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

You can't do it like that. I can't think of a single language that supports such a construct. For a more detailed explanation, see http://stackoverflow.com/a/17200368/146205. — Jensen, Sep 30 '13 at 07:57

score 3 · Accepted Answer · answered Sep 30 '13 at 07:59

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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

answered Sep 30 '13 at 07:59

Matthew Mcveigh

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The result of "HelloWorld" && "GoodbyeWorld" is "GoodbyeWorld": thanks, that I didn't quite realize. Too bad there is no way to simplify it. – Justus Romijn Sep 30 '13 at 08:02

Moritz Roessler · Answer 2 · 2013-09-30T08:20:11.840

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

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

Oswald · Answer 3 · 2013-09-30T08:05:09.380

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item.hello !== "HelloWorld" && item.hello !== "GoodbyeWorld"

is the proper way to test whether item.hello is distinct from "HelloWorld" and "GoodbyeWorld".

An expression A && B in JavaScript yields a result which is either A or B and it's that result, that is compared to your item.hello.

edited Sep 30 '13 at 08:05

answered Sep 30 '13 at 07:57

Oswald

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score 1 · Answer 4 · answered Sep 30 '13 at 08:01

No, you can't optimise the expression that way. What you are doing is elliminating one of the strings, so that you are only doing one of the comparisons.

The && operator uses short circuit evaluation, which means that if the first operand is truthy, it will just return the second operand.

So, what your code is doing is to compare the hello property value to the second one of the strings, which explains the results that you get.

Javascript how to use the and/or operator with parenthesis

4 Answers4