the last evaluated thing.

To be more explicit, you could do something like

var c = "Cat" != null && "Dog" != null

It's a little bit more wordy, but this boils down to

var c = true && true
console.log(c)
// => true

If you want a simple shortcut for the boolean, use the Boolean constructor -

var c = Boolean("Cat" && "Dog")
console.log(c)
// => true

If you just use a simple REPL or JavaScript console, you'd be able to see this output very easily.

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Per one of the comments below

Using ||, JavaScript is promising you that at least one of the sides is true. Since "Cat" is true, it stops there and returns "Cat". This is known as Short-circuit evaluation

Answer by @naomik pretty much covers it. If you want to go deep, I suggest taking a look at the ECMAScript specification, section 11.11:

http://www.ecma-international.org/ecma-262/5.1/#sec-11.11

In the "Semantics" section, we can see:

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:

1. Let lref be the result of evaluating LogicalANDExpression.
2. Let lval be GetValue(lref).
3. If ToBoolean(lval) is false, return lval.
4. Let rref be the result of evaluating BitwiseORExpression.
5. Return GetValue(rref).

I think this is because of nature of logical operator &&. Javascript executes expression "cat" && "Dog" in following way:

Checks the value "cat" - it returns something else than "", 0, null, undefined or false and is truthy. Javascript sees && and so it continues expression and faces string "Dog", which is something else than "", 0, null, undefined or false and is truthy, expression ends and expression returns the last value it checked.

If you replace && with || expressions ends at first truthy value and thats way

var boolean = "cat" || "dog"

returns first value: cat