JavaScript: empty array, [ ] evaluates to true in conditional structures. Why is this?

From http://www.sitepoint.com/javascript-truthy-falsy/

The following values are always falsy:

• false
• 0 (zero)
• "" (empty string)
• null
• undefined
• NaN (a special Number value meaning Not-a-Number!)

All other values are truthy, including "0" (zero in quotes), "false" (false in quotes), empty functions, empty arrays, and empty objects.

Regarding why this is so, I suspect it's because JavaScript arrays are just a particular type of object. Treating arrays specially would require extra overhead to test Array.isArray(). Also, it would probably be confusing if true arrays behaved differently from other array-like objects in this context, while making all array-like objects behave the same would be even more expensive.

You should be checking the .length of that array to see if it contains any elements.

if (myCollection) // always true
if (myCollection.length) // always true when array has elements
if (myCollection.length === 0) // same as is_empty(myCollection)

While [] equals false, it evaluates to true.

yes, this sounds bad or at least a bit confusing. Take a look at this:

const arr = [];
if (arr) console.log("[] is truethy");
if (arr == false) console.log("however, [] == false");

In practice, if you want to check if something is empty, then check the length. (The ?. operator makes sure that also null is covered.)

const arr = []; // or [];
if (!arr?.length) console.log("empty or null")