Logical expression

Question

Is there any difference between the below statements:

if(newValue && newValue != '') and
if(newValue != '')

I have observed expression 1 in many scripts but always got confused.

Please assist!

The first will reject all other falsy values not weakly equal to the empty string. — John Dvorak, Dec 28 '12 at 14:08

Ja͢ck · Answer 1 · 2012-12-28T14:19:44.347

3

if(newValue && newValue != '').

This guards against a value of null or undefined.

Out of the possible '', 0, false, undefined and null, only the last two are not equal to '' (using !=), requiring the extra condition.

console.log(null && null != '') // null -> falsy
console.log(null != '') // truthy

var undef = void 0;

console.log(undef && undef != '') // undefined -> falsy
console.log(undef != '') // truthy

edited Dec 28 '12 at 14:19

answered Dec 28 '12 at 14:11

Ja͢ck

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@JanDvorak What else then? – Ja͢ck Dec 28 '12 at 14:14
1

Wow... I thought `0!=''` was true – John Dvorak Dec 28 '12 at 14:16
1

@JanDvorak To be honest, I evaluated them all to be sure ;-) – Ja͢ck Dec 28 '12 at 14:17
What about `undefined`? `undefined !== null`. – John Dvorak Dec 28 '12 at 14:17
What is `var undef = void 0;`? I have never seen this syntax before but apparently it works. – John Dvorak Dec 28 '12 at 14:21
@JanDvorak It's an expression to get `undefined` literal, always :) – Ja͢ck Dec 28 '12 at 14:21
Isn't `(function(u){return u})()` or `[][0]` (or `[][[]]` if you hate alphanumerics) a better way to get `undefined`? – John Dvorak Dec 28 '12 at 14:25
Cf.: http://stackoverflow.com/questions/4806286/difference-between-void-0-and-undefined – John Dvorak Dec 28 '12 at 14:28
@JanDvorak My logic is that `void 0` is shorter than the first and clearer than the second (and even clearer than the third) :) I would use your first option if I already had a closure though. – Ja͢ck Dec 28 '12 at 14:29
jQuery uses the first approach to get `undefined`; the whole library is wrapped in a single huge IIFE to capture `window`, `document` and compute `undefined`. – John Dvorak Dec 28 '12 at 14:32
@JanDvorak Yup, it's a good option; to illustrate an example though, I feel it would distract too much ... not saying `void 0` doesn't distract, but hey, we're all here to learn ya? ;-) – Ja͢ck Dec 28 '12 at 14:34

score 1 · Answer 2 · answered Dec 28 '12 at 14:09

1

Answer is no,

1) for (newValue && newValue != ''),it checks whether newValue exist(non false values) and its not empty

2) for if(newValue != ''),it only checks whether newValue is not empty

answered Dec 28 '12 at 14:09

Milind Anantwar

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Thank you so much Milind, I got it now! – user1934643 Dec 28 '12 at 14:10
Is there any truthy value weakly equal to the empty string? – John Dvorak Dec 28 '12 at 14:11
You mean the answer is "yes" because there is a difference between the two. – David G Dec 28 '12 at 14:16

score 0 · Answer 3 · answered Dec 28 '12 at 14:59

Just one more thin I thought worth adding to the answers already posted.
As you, the OP, say: you've seen the first expression in existing code many times, so you've probably seen things like this:

if (foo && foo.bar)
{
    foo.bar();
}

In this case, this is to avoid errors in certain browsers. In most modern browsers accessing somehting like localStorage.doesNotExist won't raise errors, but not all browsers support this so we: first check if the localStorage object exists, and if so, if the property can be resolved to a non-falsy value.

The same logic applies to methods of objects, that may be different in various browsers:

var domElement = document.getElementById('foobar');//might not exist, though
if (domElement && domElement.addEventListener)
{//does element exist && does it have a method called addEventListener
    domElement.addEventListener('click', functionRef, false);
}
else if (domElement && domElement.attachEvent)
{
    domElement.attachEvent('onclick', functionRef);
}

if you were to omit the first domElement in the if statement, and simply write domElement.addEventListener, that could be the same as writing null.addEventListener which throws a TypeError (because you're trying to access a property on a primitive value)

Logical expression

3 Answers3