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

问题

I was encountering a lot of bugs in my code because I expected this expression:

Boolean([]); to evaluate to false.

But this wasn't the case as it evaluated to true.

Therefore, functions that possibly returned [] like this:

// Where myCollection possibly returned [ obj1, obj2, obj3] or []
if(myCollection)
{
  // ...

}else
{
  // ...
}

did not do expected things.

Am I mistaken in assuming that [] an empty array?

Also, Is this behavior consistent in all browsers? Or are there any gotchas there too? I observed this behavior in Goolgle Chrome by the way.

回答1:

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.

回答2:

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)

来源：https://stackoverflow.com/questions/19146176/javascript-empty-array-evaluates-to-true-in-conditional-structures-why-is