I am going through JavaScript: The Definitive Guide. In it it says
Boolean([]) // => true
But I don't understand the logic behind this. Why is the boolean of an empty array true?
3 Answers
The ECMAScript specification defines how values are cast to booleans, per the abstract ToBoolean operation: https://www.ecma-international.org/ecma-262/6.0/#sec-toboolean
That operations includes a single entry for object input:
Object: Return
true.
Thus, when you supply any object to Boolean, including an array (even an empty one), you'll get a true value back,
1 Comment
Brian Schlenker
I think the "why" here is referring to the decisions behind choosing to make “Boolean([]) === true”, not “where in the spec is this behavior described”. Javascript didn’t just evolve out of the dirt. Someone had to think that an empty array being truthy had advantages over an empty array being falsy.
Array is considered as an object, even if it's empty. That's why the Boolean has a value, means it's true.
Only false, null or undefined are values which will return false.
Comments
JavaScript (and other languages) have a concept of 'truthy' and 'falsey' values.
You said you're from a C++ background, so we can make it analogous to something like this in C++:
if (ptr) { }
which is falsey if ptr is null, and truthy otherwise.
It just so happens that in JavaScript, arrays - even empty ones, among many other things - are considered to be truthy.
NaNvalues, empty strings,false,nullandundefinedare considered false-y. Other languages make different decisions, e.g. an empty collection is false-y in Python.