Is empty array in JavaScript truthy or falsy?

Today, one of my friends argued with me about the empty array [], he think it’s falsy but in my experience I persist it should be truthy.

So, I demo it as follows:

var a = [];
if (a) console.log('empty array is truthy');

Or you can also use double logical NOT to cast it to boolean, it’s the same.

var a = [];
if (!!a) console.log('empty array is truthy');

Looking the the evidence sentence shown in the screen, I think the argument is over till he shows me the following code,

console.log([] == false);

Guess what?

The result is true. Woooooot? Well, I know there must be something wrong, escpecially the suspicous loose equality operation. I have to figure it out why it has such odd behaviour.

Type Coertion

There are both awesome articles about the equality comparison algorithm in JavaScript world.

The key point is applying ToPrimitive([]) and ToNumber(false), then compare them.

What’s ToPrimitive?

ToPrimitive is a internal operation which attempts to convert its object argument to a primitive value, by attempting to invoke varying sequences of toString and valueOf methods on the Object. The detail steps can be found here: http://bclary.com/2004/11/07/#a-9.1
So, firstly try [].valueOf() which returns itself, it’s not primitive, then try to call [].toString, for Array.toString, according MDN,

For Array objects, the toString method joins the array and returns one string containing each array element separated by commas.

This means we get the empty string ''.

What’s the ToNumber?

Basically apply the + operation on the target, for boolean,

1 if the argument is true. +0 if the argument is false.

for other types refer to :http://bclary.com/2004/11/07/#a-9.3

Problem now is simplified

Is '' == 0 truthy or falsy? Again begin with applying ToNumber to emtpy string '', the rules is complicated here but for empty string it’s simple as follows,

A StringNumericLiteral that is empty or contains only white space is converted to +0.

So, 0 === 0? Surely it’s true.

Conclusion

Do NOT use loose eqality operation, it’s complex and confusing even for experienced JavaScript developers. Although the only exception is checking with null and undefined because either is equal to itself. i.e foo != null is equivalent to foo !== null && foo !== undefined. However, I don’t recommend this way for less typing leads more confusion, the reader may even doubt whether the author forgot a = or not.