Javascript how to use the and/or operator with parenthesis

Question

I have an object that contains a string HelloWorld in the attribute hello. I want to check for two strings, and if it does not match with either one, then I want to execute certain code.

var item = { hello: "HelloWorld" }
item.hello !== "HelloWorld" && item.hello !== "GoodbyeWorld" // false, correct

However, I feel this can be optimized and made more readable:

item.hello !== ("GoodbyeWorld" && "HelloWorld") // false, correct
item.hello !== ("HelloWorld" && "GoodbyeWorld") // true   WTF?

I expected both checks to be falsy, but surely I'm missing something here. I think I do not have a correct understanding of the AND/OR operator in javascript, or I am using the parenthesis in a wrong manner. Can anyone explain?

JSFiddle example

Answer 1

The result of "HelloWorld" && "GoodbyeWorld" is "GoodbyeWorld" which is why you're getting the result you are, the previous way you were doing it is the simplest solution

Answer 2

Let's take a look at this line

item.hello !== ("HelloWorld" && "GoodbyeWorld") // true WTF?

The logical AND operator evaluates its right operand, if lVal is a truthy value.

Note, a truthy value is every value which is not falsy (null,false,0,"",undefined,NaN)

Since "HelloWorld" is indeed truthy

The expression ("HelloWorld" && "GoodbyeWorld") evaluates to "GoodbyeWorld" and you're effectively comparing

item.hello !== "GoodbyeWorld" which can be reduced to "HelloWorld" !== "GoodbyWorld"

Hence, is true

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However, if you're on an ES5 compatible environment you could use Array.prototype.indexOf to simplify it.

!~["HelloWorld","GoodbyWorld"].indexOf(item.hello) //false

the above returns true if item.hello is not contained in the array