javascript select first non falsey value shows error with array index

Question

I'm trying to use the syntax of:

someVar = otherVar || ''; 
// set someVar to otherVar, or '' if otherVar is false

when I turn otherVar into some array key,

someVar = otherVar[1] || ''; // otherVar[1] is undefined.

i get the error

Cannot read property '1' of undefined

This makes sense since otherVar[1] is undefined...But -

Question: Is the only way to prevent this, to check if otherVar[1] is truthy before setting someVar? Or can I still use this easy method to do set variables quickly like an if else?

I also tried

someVar = (!!otherVar[1]) ? otherVar[1] : ''; // didn't work either.

Thanks!

Coin_op · Accepted Answer · 2013-04-16 05:02:48Z

3

You have to first test that otherVar exists so you cannot really do it with that syntax, but you could do this:

someVar = otherVar && otherVar[1] ? otherVar[1] : '';

This works because the and statement fails before the test for the index.

answered Apr 16, 2013 at 5:02

Coin_op

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1 Comment

d-_-b Over a year ago

Awesome, thanks! Why would checking for the key instead of the whole array not work?

elclanrs · Accepted Answer · 2013-04-16 05:07:09Z

1

I'm assuming this is your case where the variable is declared but undefined. You can use this little trick, cryptic but neat:

var arr; // declared but 'undefined'
var result = (arr || [,'foo'])[1];

console.log(result); //=> "foo"

arr = [1, 2]; // declared and defined
result = (arr || [,'foo'])[1];

console.log(result); //=> 2

answered Apr 16, 2013 at 5:07

elclanrs

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