Javascript Check if Array is empty or undefined

Question

I have a function that goes something like

//denominator is a passed argument. Integer
/*priceData will either be an empty array, or an array
that looks something like this
priceData = {
 'splitBy2' : [0.5, 0.5],
 'splitBy3' : [0.34, 0.33,0.33],
 'splitBy4': [0.25, 0.25, 0.25, 0.25]
}
*/
//Returns list of values. Currency distribution. Eg if denominator = 3
//Then distribution will equal [0.34, 0.33,0.33]
distribution = distributionToArray(currency(totalPrice).distribute(denominator))
//This check doesn't work.
if(typeof (priceData['splitBy'+denominator]) !== 'undefined' && (priceData['splitBy'+denominator]).length == 0){
 priceData['splitBy'+denominator] = distribution;
 } 

What I want to do is check if priceData['splitBy'+denominator] is empty. And if it isn't, create it.

So let

denominator=5
priceData = {
 'splitBy2' : [0.5, 0.5],
 'splitBy3' : [0.34, 0.33,0.33],
 'splitBy4': [0.25, 0.25, 0.25, 0.25]
}

I want to check if priceData['splitBy5'] is set, and if it's not, run the rest of the function priceData['splitBy5] = distribution;` to create it. Even if priceData is empty.

Answer 1

You've shown this code:

if(typeof (priceData['splitBy'+denominator]) !== 'undefined' && (priceData['splitBy'+denominator]).length == 0){
 priceData['splitBy'+denominator] = distribution;
}

and that should certainly work if the property A) Either doesn't exist or has the value undefined, or B) Has an array.

What I want to do is check if priceData['splitBy'+denominator] is empty. And if it isn't, create it.

Your code should do that. But since an array (even an empty one) is a truthy value, you can do:

const key = "splitBy" + denominator;
if (!priceData[key] || priceData[key].length === 0) {
 priceData[key] = distribution;
}

!priceData[key] looks to see if the property doesn't exist (or has a falsy value; no array is falsy). priceData[key].length === 0 checks to see if the array is empty. Since they're joined with || ("or"), if the first check is true the second isn't performed and so won't cause an error.

The two key (no pun!) changes there are:

1. Building the key once. This is purely so that we avoid typos like splitby instead of splitBy in one of the three places it would be used.

2. Using a falsy rather than undefined check.

But if you know the property won't be there, will have the value undefined, or will be an array, your code should work. All of the 'splitBy'+denominator concatenations look like they're spelled the same, and the typeof check is correct (and doesn't misspell undefined, which is my particular bugaboo). You don't need any of those inner (), but they don't do any harm.

• Josh: So your code works (yay!) but I did try basically exactly that before and it didn't work. The difference being you had key as a constant. Does it not work with dynamic keys?
• T.J. Crowder: @JoshDredge - You're right, looking more closely at the code in the question, that code should work just fine. The above works with dynamic keys. You could replace key with "splitBy" + denominator in all three places in the if and it would be just the same. Something else must have been going on. Is there some possibility that the property's value is some falsy value that isn't undefined (like null, "", 0, NaN, or false)?
• T.J. Crowder: I really should have read the code in the question more closely before posting the answer, because I wouldn't have -- i'd've commented instead. Still, it looks like it's turned out useful for some reason, so...
• Josh: Definitely useful, just don't know why. I was having trouble with dynamic jQuery selectors the other day I never got working either so who knows!