What is the shortest way of checking if JS value exists?

Question

Assuming I have this :

var a = parseInt(prompt("Enter A"));
var b = parseInt(prompt("Enter B"));
var c = parseInt(prompt("Enter C"));

What is the shortest way to check if a value exists in all 3 variables ?

Doing if(a && b && c) is not accurate cause 0 is falsy.

p.s. a,b,c can be ints or strings...Also , There must be a trick here...

for the string example :

var a = (prompt("Enter A"));
var b = (prompt("Enter B"));
var c = (prompt("Enter C"));

you say they can be ints or strings, but with the `parseInt()`, they will definitely be ints. (or NaNs, I guess) — SDC, Jan 04 '13 at 15:31
Something I should mention: [check this question](http://stackoverflow.com/q/850341/913097) if you're using the `parseInt()` function. — inhan, Jan 04 '13 at 15:36
@inhan you can count on it. I know this octal problem. ( this problem does _not_ exist with chrome.) only FF and IE. — Royi Namir, Jan 04 '13 at 15:37
What's wrong with `a && b && c` for strings? "0" is not falsy .. or don't you know the types in advance? — Alex K., Jan 04 '13 at 15:39
Aye but not as a string from a prompt() cant you && if you want 3 strings & isNaN is you want 3 ints? — Alex K., Jan 04 '13 at 15:41
My point is 0 from parseInt is falsy but "0" from prompt() is not so && is ok if your not using parseInt, if you are using parseInt you can isNaN. — Alex K., Jan 04 '13 at 15:52

Dancrumb · Accepted Answer · 2013-01-04T15:40:16.493

The only way to categorically confirm whether all of these values is defined, regardless of type, is this:

if( (typeof a !== 'undefined') || 
    (typeof b !== 'undefined') || 
    (typeof c !== 'undefined')) { /* ... */ }

Unfortunately, shortcuts that rely on truthiness of values and, as you point out, this is not always an acceptable approach.

As Alex K shows, you can do some tricky stuff if you know the type of the variable beforehand, but knowing the type beforehand may require as much code as you avoid in the comparison.

Edit 1

Dave Newton pointed out that prompt always returns a defined value; it returns null if user's press cancel.

Thus, a better approach would be:

function isValid(value) {
    return typeof value !== 'undefined' && value !== null);
}

if( isValid(a) && isValid(b) && isValid(c) ){ /* ... */ }

Edit 2

Of course, the simplest thing to do is to check for validity before parsing the ints. Since

!!""  === false
!!"0" === true
!!0   === false

You can use (a && b & c) on the string/null that prompt returns and only if they are valid, do your parsing.

They'll always be defined from a prompt, no? Even if you hit escape it'd be null. — Dave Newton, Jan 04 '13 at 15:36

score 1 · Answer 2 · answered Jan 04 '13 at 15:33

1

How about this:

ok = !isNaN(a+b+c);

answered Jan 04 '13 at 15:33

Alex K.

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Alas, OP just edited with a completely different second case. – apsillers Jan 04 '13 at 15:33
@apsillers I edited now, but wrote it when first publishing.(a,b,c can be also strings). the _edit_ was written now. – Royi Namir Jan 04 '13 at 15:34
@RoyiNamir But saying "they can also be strings" didn't make any sense given the code. The code is always the defining artifact, since it's what runs. – Dave Newton Jan 04 '13 at 15:38

score 0 · Answer 3 · answered Jan 04 '13 at 15:53

Did you try (!!a && !!b && !!c)?

AFAIK, unless the value of a string variable is empty, null or undefined it should evaluate to true. 0 would also return false but I'm writing on behalf of your prompt() call since its value would not be of type Number.

score 0 · Answer 4 · answered Jan 04 '13 at 15:55

Always use:

if (typeof( variableName ) != "undefined") {
    //Do something
}

Since Javascript uses Json object notation the following code is completely valid:

var x = {};

if (x.test == undefined) {
    //Do something
}

But if "x" does not exists this produces an error. There is no way to do this safer in Javascript.

score 0 · Answer 5 · answered Jan 04 '13 at 16:10

The shortest way to check if a set of variables has valid, acceptable values is to create a helper function according to your specification which accepts a variable number of arguments. In this example, this function will return false if an argument is undefined, null, or a NaN number. Strings will return true for isNaN, so we check if the argument's type is a number before checking if it is NaN.

function isValid() {
    var i, args = Array.prototype.slice.call(arguments, 0);
    for(i = 0; i < args.length; i += 1) {
        if( typeof args[i] === 'undefined' || args[i] === null
        || (typeof args[i] === 'number' && isNaN(args[i]))
        ) {
            return false;
        }
    }
    return true;
}

Utilize it like so:

if( isValid(a, b, c) ) {
    ...
}

Justineo · Answer 6 · 2013-01-04T17:22:13.917

If the values can only be strings or numbers, you can use:

var valid = ('' + a) && ('' + b) && ('' + c); //always convert into string

Here's also a fancy but relatively short one, which deals with undefined, null and empty string:

var valid = [a].join() && [b].join() && [c].join();

In Array.prototype.join(), when an element is undefined or null it will be converted into an empty string.

But it's neither elegant nor efficient. :(

What is the shortest way of checking if JS value exists?

6 Answers6